Chapter 4 — Plant Physiology

What plant physiology studies
Plant physiology deals with the functional processes that keep plants alive: uptake of water and nutrients, photosynthesis, transport, growth and responses. It links structures such as stomata, xylem, phloem and chloroplasts to the functions they perform. Understanding these processes helps explain how plants adapt to environments and how human practices can improve plant productivity.

Water as a life-sustaining medium
Water is central to plant life. It maintains cell turgor, acts as solvent for biochemical reactions, participates directly in photosynthesis and provides the medium for transport of minerals. Cells contain a large proportion of water, and even processes like cell expansion depend on water uptake. Because water is polar and forms hydrogen bonds, it supports cohesion and adhesion that plants exploit for long-distance transport.

Water potential and its components
Water potential (symbol Ψ) predicts the movement of water. It is the combined effect of solute potential (Ψs), pressure potential (Ψp or turgor), gravity potential (Ψg) and matric potential (Ψm) in some contexts. Pure water at standard conditions has Ψ = 0; adding solute lowers Ψ (more negative) and creates a gradient that causes water to move from higher to lower water potential. Plant cells change Ψ using solute accumulation and pressure changes to absorb or release water. In tall plants gravity potential becomes significant, slightly reducing the water potential at higher points.

Osmosis, plasmolysis and turgor
Osmosis is the net movement of water across a selectively permeable membrane from higher to lower water potential. When cells are placed in hypertonic solutions they lose water and plasmolyse; in hypotonic media they swell and become turgid. Turgor pressure supports non-woody tissues, drives cell expansion and is regulated by stomatal movements in guard cells. Turgor also affects mechanical strength of green stems and movements driven by pressure changes in specialised cells.

Soil-plant-atmosphere continuum
Water moves through a continuous pathway from soil, across root tissues, through xylem, and into the leaf air spaces before evaporating into the atmosphere. This soil-plant-atmosphere continuum (SPAC) is driven by gradients of water potential. Soil moisture, root conductance, xylem properties and atmospheric demand together determine flow rates. Plants adapt by changing root architecture, adjusting stomatal aperture and altering osmolyte concentrations to regulate flow under changing conditions.

Practical consequences
Water relations set the stage for absorption, transport in xylem, stomatal behaviour and responses to drought. Mastery of water potential concepts is essential to understand how plants take up water from soil and how they maintain hydration while allowing transpiration. Practical areas such as irrigation scheduling, drought tolerance breeding and greenhouse humidity control build directly on these principles.

Root structure related to absorption
Roots are specialised for uptake. Root hairs increase surface area and are the primary sites for water and mineral absorption. The epidermis, cortex and endodermis create selective barriers. The endodermis, with its Casparian strip, forces apoplastic flow to enter symplastically before reaching the stele, allowing selective uptake. The pericycle and xylem in the stele then carry absorbed materials upward. Root architecture — length, branching and root hair density — influences how effectively a plant explores soil for water and minerals.

Pathways of water movement
Water moves to the xylem via three pathways: the apoplast (through cell walls and intercellular spaces), the symplast (through cytoplasm connected by plasmodesmata), and the transmembrane pathway (crossing membranes repeatedly). The apoplast allows rapid flow until it is blocked by the Casparian strip at the endodermis, which protects internal tissues and enables selective ion uptake. Symplastic movement permits controlled transfer of solutes between cells.

Mechanisms of mineral uptake
Minerals enter roots by diffusion (down a concentration gradient), mass flow (driven by bulk water movement) and active transport. When soil ion concentrations are low, plants use membrane-bound carriers and pumps, requiring ATP, to accumulate ions against electrochemical gradients. Proton pumps (H+-ATPase) extrude H+ to build a proton motive force that drives secondary transporters (symporters and antiporters) for uptake of NO3-, phosphate, K+ and trace elements. Specificity arises from different transporter proteins and regulation of their expression and activity.

Role of mycorrhizae and root-microbe interactions
Mycorrhizal fungi form mutualistic associations that extend the absorptive surface via hyphae, significantly improving uptake of immobile nutrients such as phosphorus and micronutrients. Rhizosphere bacteria influence nutrient availability by fixing nitrogen (in some legumes via nodules), solubilising phosphates, or producing hormones that affect root growth. These biotic interactions are vital for nutrient cycling and plant nutrition in natural and agricultural systems.

Selectivity and homeostasis
Plants selectively absorb essential ions while excluding toxic ones; they maintain cytosolic ion homeostasis by compartmentalising unwanted ions into vacuoles or exporting them back to soil. Transporters often have regulation based on internal nutrient status, pH and signalling molecules. Deficiency of specific nutrients produces diagnostic symptoms that help in field diagnosis and management — for example, potassium deficiency causes marginal chlorosis and weak stems, while phosphorus deficiency limits root growth and delays maturity.

Applications for agriculture
Understanding absorption mechanisms helps design efficient fertiliser regimes, improve root traits through breeding, and manage soils to enhance availability. Practices such as split nitrogen applications, phosphate placement near roots, and use of mycorrhizal inoculants derive from principles of root uptake and soil chemistry.

Problem to explain
Water and minerals move from roots to leaves against gravity, sometimes up trees taller than 100 m. The ascent of sap refers to this long-distance movement through the xylem and is driven by physical and biological processes. Any convincing model must account for continuity of the water column, forces generated by the plant, and conditions that cause interruptions like cavitation.

Cohesion-tension theory
The cohesion-tension theory is the widely accepted explanation. Transpiration at the leaf surface creates a negative pressure (tension) in the leaf air spaces and xylem. Water molecules cohere to each other through hydrogen bonds and adhere to xylem walls, forming a continuous water column from root to leaf. As water evaporates from mesophyll cell walls, the resulting tension is transmitted down the column to draw water from the roots. This tension can be measured as negative pressure (below atmospheric) and is sufficient to lift water to great heights without active pumping by the plant’s cells.

