Plant physiology explains how plants function: how they obtain and move water and minerals, capture and transform light energy, exchange gases, respire, grow, develop, reproduce, germinate, senesce and ultimately produce biomass and harvestable yield.

1.1 Scope

Physiology links cellular processes with whole-plant performance and environmental responses.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

1.2 Structure and function

Anatomical structures such as roots, xylem, leaves and stomata provide the physical framework for physiological processes.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

1.3 Energy and matter

Plants transform radiant energy and move carbon, water, ions and metabolites among organs.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

1.4 Regulation

Plant functions are coordinated through biochemical signals, environmental perception and developmental programmes.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

1.5 Experimental physiology

Physiological conclusions depend on controlled observations, measurement and appropriate comparisons.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

1.6 Crop relevance

Yield is an integrated outcome of photosynthesis, respiration, mineral nutrition, water relations, development and allocation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Water is the major medium for biochemical reactions, transport and cell expansion. Its polarity and hydrogen bonding explain many properties important to plants.

2.1 Polarity

Unequal charge distribution makes water an effective solvent for ions and polar molecules.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

2.2 Hydrogen bonding

Hydrogen bonds contribute to cohesion, adhesion, high heat capacity and surface tension.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

2.3 Cohesion

Attraction among water molecules helps maintain continuous columns in xylem.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

2.4 Adhesion

Attraction to hydrophilic surfaces contributes to water behaviour in cell walls and narrow conduits.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

2.5 Thermal properties

High specific heat buffers temperature changes, while evaporation removes substantial heat.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

2.6 Solvent role

Dissolved mineral ions move through aqueous phases and biochemical reactions occur largely in water.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

2.7 Turgor

Water entering cells can generate pressure against the cell wall, supporting non-woody tissues.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

2.8 Water as reactant

Water participates directly in biochemical reactions including the light reactions of oxygenic photosynthesis.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Plant water relations describe absorption, movement, loss and physiological effects of water. Water potential provides a framework for predicting spontaneous water movement.

3.1 Water potential

Water moves from higher to lower water potential when a pathway exists.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

3.2 Solute potential

Dissolved solutes lower the free energy of water; solute potential is zero for pure water under reference conditions and negative in solutions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

3.3 Pressure potential

Positive turgor pressure can raise cellular water potential.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

3.4 Osmosis

Osmosis is net water movement across a selectively permeable membrane driven by differences in water potential.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

3.5 Turgid cells

Water entry into a plant cell can increase pressure potential until equilibrium is approached.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

3.6 Plasmolysis

Sufficient water loss in a hypertonic environment can separate the protoplast from the cell wall.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

3.7 Soil-plant-atmosphere continuum

Water movement from soil through plant to atmosphere is driven by linked gradients and transport resistances.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

3.8 Water stress

Declining water availability affects stomata, cell expansion, photosynthesis and metabolism before severe dehydration occurs.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Roots absorb water from soil and deliver it to xylem, from which transpiration-driven transport supplies shoots.

4.1 Root hairs

Root hairs increase contact between absorbing surfaces and soil water.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

4.2 Apoplastic pathway

Water can move through cell walls and intercellular spaces before reaching endodermal barriers.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

4.3 Symplastic pathway

Water can move through cytoplasm connected by plasmodesmata.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

4.4 Endodermis

The endodermis imposes selective control over entry into the vascular cylinder.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

4.5 Xylem transport

Water ascends principally through tracheary elements.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

4.6 Cohesion-tension mechanism

Evaporation from leaves generates tension transmitted through cohesive xylem water columns.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

4.7 Root pressure

Positive root pressure can occur under certain conditions but does not explain the normal ascent of water in tall transpiring plants.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

4.8 Cavitation

Breakage of the water column can reduce hydraulic conductivity and influence drought responses.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Transpiration is loss of water vapour from aerial plant surfaces, predominantly through stomata in most leaves.

5.1 Stomatal transpiration

Water evaporates from moist internal surfaces and diffuses through stomatal pores.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

5.2 Cuticular loss

Water can also diffuse across the cuticle, especially when stomata are closed.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

5.3 Stomatal control

Guard-cell turgor changes regulate pore aperture.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

5.4 Benefits

Transpiration supports mineral transport, evaporative cooling and water movement through the plant.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

5.5 Costs

Excessive loss can reduce turgor, growth and photosynthesis and can lead to hydraulic failure.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

5.6 Boundary layer

Still air near the leaf surface imposes resistance to vapour movement.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

5.7 Leaf area

Total transpiring area influences whole-plant water loss.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

5.8 Water-use balance

Plants continually balance carbon dioxide uptake against water loss because both gases pass through stomatal pathways.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Transpiration rate is controlled by interacting environmental and internal factors rather than by one variable in isolation.

