Chapter 2 — Plant Physiology

Plant physiology studies the working of plants — the internal processes that sustain life, growth and reproduction. These processes include uptake of water and minerals, synthesis of food by photosynthesis, release of energy by respiration, movement of substances through vascular tissues, regulation by hormones, and responses to environmental cues. The subject links visible plant structures to invisible biochemical and physical processes, helping us understand why plants behave and grow the way they do.

Scale of study: Plant physiology ranges from molecules (pigments, enzymes, hormones) to cells (chloroplasts, mitochondria), tissues (xylem, phloem), organs (roots, stems, leaves) and whole-plant responses (flowering, dormancy). A physiological approach explains how a leaf converts sunlight into sugar, how roots extract ions from soil, how plants control water loss, and how seeds resume growth during germination.

Key physiological processes include photosynthesis — converting light to chemical energy; respiration — releasing stored energy; transpiration — water loss that drives nutrient transport; absorption and ascent of sap — moving water from roots to leaves; translocation — moving sugars to sinks; and regulation by plant hormones that control growth and development. Each process depends on structural adaptations and external conditions such as light, water availability, temperature and soil nutrients.

Practical importance is wide: farmers use physiological knowledge to time irrigation, apply fertilisers, choose crop varieties and manage pests. Horticulturists manipulate light and temperature to force flowering or fruiting, and seed technologists use dormancy and germination concepts to improve storage. Environmental scientists use physiological indicators to assess plant stress under pollution or climate change. Understanding physiology also underpins modern biotechnology and plant breeding for higher yield or stress tolerance.

Methods and experiments in this unit teach observation, measurement and interpretation. Students set up simple experiments — for example, testing requirements for photosynthesis, measuring water uptake, or comparing respiration rates — and learn to control variables and record data. Microscopy, staining, and basic biochemical tests (e.g., iodine for starch) illustrate how structure and function are linked. The aim is to build core concepts and practical competence so students can explain plant behaviour and apply this knowledge in agricultural and environmental contexts.

Autotrophic nutrition is characteristic of most green plants that produce organic compounds from inorganic substances. Using carbon dioxide from air and water from soil, and capturing light energy with chlorophyll, green plants synthesise carbohydrates and other organic molecules that form the basis of their growth. Photosynthesis is the central process of autotrophy: it provides sugars for immediate use, for storage (starch) and as building blocks for cellulose, proteins and fats after adding mineral nutrients such as nitrogen and sulphur.

Mineral nutrition complements carbon-based autotrophy. Plants obtain mineral ions dissolved in soil water through root hairs. Essential elements are classified as macronutrients — required in larger amounts (nitrogen, phosphorus, potassium, calcium, magnesium, sulphur) — and micronutrients needed in trace amounts (iron, zinc, manganese, copper, molybdenum). Each element has specific roles: nitrogen is crucial for amino acids and chlorophyll; phosphorus is part of ATP and nucleic acids; potassium regulates stomatal opening and enzyme activity; magnesium is central in chlorophyll molecule.

Heterotrophic modes occur where plants cannot obtain all nutrients via photosynthesis. Parasitic plants (e.g., Cuscuta) attach to host plants and withdraw water and organic compounds. Insectivorous plants (e.g., Nepenthes, Drosera) supplement nitrogen by trapping and digesting insects; nutrients are absorbed from breakdown products. Saprophytic plants and fungi-like organisms obtain organic matter from decaying material. Some plants form mutualistic associations such as mycorrhizae, where fungal partners increase uptake of phosphorus and minerals in exchange for carbohydrates from the plant.

Deficiency symptoms help identify which nutrient is lacking. Nitrogen deficiency often causes pale green or yellow older leaves due to insufficient chlorophyll; phosphorus shortage can cause purple or dark green leaves and stunted growth; potassium deficiency shows necrosis at leaf margins; iron deficiency gives interveinal chlorosis in young leaves. Recognising these patterns allows targeted fertiliser application, avoiding waste and environmental pollution.