Role of cohesion and adhesion
Cohesion between water molecules ensures the column remains intact under tension; adhesion to the hydrophilic walls of xylem vessels also helps to stabilise the column and reduce slippage. The narrow diameter of tracheids and vessels increases capillary forces and facilitates the maintenance of tension. The combined physical properties of water and xylem anatomy are essential for efficient sap ascent.

Contribution of root pressure and capillarity
Root pressure results from active solute accumulation in the xylem of roots, drawing water osmotically and creating a positive hydrostatic pressure that can push water a short distance. Root pressure is most notable in small plants or at night, and may cause guttation. Capillarity — rise of water in narrow pores — contributes to water movement in small-diameter xylem but cannot account for ascent in tall trees alone. Thus cohesion-tension provides the main explanation while root pressure and capillarity are supplementary.

Cavitation, embolism and safety mechanisms
Cavitation occurs when the tension is high or gas is introduced, causing air bubbles (emboli) that break the continuous water column and block conduction. Plants limit cavitation risk with small conduits, bordered pits that can isolate air-filled vessels, and redundancy in xylem pathways. Some species have repair mechanisms that can refill embolised vessels under favorable conditions or route water through alternative paths. Trade-offs exist: wide vessels transport more water but are more vulnerable to cavitation; narrow tracheids are safer but less efficient.

Evidence and practical implications
Empirical support includes direct measurements of xylem tension, correlation of transpiration with sap flow, and observations like nocturnal root pressure and guttation. For agriculture and forestry, understanding ascent of sap guides irrigation practices, selection of species for drought-prone sites, and management to reduce cavitation risks during drought or freeze-thaw events.

What is transpiration?
Transpiration is the loss of water vapour from plant aerial parts, mainly leaves. It occurs through stomata (major route), cuticle (minor) and lenticels. Transpiration is not merely wasteful loss; it serves multiple functions including cooling leaves by evaporative cooling, creating the transpirational pull that drives xylem transport, and facilitating the mass flow of minerals from roots to shoots. However, excessive transpiration under water deficit can lead to wilting and reduced productivity.

Pathway of water vapour and driving gradients
Water evaporates from the wet cell walls of mesophyll cells into intercellular air spaces and diffuses out through open stomata to the atmosphere. The driving force is the vapour pressure deficit (VPD) between internal leaf air spaces (near saturation) and external air (often drier). The magnitude of VPD depends on temperature and relative humidity; higher temperatures increase VPD and thus transpiration unless stomata close.

Stomatal control and physiological regulation
Stomatal aperture, governed by guard cell turgor, is the main physiological control over transpiration. Light, CO2 concentration, humidity, soil moisture status and internal signals like abscisic acid (ABA) affect guard cell turgor. In bright light, stomata open to allow CO2 entry needed for photosynthesis; in drought, ABA accumulates, triggering ion efflux from guard cells and stomatal closure to conserve water. Circadian rhythms and CO2 feedbacks also adjust stomatal behaviour to optimise carbon gain versus water loss.

Environmental factors influencing transpiration
External factors include light intensity (promotes opening), temperature (raises VPD), humidity (low humidity increases transpiration), wind (reduces the boundary layer thus raising transpiration) and soil water availability (low soil water causes stomatal closure). Leaf characteristics such as thickness, cuticle properties, stomatal density and distribution further modulate transpiration rates. The boundary layer — a thin layer of still air at the leaf surface — provides resistance to water loss and is affected by leaf pubescence and wind speed.

Measurement and quantification
Transpiration can be estimated using potometers that measure water uptake by an excised shoot, gas-exchange systems that measure water vapour flux, or lysimeters in field studies. These measurements help correlate plant water use with environmental conditions and guide irrigation scheduling. Transpiration rate is often expressed per unit leaf area per unit time and can be integrated to derive crop water use over time.

Adaptations to reduce transpiration
Plants of arid regions show adaptations such as reduced leaf area, thick cuticle, sunken stomata, leaf rolling, hairs (trichomes) and CAM photosynthesis. Some plants alter leaf orientation to reduce direct exposure to sun. These structural and physiological adaptations reduce water loss while allowing sufficient carbon assimilation for survival. Understanding these mechanisms informs water-conserving agricultural practices like mulching, shade management and deficit irrigation.

Essential elements and classification
Plants require a suite of chemical elements to grow and reproduce. Essential elements are those whose absence prevents a plant from completing its life cycle. They are classified into macronutrients (required in larger amounts) and micronutrients (required in trace amounts). Macronutrients include carbon, hydrogen and oxygen (obtained from air and water) and soil-derived elements such as nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S). Micronutrients include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), boron (B), chlorine (Cl) and nickel (Ni).

Roles of major nutrients
Nitrogen is central to amino acids, proteins, nucleic acids and chlorophyll, thus critical for vegetative growth. Phosphorus forms part of ATP, nucleic acids and membrane phospholipids and is vital for energy transfer and genetic material. Potassium regulates osmotic balance, stomatal movement and activates enzymes. Calcium stabilises cell walls and membranes and participates in signalling. Magnesium is the central atom in chlorophyll and acts as a cofactor for many enzymes. Sulfur is a component of certain amino acids and coenzymes.

Functions of micronutrients and deficiency effects
Micronutrients act mainly as cofactors in redox reactions and enzyme activities: iron is required for cytochromes and chlorophyll synthesis, manganese participates in water-splitting in PSII, zinc is involved in transcription factors and enzyme activation. Deficiency symptoms are often characteristic: iron deficiency causes interveinal chlorosis of young leaves because iron is not mobile; nitrogen deficiency causes uniform chlorosis beginning in older leaves due to N mobility. Recognising deficiency patterns helps manage fertilisation.