6.1 Light

Light commonly promotes stomatal opening and raises leaf energy input, though responses depend on species and conditions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

6.2 Humidity

A larger vapour-pressure difference between leaf and air generally increases evaporative demand.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

6.3 Temperature

Temperature affects saturation vapour pressure, evaporation and stomatal responses.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

6.4 Wind

Moderate air movement can reduce boundary-layer resistance, whereas strong wind may induce stomatal responses or alter leaf temperature.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

6.5 Soil water

Water deficit commonly triggers stomatal closure and reduces leaf expansion.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

6.6 Stomatal density

Number and distribution of stomata influence potential conductance.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

6.7 Leaf morphology

Cuticle, hairs, rolling and leaf dimensions alter water-loss pathways and boundary layers.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

6.8 Plant hormones

Abscisic acid is an important signal in stomatal responses to water deficit.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Photosynthesis converts light energy into chemical energy and uses that energy to support carbon assimilation.

7.1 Overall concept

Oxygenic photosynthesis uses light-driven electron transfer to generate ATP and reducing power and releases oxygen from water.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

7.2 Chloroplast

Thylakoid membranes contain photosynthetic electron-transfer machinery; the stroma contains enzymes of carbon assimilation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

7.3 Light reactions

Photochemical reactions capture energy and produce ATP and NADPH.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

7.4 Carbon reactions

The Calvin cycle uses ATP and NADPH to reduce carbon dioxide into organic carbon.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

7.5 Gas exchange

Carbon dioxide enters leaves mainly through stomata and diffuses to photosynthetic cells.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

7.6 Limiting factors

Light, carbon dioxide, temperature, water and nutrient status can constrain photosynthetic rate.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

7.7 Net photosynthesis

Measured net carbon gain reflects gross photosynthetic fixation minus respiratory and related carbon losses.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

7.8 Physiological integration

Photosynthesis is connected to water relations, mineral nutrition, respiration and source-sink demand.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Pigments absorb particular wavelengths and transfer excitation energy within photosynthetic complexes.

8.1 Chlorophyll a

Chlorophyll a is central to oxygenic photosynthetic reaction centres.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

8.2 Chlorophyll b

Chlorophyll b expands the usable absorption spectrum in green plants and transfers excitation energy.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

8.3 Carotenoids

Carotenoids contribute light harvesting and photoprotection.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

8.4 Absorption spectrum

An absorption spectrum shows wavelengths absorbed by a pigment.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

8.5 Action spectrum

An action spectrum relates wavelength to the effectiveness of a biological process such as photosynthesis.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

8.6 Pigment-protein complexes

Pigments are precisely organised with proteins in thylakoid membranes.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

8.7 Leaf colour

Green appearance reflects stronger reflection/transmission of some green wavelengths relative to strongly absorbed red and blue regions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

8.8 Chromatographic separation

Pigments can be separated experimentally according to differential interactions with stationary and mobile phases.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Reaction centres convert excitation energy into stable charge separation and electron flow.

9.1 Photosystem II

PSII initiates electron transfer associated with oxidation of water and oxygen evolution.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

9.2 Water splitting

The oxygen-evolving complex extracts electrons from water, releasing protons and molecular oxygen.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

9.3 Electron carriers

Electrons pass through membrane-associated carriers toward photosystem I.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

9.4 Photosystem I

PSI re-energises electrons using light and supports reduction of NADP+ to NADPH.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

9.5 Proton gradient

Electron transfer contributes to proton accumulation across the thylakoid membrane.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

9.6 ATP synthase

Proton flow through ATP synthase drives ATP formation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

9.7 Linear electron flow

Electrons move from water through PSII and PSI toward NADP+, producing ATP, NADPH and oxygen.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

9.8 Photoprotection

Plants dissipate excess excitation and regulate electron flow to limit photodamage.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Photophosphorylation is light-driven formation of ATP associated with photosynthetic electron transport.

10.1 Chemiosmosis

Electron transport establishes an electrochemical proton gradient across the thylakoid membrane.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

10.2 ATP formation

ATP synthase couples proton movement to phosphorylation of ADP.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

10.3 Non-cyclic flow

Linear flow through both photosystems supports formation of NADPH and ATP and releases oxygen.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

10.4 Cyclic flow

Cyclic electron flow around PSI can contribute ATP production without net NADPH formation or oxygen evolution from that cyclic route.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

10.5 Energy balance

Relative ATP and reducing-power requirements can influence electron-flow regulation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

10.6 Membrane integrity

Thylakoid organisation is essential because chemiosmosis requires separation of proton concentrations.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

10.7 Light dependence

Photophosphorylation depends on light-driven electron transfer rather than direct substrate oxidation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

10.8 Connection to Calvin cycle

ATP and NADPH generated in the light reactions supply energy and reducing equivalents for carbon assimilation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

The Calvin-Benson cycle fixes carbon dioxide in the chloroplast stroma through carboxylation, reduction and regeneration phases.