Fertiliser management balances nutrient supply. Organic sources like compost and farmyard manure release nutrients slowly and improve soil structure. Chemical fertilisers supply precise amounts of N, P and K (listed as N-P-K on packets). Soil testing provides data to choose appropriate fertiliser rates. Overuse of fertilisers can cause leaching of nitrates into groundwater and eutrophication in water bodies, so careful management is essential.

Practical methods include hydroponics and solution culture to study nutrient roles by omitting specific elements and observing plant responses. This controlled approach clearly links deficiencies to symptoms, and helps students understand how minerals integrate with carbon-based growth to produce healthy plants.

Photosynthesis is the fundamental biochemical process

Basic stages: Photosynthesis consists of light-dependent reactions and light-independent reactions. In light reactions, occurring on thylakoid membranes, photons are absorbed by chlorophyll, driving electron transport and generating ATP and NADPH. Water molecules may be split in photolysis, releasing oxygen. In the light-independent stage (Calvin cycle) in the stroma, enzyme-driven steps use ATP and NADPH to fix CO2 into three-carbon compounds that are subsequently converted into sugars like glucose. The overall balanced reaction commonly written is: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2.

Where it occurs: Anatomically, photosynthesis is most active in mesophyll cells of leaves where chloroplasts are abundant. Palisade mesophyll cells, arranged beneath the upper epidermis, have many chloroplasts and are specialised for light capture. Spongy mesophyll allows gas diffusion. Leaf adaptations such as thin lamina, large surface area, stomatal distribution and vascular supply support efficient photosynthesis.

Ecological and practical importance: Photosynthesis removes carbon dioxide from the atmosphere and produces oxygen, balancing atmospheric composition. It forms the base of food chains: plant biomass supports herbivores and through them higher trophic levels. In agriculture and forestry, photosynthetic efficiency determines crop productivity. Enhancing photosynthesis—by breeding, controlling environment (light, CO2, temperature), or improving nutrient supply—can increase yields. Understanding photosynthesis is also central to climate science, bioenergy research and ecological management.

Limiting factors: The rate of photosynthesis depends on light intensity, CO2 concentration, temperature, water availability and chlorophyll amount. Each factor has an optimum range; when one is below optimal, it limits the overall rate (limiting factor principle). For example, even with abundant light, low CO2 restricts sugar formation. Similarly, enzyme-mediated steps depend on temperature: low temperatures slow reactions, high temperatures may denature enzymes.

Experimental evidence and skills: Simple school experiments—iodine tests for starch, variegated leaf tests, CO2 removal tests and oxygen evolution measurements—demonstrate requirements and stages of photosynthesis. Students learn to design controls and interpret results, linking biochemical reactions to observable outcomes in leaves and plants.

Leaf structure is intricately linked to photosynthetic function.

Upper epidermis and cuticle: The upper epidermis is usually a single layer of transparent cells with a waxy cuticle. The cuticle reduces water loss while transparency allows light to reach underlying chloroplast-rich cells. Sun leaves often have thicker cuticles and more layers of palisade cells to handle higher light intensity and reduce water loss.

Palisade and spongy mesophyll: Palisade parenchyma cells are elongated and densely packed with chloroplasts; they absorb a major portion of incoming light and are the principal sites of photosynthesis. Below them, spongy parenchyma has loosely arranged cells and large intercellular air spaces that allow rapid diffusion of CO2 from stomata to mesophyll cells and facilitate the exit of oxygen and water vapour.

Vascular tissues: Xylem and phloem in the veins provide essential supplies and transport: xylem brings water and dissolved minerals to mesophyll cells for photosynthesis, while phloem transports synthesized sugars to sinks. Veins also provide mechanical support and help maintain leaf shape for maximum light exposure.