Soil chemistry and nutrient availability
Soil properties such as pH, redox potential, cation exchange capacity and organic matter influence nutrient availability. For instance, phosphorus can become fixed and unavailable in very acidic or alkaline soils through formation of insoluble complexes. Soil pH affects the solubility of micronutrients; alkaline soils can induce iron deficiency. Organic matter improves nutrient retention and acts as a reservoir of slowly mineralisable nutrients.

Uptake mechanisms and homeostasis
Uptake involves passive processes (diffusion and mass flow) and active transport mediated by specific carrier proteins and channels. Proton pumps create electrochemical gradients that drive secondary active transport. Plants maintain internal homeostasis by compartmentalising excess ions in vacuoles, chelating metals with organic molecules, or limiting uptake. Some species (hyperaccumulators) tolerate and store unusually high levels of particular elements.

Practical management
Understanding mineral nutrition informs fertiliser application rates, timing (split applications), forms (nitrate versus ammonium), and methods (soil versus foliar). Soil testing, balanced use of NPK with micronutrients and use of organic amendments improve nutrient efficiency and reduce environmental pollution. Mycorrhizae and crop rotations with legumes enhance nutrient availability sustainably.

Phloem anatomy and cell types
Phloem is the living vascular tissue responsible for long-distance transport of organic solutes, primarily sucrose, from source tissues (mature leaves) to sinks (roots, growing buds, developing fruits). Phloem consists of sieve tube elements, companion cells, phloem parenchyma and fibres. Sieve tube elements are elongated cells joined end to end by sieve plates that allow movement of sap; they are specialised for transport and often lack a nucleus in mature state. Companion cells are metabolically active, containing nuclei and many organelles; they support sieve tubes by managing loading and unloading of solutes and maintaining metabolic functions.

Principles of translocation — pressure-flow model
The pressure-flow hypothesis (mass flow theory) explains phloem translocation by osmotically generated pressure differences. Sugars produced in mesophyll (source) are actively loaded into sieve tubes, increasing solute concentration and lowering water potential. Water follows osmotically from xylem, creating high turgor pressure in source regions. At sinks, sugars are unloaded and used or stored, reducing solute concentration and turgor; water exits to xylem. The resulting pressure gradient drives bulk flow of phloem sap from source to sink. This model accounts for bidirectional flow and the coupling of phloem transport with source-sink dynamics.

Mechanisms of loading and unloading
Loading into phloem can be symplastic (through plasmodesmata) or apoplastic (requiring crossing cell membranes). Apoplastic loading involves sucrose transporters and H+-ATPase-driven proton gradients to co-transport sucrose into sieve elements or companion cells. Unloading at sinks may be passive diffusion, active transport or involve enzymatic conversion (e.g., sucrose cleaved by invertase) to maintain gradient. The mode of loading affects phloem sap composition and flow rates.

Role of companion cells and plasmodesmata
Companion cells are essential for phloem function; they synthesise ATP and proteins required for loading, regulate plasmodesmatal conductivity and influence sieve element metabolism. Plasmodesmata provide symplastic continuity for movement of small molecules and signalling compounds between cells, but their conductivity can be regulated to control flow.

Evidence and physiological implications
Tracer experiments using labelled carbon (14C) show rapid movement of photosynthates along phloem. The pressure-flow model is supported by measurements of phloem pressure and observations that disrupting phloem halts translocation. Phloem transport is crucial during development, supporting growing tissues and storage organs; it also plays a role in defence by moving signalling molecules and secondary metabolites. Understanding phloem function has applications in crop storage, fruit development and controlling resource allocation for yield improvement.

Photosynthesis as the central plant process
Photosynthesis converts light energy into chemical energy, producing organic compounds from CO2 and water. It sustains autotrophic life and forms the basis of food chains. Photosynthesis occurs primarily in chloroplasts of leaf mesophyll cells, where pigments absorb light and biochemical reactions fix carbon. The overall process couples photochemistry (light reactions) with carbon fixation (the Calvin cycle) so that light energy is converted into reduced carbon skeletons for growth and storage.

Overall chemical equation and partitioning
The simplified overall chemical equation is: 6CO2 + 6H2O + light → C6H12O6 + 6O2. This equation represents the net outcome but the process is split into two linked sets of reactions. Light reactions harvest photons to produce ATP and NADPH and evolve oxygen from water. The Calvin cycle uses ATP and NADPH to fix CO2 into triose phosphates, which can be assembled into sugars, starch or used as precursors for other biomolecules.

Chloroplast structure and functional zones
Chloroplasts contain an inner membrane system of thylakoids organised into grana (stacks) and intergranal lamellae. Thylakoid membranes host photosystems, electron carriers and ATP synthase where light energy is converted to chemical energy. The stroma surrounds the thylakoids and contains enzymes for the Calvin cycle, ribosomes and chloroplast DNA. Pigments — chlorophyll a, chlorophyll b and carotenoids — are bound to proteins and form light-harvesting complexes that capture and channel light energy to reaction centres.

Spectral properties and photoprotection
Different pigments absorb light in different wavelength ranges; chlorophylls absorb strongly in red and blue regions while carotenoids absorb blue-green light and protect against photooxidative damage. Excess light can generate reactive oxygen species; plants dissipate excess energy through non-photochemical quenching, adjust antenna size and deploy antioxidants to avoid damage. Photoacclimation modifies pigment composition and thylakoid organisation depending on growth light.

Environmental limitations and regulation
Photosynthesis rate depends on light intensity, CO2 concentration, temperature and water availability. Light increases photosynthetic rate up to a saturation point; CO2 availability limits the Calvin cycle when stomata are closed or when atmospheric CO2 is low; temperature affects enzyme kinetics and membrane properties. Plants balance stomatal opening for CO2 uptake against water loss and modulate their physiology in response to environmental changes. Understanding these dependencies is important for crop management, greenhouse operation and improving plant productivity under changing climates.