11.1 Carboxylation

Rubisco catalyses addition of carbon dioxide to ribulose-1,5-bisphosphate, producing unstable six-carbon material that yields 3-phosphoglycerate.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

11.2 Reduction

ATP and NADPH convert 3-phosphoglycerate through phosphorylated intermediates to triose phosphate.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

11.3 Product withdrawal

A fraction of triose phosphate can contribute to carbohydrate and other biosynthesis.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

11.4 Regeneration

Most cycle carbon regenerates ribulose-1,5-bisphosphate so fixation can continue.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

11.5 Rubisco

Rubisco is the key carboxylating enzyme but also catalyses oxygenation, leading to photorespiration.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

11.6 Stoichiometric concept

Fixation of three CO2 yields one net triose-phosphate equivalent while the remainder regenerates acceptor molecules.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

11.7 Energy requirement

Carbon reduction requires ATP and NADPH supplied principally by photosynthetic light reactions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

11.8 Regulation

Calvin-cycle activity is coordinated with illumination, metabolite availability and chloroplast conditions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

C3 and C4 plants differ in the initial route of carbon fixation and spatial organisation of carbon assimilation.

12.1 C3 pathway

In C3 plants, Rubisco directly fixes atmospheric carbon dioxide into Calvin-cycle intermediates in mesophyll chloroplasts.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

12.2 C4 initial fixation

PEP carboxylase initially fixes bicarbonate into four-carbon compounds in mesophyll cells.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

12.3 Bundle sheath

Four-carbon acids deliver concentrated carbon to bundle-sheath cells, where CO2 is released for the Calvin cycle.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

12.4 Kranz anatomy

Many C4 leaves show characteristic organisation of mesophyll around prominent bundle-sheath tissues.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

12.5 Photorespiration

Carbon concentration around Rubisco suppresses photorespiration in C4 plants.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

12.6 Energy cost

The C4 concentrating mechanism requires additional energy.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

12.7 Environmental performance

C4 photosynthesis is often advantageous under high light, high temperature and conditions favouring photorespiration, though ecological performance depends on many factors.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

12.8 Crop examples

Important C4 crops include maize, sorghum and sugarcane.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Photorespiration begins when Rubisco acts as an oxygenase rather than a carboxylase.

13.1 Oxygenation

RuBP oxygenation produces one normal Calvin-cycle product and one phosphoglycolate-related product requiring recovery.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

13.2 Organelle cooperation

Recovery involves chloroplasts, peroxisomes and mitochondria.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

13.3 Carbon cost

The pathway releases previously fixed carbon dioxide and consumes energy.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

13.4 Temperature

High temperature can favour conditions that increase Rubisco oxygenation relative to carboxylation in C3 leaves.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

13.5 Stomatal closure

Reduced internal CO2 during water stress can increase the relative importance of oxygenation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

13.6 C4 suppression

C4 carbon concentration reduces photorespiration around Rubisco.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

13.7 Not simple respiration

Photorespiration is biochemically distinct from mitochondrial respiratory oxidation of sugars.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

13.8 Physiological context

Although costly for carbon assimilation, the pathway is integrated with nitrogen metabolism and cellular redox/photoprotection processes.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

CAM is a carbon-concentrating adaptation in which initial CO2 fixation and Calvin-cycle processing are separated mainly in time.

14.1 Night opening

Many CAM plants open stomata predominantly at night, reducing evaporative water loss.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

14.2 Initial fixation

PEP carboxylase fixes carbon into organic acids during the night.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

14.3 Acid storage

Organic acids accumulate, commonly in vacuoles.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

14.4 Day decarboxylation

Stored acids release CO2 during the day.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

14.5 Calvin cycle

Released CO2 is reassimilated by Rubisco while stomata can remain relatively closed.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

14.6 Water-use efficiency

Temporal separation can improve carbon gain per unit water lost in arid or epiphytic environments.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

14.7 Energetic trade-off

CAM imposes biochemical and storage constraints and often supports slower growth than unrestricted high-carbon assimilation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

14.8 Plasticity

Some plants can vary CAM expression with environmental conditions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Glycolysis converts glucose to pyruvate through a sequence of cytosolic reactions that capture part of the substrate's energy as ATP and NADH.

15.1 Activation phase

ATP is invested to phosphorylate and rearrange the six-carbon substrate.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

15.2 Cleavage

A six-carbon intermediate is divided into three-carbon units.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

15.3 Oxidation

Glyceraldehyde-3-phosphate is oxidised and NAD+ is reduced.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

15.4 Substrate-level phosphorylation

High-energy intermediates transfer phosphate directly to ADP.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

15.5 Pyruvate formation

Phosphoenolpyruvate is converted to pyruvate with ATP production.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

15.6 Net yield

Per glucose, glycolysis has a net yield of two ATP and two NADH, with two pyruvate produced.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

15.7 Anaerobic context

When oxidative metabolism is limited, fermentation can regenerate NAD+ so glycolysis continues.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

15.8 Plant integration

Glycolytic intermediates also supply carbon skeletons for biosynthesis.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Cellular respiration oxidises organic substrates and conserves energy in ATP while providing metabolic intermediates.

16.1 Pyruvate oxidation

Pyruvate can enter mitochondria and be converted to acetyl-CoA with production of CO2 and reducing equivalents.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

16.2 Citric acid cycle

Acetyl units are oxidised, producing CO2, NADH, FADH2 and nucleotide triphosphate equivalents.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

16.3 Electron transport

Electrons from reduced cofactors pass through respiratory complexes to oxygen.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

16.4 Oxidative phosphorylation

Electron transport establishes a proton gradient used by ATP synthase.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

16.5 Respiration in plants

Respiration occurs in living plant cells by day and night; it does not stop simply because photosynthesis occurs in light.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

16.6 Alternative pathways

Plant mitochondria possess alternative respiratory components that can alter energy conservation and redox balance.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

16.7 Respiratory substrates

Carbohydrates are major substrates, while lipids and proteins can also contribute under particular conditions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

16.8 Growth and maintenance

Respiratory energy supports biosynthesis, transport, ion uptake, repair and cellular maintenance.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Plant growth involves irreversible increases in size or dry mass produced through cell division, expansion and differentiation.

17.1 Cell division

Meristems generate new cells.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

17.2 Cell expansion

Water uptake, wall loosening and synthesis enlarge cells.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

17.3 Differentiation

Cells acquire specialised structures and functions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

17.4 Growth measurement

Length, area, fresh mass and dry mass measure different aspects of growth.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

17.5 Auxin

Auxins regulate processes including cell elongation, apical dominance, tropisms and root development depending on concentration and tissue.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

17.6 Gibberellins

Gibberellins promote processes such as stem elongation and can influence germination and reproductive development.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

17.7 Cytokinins

Cytokinins promote cell division and interact with other signals in shoot/root development and senescence.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

17.8 Hormone interaction

Plant development is controlled by networks of hormones and environmental signals rather than one hormone acting alone.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Growth regulation includes signals that coordinate stress responses, dormancy, ripening and developmental transitions.

18.1 Abscisic acid

ABA participates strongly in drought responses, stomatal regulation and seed dormancy.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

18.2 Ethylene

Ethylene regulates processes including fruit ripening in climacteric fruits, abscission and responses to mechanical or flooding signals.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

18.3 Signal interaction

ABA, ethylene, auxin, gibberellins and cytokinins interact in context-dependent ways.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

18.4 Hormone sensitivity

Response depends on tissue competence and receptor/signalling systems, not only hormone concentration.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

18.5 Transport

Some hormones are transported over long distances while others act locally or through precursor movement.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

18.6 Environmental integration

Light, temperature, water and nutrient status modify hormonal responses.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

18.7 Agricultural use

Plant growth regulators can be used to manipulate rooting, fruiting, ripening or plant stature when applied appropriately.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

18.8 Caution

A hormone should not be described as having only one universal effect; dose, tissue, developmental stage and interactions matter.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Flowering is a developmental transition coordinated by endogenous state and environmental cues.

19.1 Competence

Plants must reach a developmental condition capable of responding to flowering signals.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

19.2 Photoperiodism

Some species respond to relative day/night length.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

19.3 Dark period

For many photoperiodic responses, uninterrupted night length is particularly important.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

19.4 Phytochrome

Phytochrome photoreceptors participate in red/far-red light sensing and developmental regulation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

19.5 Vernalization

Some plants require prolonged exposure to low temperature to acquire competence for flowering.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

19.6 Floral signalling

Signals generated in leaves can influence the shoot apex and promote floral transition.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

19.7 Autonomous pathways

Internal developmental programmes also contribute to flowering time.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

19.8 Crop importance

Flowering time determines adaptation, reproductive success and timing of yield formation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Dormancy prevents germination even when some apparently favourable environmental conditions are present, thereby regulating timing of seedling establishment.

20.1 Physical dormancy

Impermeable seed coverings can restrict water entry.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

20.2 Physiological dormancy

Embryonic physiological mechanisms can prevent germination until appropriate cues are received.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

20.3 Morphological dormancy

An underdeveloped embryo may require further growth before germination.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

20.4 Hormonal balance

ABA commonly promotes dormancy, whereas gibberellin can promote processes associated with germination.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

20.5 After-ripening

Dry storage can gradually reduce dormancy in some seeds.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

20.6 Stratification

Exposure to defined moist temperature conditions can release dormancy in certain species.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

20.7 Scarification

Weakening an impermeable coat can permit water entry in physically dormant seeds.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

20.8 Ecological significance

Dormancy distributes germination through time and can reduce establishment during unsuitable periods.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Germination begins with water uptake by a viable non-dormant seed and culminates in renewed embryo growth and radicle emergence.