Location, components and pigment organisation
Light reactions occur in the thylakoid membranes of chloroplasts. Pigments and protein complexes — Photosystem II (PSII), cytochrome b6f complex, Photosystem I (PSI), plastoquinone, plastocyanin and ATP synthase — are arranged to capture light and pass electrons along the chain. Light-harvesting complexes contain chlorophyll a and b and carotenoids that broaden the spectrum of absorbed light and transfer excitation energy to reaction centres where charge separation begins.

Sequence of events and electron flow
Photon absorption excites electrons in PSII reaction centre chlorophyll, which pass to primary acceptors and then to plastoquinone. Electrons traverse the cytochrome b6f complex to plastocyanin and to PSI, where further excitation by light raises them to reduce ferredoxin. Ferredoxin then reduces NADP+ to NADPH via ferredoxin-NADP+ reductase. The loss of electrons from PSII is compensated by photolysis of water at the oxygen-evolving complex, releasing O2 and contributing protons to the thylakoid lumen. The electron flow therefore couples water oxidation to NADP+ reduction.

Generation of proton motive force and ATP synthesis
Electron transport moves protons into the thylakoid lumen from both water splitting and plastoquinone-mediated transport, creating a proton gradient (ΔpH) and an electrical potential across the thylakoid membrane. ATP synthase uses the proton motive force to drive synthesis of ATP from ADP and inorganic phosphate — a process called photophosphorylation. The resulting ATP and NADPH provide the energy and reducing power for the Calvin cycle.

Cyclic versus non-cyclic photophosphorylation
Non-cyclic (linear) electron flow involves both PSII and PSI, producing ATP, NADPH and O2 — this supports carbon fixation. Cyclic electron flow involves PSI only; electrons are cycled back to the plastoquinone pool, increasing proton pumping and ATP production without NADPH or O2 formation. Cyclic flow helps balance the ATP:NADPH ratio to match the demands of the Calvin cycle and other metabolic processes, particularly under high light or specific stress conditions.

Photoprotection and regulation
Excess light can over-reduce electron carriers and produce reactive oxygen species. Plants regulate light harvesting through state transitions, non-photochemical quenching and xanthophyll cycle activity. Repair mechanisms like D1 protein turnover in PSII restore function after photodamage. These regulatory mechanisms maintain efficiency while preventing oxidative damage.

Overview and cellular location
The Calvin cycle, also called the C3 pathway or Calvin–Benson cycle, takes place in the stroma of chloroplasts and fixes atmospheric CO2 into organic molecules using ATP and NADPH generated by light reactions. It operates in three interconnected phases: carboxylation of RuBP, reduction of 3-phosphoglycerate (3-PGA) to triose phosphates, and regeneration of ribulose-1,5-bisphosphate (RuBP) to allow continuous operation.

Carboxylation: the role of RuBisCO
CO2 is combined with RuBP (a 5-carbon sugar) by the enzyme RuBisCO to form an unstable 6-carbon intermediate that immediately splits into two molecules of 3-PGA (3-phosphoglycerate). This carboxylation is the primary entry point of CO2 into organic metabolism. RuBisCO is abundant and catalyses both carboxylation and oxygenation reactions; its kinetic properties and specificity have major effects on photosynthetic efficiency.

Reduction phase
Each 3-PGA molecule is phosphorylated by ATP and then reduced by NADPH to form glyceraldehyde-3-phosphate (G3P), a triose phosphate. G3P serves as a precursor for synthesis of glucose, sucrose, starch and other biosynthetic pathways. For every three CO2 molecules fixed, six G3P molecules are produced but only one net G3P exits the cycle for biosynthesis; the remaining five are used to regenerate RuBP.

Regeneration of RuBP and stoichiometry
A series of enzyme-catalysed rearrangements use ATP to convert three-carbon intermediates back into the five-carbon acceptor RuBP. The stoichiometry shows that fixing three CO2 molecules requires nine ATP and six NADPH to yield one net G3P. Thus the energy and reducing power provided by light reactions must meet the demands of the Calvin cycle for the plant to sustain carbon gain.

Regulation and integration
Calvin cycle enzymes are regulated by substrate availability, stromal pH and Mg2+ concentration (which change in light), and by specific regulatory proteins such as Rubisco activase. The cycle is integrated with other metabolic pathways: intermediates feed into amino acid synthesis, lipid synthesis and the pentose phosphate pathway. Rubisco’s oxygenase activity leads to photorespiration under certain conditions, which reduces net carbon fixation and is a major constraint on efficiency.

Importance and applications
Understanding the Calvin cycle is central to improving photosynthetic efficiency and crop yield. Strategies to enhance carbon fixation include increasing Rubisco specificity or expression, engineering alternative pathways, or introducing CO2-concentrating mechanisms such as those in C4 plants. These approaches aim to increase biomass production and resource-use efficiency in agriculture.

Why alternative pathways evolved
RuBisCO's affinity for O2 leads to photorespiration, which wastes energy and fixed carbon. In environments that are hot and dry, stomata close to conserve water, reducing CO2 availability and enhancing photorespiration. C4 and CAM pathways evolved to concentrate CO2 at the site of RuBisCO or to separate CO2 uptake in time, thereby reducing photorespiration and improving water-use efficiency under such conditions.