21.1 Imbibition

Dry seed tissues rapidly absorb water because of their matric properties.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

21.2 Metabolic reactivation

Hydration restores membrane organisation and activates enzymes and respiration.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

21.3 Reserve mobilisation

Stored starch, lipid or protein is converted into transportable substrates for the growing embryo.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

21.4 Gibberellin role

In cereal grains, gibberellin from the embryo can stimulate enzyme production in surrounding tissues that mobilise reserves.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

21.5 Oxygen

Aerobic respiration requires adequate oxygen in most germinating seeds.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

21.6 Temperature

Each species has a temperature range within which germination proceeds.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

21.7 Light

Some seeds respond positively or negatively to light through photoreceptor systems.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

21.8 Seedling establishment

After radicle emergence, successful establishment depends on reserve use until photosynthetic tissues become functional.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Senescence is a regulated developmental process involving progressive remobilisation and eventual loss of function in cells, organs or the whole plant.

22.1 Leaf senescence

Chlorophyll breakdown, protein degradation and nutrient export accompany ageing leaves.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

22.2 Nutrient remobilisation

Nitrogen, phosphorus and other resources can be transferred to young organs, storage tissues or seeds.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

22.3 Hormonal regulation

Ethylene, ABA, cytokinins and other signals interact in senescence regulation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

22.4 Environmental acceleration

Drought, nutrient stress, shading and pathogen damage can accelerate senescence-like processes.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

22.5 Whole-plant senescence

Annual plants may undergo coordinated senescence after reproduction.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

22.6 Abscission

Organ separation can follow development of an abscission zone and is related to, but not identical with, senescence.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

22.7 Agricultural significance

Timing of leaf senescence affects grain filling, forage quality and harvest.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

22.8 Programmed process

Senescence is not merely passive decay; it involves regulated gene expression and resource recovery.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Plants require mineral elements for structural, catalytic, osmotic and regulatory functions.

23.1 Essentiality

An essential element is required for completion of the life cycle or has an irreplaceable role in plant metabolism under classical criteria.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

23.2 Macronutrients

Elements required in relatively large amounts include nitrogen, phosphorus, potassium, calcium, magnesium and sulfur.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

23.3 Micronutrients

Elements such as iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel are required in smaller quantities.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

23.4 Root uptake

Ions cross root membranes through channels, carriers and active transport systems.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

23.5 Rhizosphere

Root activity, microbes, pH and soil chemistry influence nutrient availability.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

23.6 Mobility

Nutrient mobility within plants affects where deficiency symptoms appear.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

23.7 Deficiency

Deficiency symptoms reflect disrupted function but visual diagnosis can be ambiguous.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

23.8 Toxicity

Excess nutrients or non-essential ions can impair growth and metabolism.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Each essential mineral contributes particular structural or metabolic roles, while balanced nutrition is necessary because excess of one element can interfere with another.

24.1 Nitrogen

Nitrogen is required for amino acids, proteins, nucleic acids, chlorophyll and many metabolites.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

24.2 Phosphorus

Phosphorus occurs in nucleic acids, phospholipids, phosphorylated intermediates and energy-transfer compounds.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

24.3 Potassium

Potassium contributes osmotic regulation, stomatal function and enzyme activation.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

24.4 Calcium

Calcium contributes cell-wall structure, membrane function and signalling.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

24.5 Magnesium

Magnesium is central to chlorophyll and functions in many enzyme reactions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

24.6 Sulfur

Sulfur occurs in particular amino acids and many coenzymes and metabolites.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

24.7 Iron and micronutrients

Micronutrients commonly function in redox proteins, enzymes, cell-wall processes or regulatory systems.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

24.8 Balanced supply

Nutrient management should match plant demand, soil supply and environmental consequences.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Crop yield emerges from interception of resources, conversion into biomass and allocation of biomass to harvested organs.

25.1 Canopy photosynthesis

Yield depends on integrated carbon gain across leaves and time, not the maximum photosynthetic rate of one leaf alone.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

25.2 Leaf area

Leaf-area development influences light interception but excessive self-shading can reduce efficiency.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

25.3 Respiration

Maintenance and growth respiration consume part of assimilated carbon.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

25.4 Source-sink relation

Sources export assimilates; sinks import them for growth or storage.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

25.5 Partitioning

The fraction of biomass allocated to grain, tuber, fruit or other harvested organs strongly affects economic yield.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

25.6 Harvest index

Harvest index expresses economic yield as a fraction of total biological yield.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

25.7 Water and nutrients

Deficiencies reduce leaf expansion, photosynthesis, reproductive development and sink filling.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

25.8 Phenology

Timing of flowering, grain filling and maturity determines how the crop encounters environmental conditions.