C4 pathway — spatial separation and Kranz anatomy
C4 plants (for example, maize, sugarcane, sorghum) separate initial CO2 fixation and the Calvin cycle into different cell types. In mesophyll cells, PEP carboxylase fixes CO2 (as bicarbonate) to phosphoenolpyruvate (PEP) to form a four-carbon acid (oxaloacetate), which is converted to malate or aspartate. These C4 acids move into bundle sheath cells where they are decarboxylated, releasing CO2 at high concentration around RuBisCO so that carboxylation predominates over oxygenation. This anatomical and biochemical compartmentation (Kranz anatomy) minimises photorespiration and gives C4 plants an advantage under high light and temperature, particularly when water is limiting.

CAM pathway — temporal separation
CAM (Crassulacean Acid Metabolism) plants (such as cacti, succulents and pineapples) separate CO2 uptake and fixation temporally. They open stomata at night to take up CO2, which is fixed by PEP carboxylase into organic acids (mainly malic acid) and stored in vacuoles. During the day, stomata remain closed to conserve water; malic acid is decarboxylated to supply CO2 for the Calvin cycle. CAM confers very high water-use efficiency but limits the rate of carbon gain because CO2 uptake is confined to night periods.

Biochemical differences and energetic costs
C4 metabolism consumes additional ATP for regenerating PEP, so it trades higher energy cost for greater photosynthetic efficiency under conditions where photorespiration would otherwise be severe. CAM also uses extra energy for nocturnal acid synthesis and daytime decarboxylation. The net benefit depends on environmental conditions: C4 and CAM perform best in hot, bright or arid habitats where the reduction in photorespiration and water loss outweighs the extra energy demands.

Ecological and agricultural significance
C4 crops are major staples in tropical agriculture due to high productivity and resource-use efficiency. CAM species dominate many arid landscapes and are important in horticulture and restoration of degraded lands. Understanding these pathways guides crop selection, irrigation strategies and breeding efforts; research into engineering C4 traits into C3 crops aims to boost yields under warming climates by reducing photorespiration and improving water use.

Definition and biochemical start point
Photorespiration is a metabolic pathway that begins when RuBisCO acts as an oxygenase, using O2 instead of CO2 with ribulose-1,5-bisphosphate (RuBP). This reaction yields one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate, the latter being a metabolically problematic compound that must be recycled. The salvage pathway involves chloroplasts, peroxisomes and mitochondria and results in release of CO2 and consumption of ATP and reducing power.

Environmental conditions that promote photorespiration
High temperatures, drought and high O2:CO2 ratios favour the oxygenase activity of RuBisCO. When stomata close to conserve water, internal CO2 concentration falls and O2 generated by light reactions accumulates, increasing the probability that RuBisCO will fix O2. Therefore hot, dry conditions that induce stomatal closure generally increase photorespiration rates and reduce net carbon gain.

Metabolic costs and consequences
Photorespiration reduces photosynthetic efficiency because it consumes ATP and reducing equivalents while releasing previously fixed CO2, meaning that some of the energy invested in CO2 fixation is wasted. The recycling of 2-phosphoglycolate requires enzymes in multiple organelles and results in loss of fixed carbon. Despite these costs, photorespiration has protective roles: it helps in dissipation of excess energy and reducing power under stress, preventing over-reduction of the photosynthetic electron transport chain and limiting reactive oxygen species formation.

Physiological and ecological effects
In many C3 crops, photorespiration can significantly lower yields under hot climates. Plants adapted to such climates have evolved mechanisms to minimise photorespiration, such as C4 and CAM photosynthesis. Photorespiration also interacts with nitrogen metabolism, as glycine decarboxylation in mitochondria links photorespiration to amino acid pools and respiration. This integration influences nitrogen use efficiency and responses to environmental change.

Approaches to reduce photorespiration
Agricultural and biotechnological strategies aim to reduce photorespiration through breeding for improved Rubisco specificity, introducing CO2 concentrating mechanisms, or modifying photorespiratory pathways to recapture carbon more efficiently. Each approach must consider trade-offs, because photorespiration also serves protective and regulatory functions under fluctuating environmental conditions.

Purpose and general outline
Plant respiration oxidises organic compounds to release energy stored as ATP, which is required for maintenance, growth, nutrient uptake and biosynthesis. Respiration operates continually in plants, both in light and dark. The major stages are glycolysis in the cytosol, the link reaction (pyruvate to acetyl-CoA), the Krebs cycle (citric acid cycle) in the mitochondrial matrix, and oxidative phosphorylation via the electron transport chain on the inner mitochondrial membrane.

Glycolysis and substrate-level phosphorylation
Glycolysis converts glucose (6C) into two molecules of pyruvate (3C each) while producing a small yield of ATP and NADH. It occurs in the cytosol and provides intermediates for multiple biosynthetic pathways as well as pyruvate for mitochondrial oxidation. Under aerobic conditions pyruvate enters mitochondria; under anaerobic conditions it can be reduced to lactate or ethanol depending on plant tissue and species.

Krebs cycle and respiratory substrates
In mitochondria pyruvate is oxidised to acetyl-CoA which enters the Krebs cycle. Each turn of the cycle oxidises acetyl units to CO2, producing NADH and FADH2, and a small amount of ATP (or GTP). The Krebs cycle also supplies carbon skeletons for amino acid and lipid biosynthesis, linking respiration to metabolic demands and anabolic processes.

Electron transport chain and oxidative phosphorylation
NADH and FADH2 donate electrons to respiratory complexes I–IV in the inner mitochondrial membrane, transferring electrons to molecular oxygen, the final electron acceptor, to form water. The flow of electrons pumps protons across the inner membrane, creating an electrochemical gradient. Protons re-enter the matrix via ATP synthase, driving ADP phosphorylation to ATP. This stage yields the majority of ATP from respiration and is sensitive to inhibitors and uncouplers that disrupt proton gradients.

Alternative pathways and regulation
Plants possess alternative oxidase pathways that bypass parts of the electron transport chain; these pathways can help prevent over-reduction of electron carriers and reduce reactive oxygen formation during stress, though they yield less ATP. Respiratory rates adjust to temperature and substrate availability; maintenance respiration supports essential cell functions, while growth respiration provides energy for biosynthesis. During the day, respiration interacts with photosynthesis: photosynthetic products supply substrates, while respiration supplies ATP for energy-demanding processes in chloroplasts and cytosol.

Physiological importance
Respiration supplies energy for seed germination, root growth, phloem loading, active transport and thermogenesis in some flowers. Measurement of respiratory rates informs storage, post-harvest handling and understanding of plant responses to stress. Balancing storage of photosynthates and their utilisation by respiration determines net biomass accumulation.

Hormones as integrators of growth and environment
Plant growth regulators (hormones) are organic substances produced in low amounts that modify or control physiological processes at sites distant from their production. They coordinate growth, development, stress responses and environmental adaptation. Although plants lack a circulatory endocrine system like animals, hormones travel via phloem, xylem, cell-to-cell transport or diffusion and act locally or systemically to regulate processes such as cell division, elongation, differentiation, dormancy and senescence.

Auxins (IAA and related compounds)
Auxins promote cell elongation by stimulating proton pumps that acidify the cell wall, activating expansin proteins and loosening the cell wall to allow turgor-driven expansion. Auxins are involved in apical dominance, vascular differentiation and root initiation. Their polar transport (directional movement from shoot apex downward) creates concentration gradients that pattern organ development and mediate tropic responses to light and gravity. Auxin also interacts with other hormones to regulate branching and fruit development.

Gibberellins (GAs)
Gibberellins stimulate stem elongation by promoting cell division and elongation, break seed dormancy and promote germination by mobilising stored reserves. They also affect flowering in some species and contribute to fruit growth. GA biosynthesis and signalling influence plant stature; mutants defective in GA biosynthesis or response are dwarfs. Exogenous gibberellin application can be used to increase fruit size or break dormancy in seeds.

Cytokinins
Cytokinins promote cell division, delay leaf senescence, and influence nutrient mobilisation and shoot formation. Synthesised largely in root tips and transported upward, they interact with auxin to determine organogenesis: a high cytokinin:auxin ratio favours shoot formation in tissue culture, while a low ratio favours root formation. Cytokinins modulate sink strength and nutrient allocation and are key in coordinating root-to-shoot signalling.

Ethylene and abscisic acid (ABA)
Ethylene is a gaseous hormone involved in fruit ripening, leaf abscission and stress responses; it modifies cell wall properties and gene expression associated with senescence. ABA is central to stress physiology: it accumulates under drought and induces stomatal closure to limit water loss, promotes seed dormancy and mediates gene expression for stress tolerance. ABA acts as a long-distance drought signal from roots to shoots and as a local regulator of guard cell ion channels.

Interactions and applications
Hormones interact in complex networks; their relative concentrations and sensitivity determine developmental outcomes. For example, auxin and cytokinin ratios guide organogenesis, while ABA antagonises GA during seed dormancy. Agricultural uses include auxin-based rooting powders, gibberellin treatments to improve fruit size, cytokinins to delay senescence, and ethylene inhibitors to prolong shelf life. Understanding hormone action underlies practices in propagation, crop enhancement and post-harvest management.

Classification of plant movements
Plants display both irreversible growth movements (tropisms) and reversible, often rapid movements (nastic movements). Tropisms are directional growth responses toward or away from a stimulus — phototropism (light), gravitropism (gravity), hydrotropism (water), chemotropism (chemicals) and thigmotropism (touch). Nastic movements are non-directional with respect to the stimulus and are typically driven by turgor changes or growth, for example nyctinastic leaf movements or the rapid folding of Mimosa pudica.

Phototropism — sensing and response
Phototropism results from lateral redistribution of auxin such that cells on the shaded side elongate more than those on the illuminated side, causing curvature toward light. Photoreceptors such as phototropins perceive blue light and initiate signalling cascades that alter auxin transporters (PIN proteins), leading to asymmetric auxin distribution. Phototropism helps seedlings orient to light for efficient photosynthesis.

Gravitropism and statoliths
Gravitropic responses orient shoots upward (negative gravitropism) and roots downward (positive gravitropism). Specialized cells called statocytes contain starch-filled plastids (statoliths) that sediment under gravity, triggering signalling that redistributes auxin and modulates cell elongation. In roots, higher auxin concentration on the lower side inhibits elongation leading to downward curvature; in shoots, auxin stimulates cell elongation on the lower side, bending the shoot upward.

Thigmotropism, tendril coiling and rapid movements
Contact with a support induces thigmotropic responses in tendrils and climbing stems; mechanical stimulation causes local changes in growth rates and auxin distribution, resulting in coiling. Rapid movements like Venus flytrap closure or Mimosa leaf folding rely on quick ion fluxes that change turgor in motor cells; action-potential-like electrical signals often precede these movements. These fast responses serve in defence, prey capture or avoidance.

Nastic movements and circadian control
Nyctinasty (sleep movements) involves rhythmic leaf folding and unfolding driven by turgor changes in pulvini and regulated by endogenous circadian clocks. Stomatal rhythms also show daily patterns. These movements enable optimization of light capture, water economy and protection from nocturnal stresses.

Significance and applications
Tropic and nastic responses influence plant architecture, resource acquisition and interactions with the environment. Knowledge of these mechanisms informs practices in horticulture and training vines, supports breeding for desired growth forms and aids understanding of how plants cope with mechanical and environmental challenges.

Photoperiodism — the daylength response
Photoperiodism is a plant's physiological reaction to the relative lengths of day and night. It is crucial in timing seasonal events such as flowering, tuber formation and dormancy. Plants are categorised as short-day, long-day or day-neutral depending on whether they flower when nights are long, nights are short, or independently of daylength. Critical photoperiod thresholds determine whether a particular species will initiate flowering under given conditions.