The process should be interpreted at cellular and whole-plant levels. In experimental work, environmental conditions, developmental stage and the measured variable must be stated because physiological responses are strongly context dependent.

Water Potential

Direction of Water Movement

Transpiration Rate

Photosynthetic Carbon Balance

Calvin-Cycle Carbon Accounting

Glycolytic ATP

Germination Percentage

Relative Growth Rate Concept

Harvest Index

A correct Calvin-cycle diagram should show three functional phases rather than an unexplained circle. Begin with carbon dioxide entering the carboxylation step. Rubisco catalyses reaction of CO2 with ribulose-1,5-bisphosphate (RuBP). The unstable six-carbon intermediate yields 3-phosphoglycerate. During reduction, ATP and NADPH from the light reactions support conversion toward triose phosphate. Some triose phosphate leaves the cycle for biosynthesis; most carbon is rearranged, with ATP consumption, to regenerate RuBP.

For three CO2 molecules fixed, the carbon accounting can be followed through six molecules of 3-phosphoglycerate, then six triose-phosphate equivalents after reduction. One net triose-phosphate equivalent can be withdrawn, while five are rearranged to regenerate three RuBP molecules. The exact biochemical sequence contains several intermediates; the conceptual drawing should preserve carbon balance and the three phases.

The three categories should not be treated as rankings. Each pathway has costs and advantages that depend on temperature, light, water, carbon dioxide, nutrient supply and growth strategy.

Practical 1: Osmosis Using Plant Tissue

Prepare equal-sized tissue pieces and place them in solutions of different solute concentration. Measure initial and final mass or length. Use replicates and identical exposure times. Interpret changes as water movement associated with differences in water potential, while recognising that damaged tissues and solute permeability can affect results.

Practical 2: Demonstrating Transpiration

Use an approved potometer arrangement to estimate water uptake as an indirect index of transpiration. Ensure airtight connections, allow acclimation and change one environmental variable at a time. A potometer measures water uptake, which approximates but is not identical to water loss because some absorbed water is retained or used metabolically.

Practical 3: Environmental Factors and Transpiration

Compare uptake under moving air, altered light or humidity while keeping other conditions as constant as practicable. Record temperature and time. Explain results through boundary-layer resistance, stomatal behaviour and vapour-pressure demand rather than simply saying that one treatment “increases transpiration.”

Practical 4: Photosynthetic Pigment Extraction

Extract pigments from fresh leaves using the laboratory-approved solvent and safety procedure. Separate pigments chromatographically if required. Compare bands by colour and migration while avoiding unsupported identification when reference standards are absent.

Practical 5: Light Requirement for Photosynthesis

Destarch a suitable plant where the teaching protocol requires it, expose defined leaf regions to light while shielding others, and test for accumulated starch using the approved procedure. Interpret starch formation as evidence of net photosynthetic carbohydrate accumulation under the experimental conditions.

Practical 6: Carbon Dioxide Requirement

Compare suitable leaf material under conditions differing in CO2 availability while controlling light and other factors. Appropriate controls are essential because enclosure itself can alter humidity and temperature.

Practical 7: Respiration in Germinating Seeds

Use a respirometer or approved indicator method to demonstrate respiratory gas exchange. Include a non-respiring control. Temperature must be controlled because gas volume changes independently with temperature.

Practical 8: Growth Measurement

Measure plant height, leaf area or dry mass at defined intervals. Distinguish absolute increase from relative growth. For destructive dry-mass sampling, independent replicate plants are needed at each time point.

Practical 9: Hormonal Effects

Using instructor-approved concentrations, compare growth responses to a plant growth regulator against a solvent or untreated control. Dose-response relationships are often non-linear; an effective concentration in one organ may inhibit another.

Practical 10: Seed Dormancy and Germination

Compare untreated seeds with an appropriate dormancy-breaking treatment such as scarification or stratification where biologically relevant. Record final germination and timing. Do not apply a treatment merely because it is common; the dormancy mechanism should guide treatment choice.

Practical 11: Mineral Deficiency Culture

Grow comparable plants in complete nutrient solution and solutions lacking one essential element, using a suitable teaching system. Record growth and symptoms. Deficiency treatments should be interpreted cautiously because nutrient interactions and contamination can obscure symptoms.

Practical 12: Crop Yield Components

Measure total dry biomass and economic yield of sample plants. Calculate harvest index and discuss how leaf area, water status, mineral nutrition, flowering and assimilate partitioning contribute to the final value.