Phytochrome system and light quality
Photoperiodic responses are mediated by photoreceptors, notably phytochromes that absorb red and far-red light, and by cryptochromes that detect blue light. Phytochrome interconverts between Pr (red-absorbing) and Pfr (far-red-absorbing) forms; Pfr is generally the biologically active form. It is the duration of uninterrupted dark (night) that largely determines the level of Pfr at dawn and thus the floral induction signal. Night interruption experiments demonstrate that a short flash of light during the night can reverse the effect of a long night for many species, emphasising the importance of continuous darkness.

Florigen and long-distance signalling
Leaves perceive photoperiod and produce a mobile flowering signal often referred to as florigen, which moves through the phloem to the shoot apical meristem to trigger floral development. Molecular studies have identified FLOWERING LOCUS T (FT) and related proteins as components of this mobile signal in many species. The integration of photoperiod with other cues such as temperature and internal developmental state determines flowering timing.

Vernalization — cold requirement for flowering
Vernalization is the acquisition of a plant's ability to flower after exposure to prolonged cold. Many biennial and some annual plants require vernalization to ensure that flowering occurs after winter and in favourable season. Cold exposure leads to epigenetic changes and repression of flowering repressors, enabling activation of flowering genes when temperatures rise. Practical treatments, such as cold storage of bulbs or seedlings, exploit vernalization to synchronise flowering.

Agricultural relevance
Understanding photoperiodism and vernalization permits manipulation of flowering times through controlled lighting regimes, sowing dates, and artificial cold treatments. This knowledge is used in breeding, seed production, greenhouse management and scheduling of harvests to match market demand or avoid adverse climatic periods.

Types of abiotic stress and their impacts
Plants face many abiotic stresses — drought, salinity, extreme temperatures, waterlogging and heavy metals — that disrupt water relations, ion balance, metabolism and growth. Stress reduces photosynthesis, impairs nutrient uptake, alters membrane integrity and can lead to oxidative damage. Plant stress physiology studies how plants perceive stress, signal internally and enact protective and repair mechanisms to survive or acclimate.

Drought stress: sensing and responses
Drought reduces soil water availability and plant water potential, causing stomatal closure to limit water loss and consequently reducing CO2 uptake and photosynthesis. At the cellular level, osmotic stress triggers synthesis of compatible solutes (osmolytes) such as proline, glycine betaine and soluble sugars that lower cell osmotic potential and help maintain turgor. Root-to-shoot signalling, notably via abscisic acid (ABA), coordinates whole-plant responses including stomatal closure, growth adjustment and gene expression changes to enhance drought tolerance.

Salinity stress: osmotic and ionic components
Salinity imposes osmotic stress similar to drought and creates ionic toxicity, notably from Na+ and Cl-. Excess Na+ disrupts K+ homeostasis and enzyme activities. Plants employ strategies such as selective ion transport to limit Na+ uptake, extrusion of Na+ from root cells via plasma membrane Na+/H+ antiporters, and sequestration of Na+ into vacuoles by tonoplast Na+/H+ exchangers driven by vacuolar proton pumps. Production of osmoprotectants, antioxidant responses and structural changes (e.g., succulence) also contribute to salinity tolerance. Halophytes have specialised adaptations for high-salt environments.

Temperature stress: heat and cold responses
High temperatures can denature proteins, increase membrane fluidity and accelerate respiration relative to photosynthesis. Plants respond by synthesising heat-shock proteins that act as molecular chaperones, altering membrane lipid composition and activating antioxidant defences. Low temperatures reduce membrane fluidity and enzyme kinetics and can cause ice formation; cold acclimation involves accumulation of cryoprotectants, changes in membrane lipids, and expression of antifreeze proteins to mitigate freezing injury.

Oxidative stress and antioxidant systems
Many stresses lead to overproduction of reactive oxygen species (ROS) such as superoxide, hydrogen peroxide and hydroxyl radicals. Plants deploy enzymatic antioxidants (superoxide dismutase, catalase, peroxidases) and non-enzymatic antioxidants (ascorbate, glutathione, carotenoids) to scavenge ROS. Maintaining redox balance is crucial for preventing damage to lipids, proteins and nucleic acids.

Applications: breeding and management
Understanding stress physiology guides breeding for tolerance traits (deep roots, efficient osmotic adjustment, Na+ exclusion), and informs agronomic measures like mulching, controlled irrigation, salt management and timing of planting to avoid stress periods. Physiological screening and molecular markers accelerate development of stress-tolerant varieties, while integrated management reduces yield losses in adverse environments.

Basic principles of membrane transport
Transport across plant cell membranes occurs via passive diffusion, facilitated diffusion through channels, active transport using pumps and via vesicular trafficking. The plasma membrane and the tonoplast (vacuolar membrane) host many specialised transport proteins that maintain ion gradients, pH differences, and cellular turgor. These transport processes are essential for nutrient uptake, cell expansion, stomatal movements and stress responses.

Proton pumps as primary energy providers
Plasma membrane H+-ATPases hydrolyse ATP to export H+ from the cytosol to the apoplast, creating both a pH gradient and an electrical potential across the membrane. This proton motive force is then used by secondary transporters — symporters and antiporters — to move nutrients such as sucrose, nitrate and phosphate against their concentration gradients by coupling their uptake to the downhill movement of H+. In the tonoplast, vacuolar H+-ATPases and H+-pyrophosphatases (H+-PPases) generate proton gradients that drive sequestration of ions into vacuoles.

Aquaporins and control of water flow
Aquaporins are integral membrane channel proteins that facilitate rapid water movement across membranes, greatly increasing membrane permeability compared to simple diffusion. They are regulated by gating mechanisms sensitive to pH, calcium, phosphorylation state and reactive oxygen species. Aquaporins modulate hydraulic conductivity of roots and leaves and participate in rapid turgor-driven movements and recovery from dehydration.

Ion transport under salinity and nutrient stress
Under saline conditions, maintaining low cytosolic Na+ is critical. Plants employ plasma membrane Na+/H+ antiporters to extrude Na+ and tonoplast Na+/H+ exchangers to sequester Na+ into vacuoles, both powered by proton gradients. Selective K+ uptake is vital because K+ is required for enzyme activation and osmotic balance; high Na+ competes with K+ at membranes, so transporters and channels with selectivity for K+ are important for tolerance. Regulation of transporter abundance and activity by phosphorylation, ubiquitination and interaction with regulatory proteins allows dynamic responses to changing conditions.

Signal integration and physiological outcomes
Membrane transport is tightly linked with signalling networks. Calcium acts as a second messenger modulating channel activity; ABA signalling targets guard cell ion channels to induce stomatal closure; redox signals alter transporter activity during stress. These integrated responses control water relations, nutrient distribution and whole-plant homeostasis and are targets for breeding and biotech interventions to improve crop performance.

Senescence as a regulated developmental process
Senescence is the genetically controlled ageing and orderly degradation of cells, organs or whole plants. Leaf senescence is a coordinated process that reallocates nutrients (nitrogen, phosphorus, carbon) from ageing tissues to developing organs, seeds or storage tissues. Rather than simple decay, senescence is an active, energy-dependent programme involving changes in gene expression, protease and nuclease activities, chlorophyll breakdown and remobilisation of macromolecules.

Hormonal and environmental regulation
Senescence is influenced by hormonal balances: ethylene and abscisic acid generally promote senescence, while cytokinins delay it. Nutrient deficiency, shading, pathogen attack and developmental cues such as seed maturation can induce senescence. Environmental factors and internal signals integrate through transcriptional networks to time senescence for maximal reproductive success and resource economy.

Biochemical changes and nutrient remobilisation
During senescence chlorophyll is degraded, revealing carotenoids and leading to yellowing. Proteases and nucleases break down proteins and nucleic acids into transportable forms such as amino acids and nucleotides, which are mobilised through the phloem to sinks. Lipids are catabolised and sugars are redistributed. Controlled autophagy and compartmentalisation help recycle organelles and macromolecules efficiently while minimising cellular damage.

Dormancy in seeds and buds
Dormancy is a reversible state of arrested growth that enables survival during unfavourable periods. Seed dormancy prevents germination until environmental conditions are suitable; bud dormancy in perennials prevents premature growth during winter. Hormonal control is central: ABA promotes induction and maintenance of dormancy, while gibberellins and favourable environmental cues break dormancy. Dormancy can be enforced by physical barriers (seed coats), physiological inhibitors, or requirement for environmental cues like stratification (cold) or after-ripening.

Agricultural importance and manipulation
Controlling senescence and dormancy has direct agricultural applications. Delaying leaf senescence (stay-green traits) can prolong photosynthetic activity and increase yield, while timely senescence aids nutrient remobilisation into seeds for better grain filling. Breaking seed dormancy uniformly is important for crop establishment and can be achieved by treatments such as stratification, scarification or application of growth regulators. Understanding these processes helps breeders develop varieties with desirable maturation, storage and stress-response traits.

Using physiological knowledge in farming
Applied plant physiology translates understanding of plant functions into practices that improve crop yield, quality and resource use. Farmers and agronomists apply principles of water relations, nutrient cycling, photosynthesis, growth regulation and stress responses to decide irrigation schedules, fertiliser regimes, planting dates and use of growth regulators. Integrating physiology with soil science and weather forecasts allows more efficient and sustainable crop management.

Irrigation strategies and water management
Knowledge of transpiration, root distribution and crop water requirements enables scientists to design irrigation systems such as drip, sprinkler or surface irrigation tailored to crop needs. Techniques like deficit irrigation deliberately supply less water than full crop evapotranspiration during non-critical growth stages to save water with minimal yield loss. Soil moisture sensors, tensiometers and crop coefficients guide timing and amount of irrigation to match plant demand while reducing waste and salinisation risk.

Fertiliser management based on nutrient physiology
Understanding mineral uptake, nutrient mobility and crop demand curves informs fertiliser placement, timing and form. Split application of nitrogen reduces leaching losses and matches supply with crop uptake. Foliar sprays can correct micronutrient deficiencies rapidly during critical growth stages. Integrated nutrient management combines organic manures, biofertilisers, and inorganic fertilisers to maintain soil health and nutrient availability over time.

Growth regulators and post-harvest handling
Plant hormones and their analogues have many agricultural uses: auxins for rooting cuttings, gibberellins to increase fruit size or break dormancy, cytokinins to delay senescence, and ethylene or its inhibitors to control ripening. Controlled application of regulators enhances propagation, fruit set and post-harvest life. Understanding hormone action prevents misuse and unwanted side effects.

Breeding and biotechnology guided by physiology
Breeding programs use physiological traits like deep root systems, high photosynthetic capacity, efficient nutrient use and stress tolerance as selection criteria. Biotechnological approaches aim to modify photosynthetic pathways, improve nitrogen fixation, or enhance stress-protective systems. Physiological screening and marker-assisted selection speed development of varieties adapted to local agro-climatic conditions.

Sustainability and integrated approaches
Applied physiology contributes to sustainable agriculture by improving resource use efficiency, reducing chemical inputs and designing resilient cropping systems. Practices such as conservation tillage, cover cropping, crop rotation and precision nutrient management rely on physiological insights to maintain soil fertility, conserve water and reduce environmental impacts while sustaining productivity.