Deficiency diagnosis should combine symptoms with tissue or soil analysis where possible. Similar visual symptoms can arise from different nutrients, root damage, pH problems, drought or disease.

Radiation Capture

A crop must first develop a canopy capable of intercepting photosynthetically useful radiation. Leaf area develops through cell division and expansion, both of which depend on water and nutrients. Too little leaf area wastes available light; excessive overlapping foliage can increase respiratory cost and self-shading.

Radiation Use and Carbon Gain

Intercepted radiation must be converted into biomass. Photosynthetic capacity, stomatal conductance, nitrogen distribution, temperature and sink demand all influence conversion. Stress during critical reproductive stages can reduce yield even if earlier vegetative growth was strong.

Partitioning

Economic yield depends on allocation. Grain crops must move assimilates to developing seeds; root and tuber crops allocate substantial biomass below ground; fruit crops support reproductive sinks. Harvest index captures this allocation in a simple ratio but does not by itself explain the physiological causes.

Water Limitation

Water deficit can reduce cell expansion before photosynthesis collapses, limiting leaf area. Stomatal closure conserves water but restricts CO2 entry. Severe stress disrupts metabolism, reproductive development and hydraulic function. Timing therefore matters as much as total water supply.

Nutrient Limitation

Nitrogen shortage reduces photosynthetic proteins and canopy development; phosphorus shortage affects energy and growth processes; potassium shortage impairs osmotic and stomatal functions. Balanced nutrition is required because yield responses depend on the most limiting physiological resource.

Source and Sink

A leaf is a source when it exports assimilates, whereas growing seeds, roots, fruits and young leaves are sinks. Sink demand can feed back on photosynthetic activity. Yield improvement can therefore involve both greater assimilate supply and greater capacity of harvestable organs to receive and store assimilates.

Senescence and Yield

Premature senescence shortens the period of carbon assimilation, whereas delayed senescence can extend canopy function if water and nutrients remain adequate. During grain filling, controlled senescence also remobilises nitrogen and other nutrients to seeds. Productive timing rather than simply “more green leaves” is the important physiological principle.

1. National Universities Commission. Core Curriculum and Minimum Academic Standards for the Nigerian University System: Sciences. BOT 303 Plant Physiology.
2. Taiz, L. et al. Plant Physiology and Development.
3. Hopkins, W. G. & Hüner, N. P. A. Introduction to Plant Physiology.
4. Buchanan, B. B., Gruissem, W. & Jones, R. L. Biochemistry and Molecular Biology of Plants.
5. Salisbury, F. B. & Ross, C. W. Plant Physiology.
6. Raven, P. H., Evert, R. F. & Eichhorn, S. E. Biology of Plants.

Case Study 1: Midday Wilting Despite Moist Soil

A crop may temporarily wilt at midday even when soil contains usable water. High radiation, temperature and atmospheric dryness can create evaporative demand faster than the root–xylem system can replace leaf water. Leaf water potential declines, guard-cell behaviour changes and cell turgor falls. As afternoon demand decreases, water status can recover. The case demonstrates that plant water status depends on the whole soil–plant–atmosphere pathway, not soil moisture alone.

Case Study 2: Drought and Photosynthesis

During developing drought, one of the earliest limitations to carbon gain is often reduced stomatal conductance. Closing stomata conserves water but lowers internal CO2 supply. If stress intensifies, biochemical limitations, impaired electron transport, loss of leaf area and hydraulic dysfunction can become important. Therefore a measured decline in photosynthesis should not automatically be attributed to chlorophyll destruction.

Case Study 3: High Temperature in C3 and C4 Crops

Under hot bright conditions, photorespiration can impose a larger carbon cost on a C3 leaf, particularly when stomata restrict CO2 entry. A C4 crop uses an additional carbon-concentrating mechanism that elevates CO2 around Rubisco and suppresses oxygenation. This helps explain the strong performance of crops such as maize and sorghum in many warm high-light environments, although yield still depends on water, nutrients, genotype and management.

Case Study 4: Waterlogged Germination

Seeds need water for imbibition, but complete waterlogging can restrict oxygen diffusion. Respiration then becomes constrained, ATP supply falls and seedling establishment may fail even though water is abundant. This illustrates why “more water” is not equivalent to “better germination.” Suitable water, oxygen and temperature conditions must coincide.

Case Study 5: Nitrogen Deficiency and Yield

Nitrogen shortage can reduce chlorophyll, photosynthetic protein, leaf-area development and sink formation. Because nitrogen is relatively mobile within many plants, deficiency symptoms often appear first on older leaves as resources are remobilised. The final yield penalty results from interacting reductions in radiation capture, carbon assimilation and reproductive growth rather than leaf colour alone.

Case Study 6: Premature Senescence

A grain crop experiencing severe stress during grain filling may lose green leaf area rapidly. Shortened photosynthetic duration reduces new assimilate supply, while nutrients are remobilised from ageing tissues. Grain filling may then depend more strongly on stored reserves. The physiological value of delayed senescence depends on maintaining functional photosynthesis, not simply retaining chlorophyll without productive carbon assimilation.

Case Study 7: Sink Limitation

A plant with a large healthy canopy may fail to convert high photosynthetic capacity into economic yield if reproductive sinks are few or poorly developed. Accumulated carbohydrates can feed back on source activity. Crop productivity therefore requires coordination between source capacity and sink number, size and filling duration.

Case Study 8: Mineral Toxicity versus Deficiency

Applying increasing fertilizer does not guarantee increasing yield. Excess salts can lower soil water potential, specific ions can become toxic, and nutrient imbalance can restrict uptake of other elements. Physiological nutrient management aims for adequate balanced supply rather than the largest possible application.

Source Formation

A young expanding leaf initially imports carbon and nutrients. As photosynthetic machinery develops and carbon production exceeds local demand, the leaf becomes a net exporter. Source status is therefore developmental rather than permanently assigned by organ name.

Sink Strength

Sink strength combines sink size with the activity by which assimilates are imported and used or stored. Developing seeds, fruits, tubers, roots and meristems can become strong sinks. Hormonal regulation, vascular connections and metabolic capacity influence this strength.

Phloem Loading and Unloading

Assimilates enter the phloem through pathways that differ among species and tissues. At sinks, sugars are unloaded and metabolised or stored. Movement between source and sink is closely linked to water relations because pressure differences in phloem arise from osmotic loading and unloading processes.

Competition among Sinks

When assimilate supply is limited, rapidly growing sinks compete. Flower or fruit abortion can therefore reflect inadequate carbon supply, water stress, nutrient shortage or hormonal effects. Agricultural practices that change sink number can alter final organ size.

Remobilisation

Stored carbohydrates and nutrients can later be exported to reproductive organs. During senescence, proteins and cellular constituents are dismantled in a regulated manner, permitting valuable nitrogen and minerals to move to seeds. Efficient remobilisation is an important component of reproductive physiology.

Yield Interpretation

Yield should be analysed as a sequence: resource capture, photosynthetic conversion, respiratory expenditure, biomass accumulation, sink establishment and partitioning. A limitation at any stage can reduce harvestable production even when another stage performs well.

Question and Hypothesis

A useful experiment begins with a precise question. “Does wind affect transpiration?” can be developed into a testable prediction about water uptake or mass loss under defined air movement while temperature, light, leaf area and other conditions are controlled as far as possible.

Controls

A control establishes the baseline against which a treatment is interpreted. In hormone experiments, the control should account for the solvent or handling procedure. In respiration experiments, a non-respiring control helps distinguish biological gas exchange from temperature-driven volume changes.

Replication

Biological material varies naturally. Replicates permit estimation of this variation and prevent one unusual plant from representing an entire treatment. Repeated measurements on the same individual are not always equivalent to independent biological replicates.

Standardisation

Leaf age, plant size, time of day, temperature and pretreatment can strongly alter physiological measurements. Standardisation reduces unwanted variation and makes treatment differences more interpretable.

Rate versus Total

A total change must be related to time when a rate is required. Comparisons of transpiration may also require correction for leaf area, while growth comparisons may use initial size or relative growth measures.

Correlation and Mechanism

If two physiological variables change together, one does not automatically cause the other. Mechanistic experiments manipulate a proposed cause and measure predicted consequences while controlling alternatives.

Reporting

A complete practical report states aim, materials, method, results, calculations, interpretation, limitations and conclusion. Results should be separated from explanation: the result records what was measured; the discussion explains what the measurements may mean.

Plant physiology is an integrated science. Water absorbed by roots supplies expanding cells and replaces transpiration. Stomata regulate both water loss and CO2 entry. Photosynthetic electron transport converts light energy into ATP and reducing power; the Calvin cycle uses these products to assimilate carbon. C4 and CAM mechanisms alter the context in which Rubisco functions, while photorespiration illustrates the consequences of its oxygenase activity.

The carbohydrates produced by photosynthesis enter a wider metabolic network. Glycolysis and mitochondrial respiration release usable energy and intermediates for growth and maintenance. Hormones and environmental signals coordinate cell division, expansion, flowering, dormancy, germination and senescence. Mineral elements provide structural components, osmotic functions and catalytic cofactors required throughout these pathways.

Crop yield is therefore not a separate topic added at the end of physiology. It is the integrated expression of the entire course. Productive crops must acquire water and minerals, intercept light, assimilate carbon, control respiratory costs, establish reproductive or storage sinks and partition sufficient biomass to the harvested organ. Understanding these connections allows students to move beyond memorising isolated pathways and explain how plants function as coordinated organisms.