Stomata and guard cells: Stomata are pores mainly on the lower epidermis of dicot leaves, flanked by guard cells that regulate their opening. When open, stomata allow CO2 into the leaf and water vapour exit; guard cells change turgor in response to light, CO2, humidity and hormones, balancing CO2 uptake with water loss. In many monocots and aquatic plants, stomatal distribution and behaviour differ: hydrophytes may have stomata on upper surfaces and thin cuticles, while xerophytes show sunken stomata, thick cuticles and reduced leaf area to conserve water.

Anatomical adaptations: Leaves show diverse adaptations to habitat. Shade leaves are thinner, with fewer palisade layers and more surface area relative to thickness, optimised for low light. Sun leaves are thicker with more palisade development. Xerophytic leaves (e.g., succulents) have thickened mesophyll for water storage, reduced surface area, and sunken stomata to reduce transpiration. Kranz anatomy in C4 plants concentrates chloroplast-rich bundle sheath cells around veins to perform the Calvin cycle under high CO2 conditions, minimising photorespiration.

Microscopic study and practical work: Preparing a transverse section of a leaf and observing under light microscope reveals these layers and chloroplast distribution. Staining techniques can highlight vascular bundles. Drawing a labelled diagram of the transverse section helps students link structure to function: palisade for light capture, spongy for gas diffusion, stomata for exchange, and veins for transport and structural support.

Photosynthesis consists of two linked but distinct stages: the light-dependent reactions (light reactions) and the light-independent reactions (commonly called the Calvin cycle or dark reactions). The light reactions capture solar energy and convert it into chemical forms (ATP and NADPH) while producing oxygen as a by-product. The dark reactions use ATP and NADPH to fix atmospheric CO2 into organic molecules that can be built into sugars.

Light reactions — sequence and components: These occur on the thylakoid membranes of chloroplasts where photosystems I and II, electron carriers and ATP synthase are arranged. When photons strike chlorophyll in Photosystem II (PSII), electrons are excited and passed along an electron transport chain. The lost electrons in PSII are replaced by electrons from water, which is split in photolysis to yield O2, protons and electrons. As electrons move along carriers to Photosystem I (PSI), energy released pumps protons into the thylakoid lumen, creating an electrochemical gradient. This proton motive force drives ATP synthase to produce ATP (photophosphorylation). At PSI, electrons are re-excited and used to reduce NADP+ to NADPH. Two modes of photophosphorylation exist: non-cyclic (produces ATP, NADPH and O2) and cyclic (produces ATP only, no NADPH or O2) — cyclic flow helps balance ATP/NADPH ratio.

Photolysis and oxygen evolution: The splitting of water is essential to sustain electron flow and explains why photosynthesis produces oxygen. Photolysis also releases protons that contribute to the proton gradient and thus to ATP formation.

Dark reactions — the Calvin cycle: Occurring in the stroma, the Calvin cycle fixes CO2 into organic molecules in a series of enzyme-catalysed steps. The enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) fixes CO2 by combining it with ribulose-1,5-bisphosphate (RuBP) to make two molecules of 3-phosphoglycerate (3-PGA). ATP and NADPH from the light reactions convert 3-PGA to glyceraldehyde-3-phosphate (G3P). Some G3P exits the cycle to be used in carbohydrate synthesis; the remainder regenerates RuBP using additional ATP so the cycle can continue. The Calvin cycle is sometimes described in three phases: carbon fixation, reduction, and regeneration of RuBP.

Interdependence and regulation: Light reactions supply the energy and reducing power required by the Calvin cycle; without light-derived ATP and NADPH, CO2 fixation cannot proceed at full rate. Enzymes of the Calvin cycle are temperature-sensitive; thus temperature influences dark reactions while light intensity and chlorophyll content influence light reactions. Plants regulate photosynthetic activity by controlling stomatal opening (affecting CO2 availability), adjusting photosystem activity and sometimes by modulating cyclic electron flow under stress.

Practical demonstrations: In laboratory exercises, oxygen evolution from illuminated algal cultures or the starch test on leaves illustrates operation of light and dark phases. Knowledge of these stages helps explain plant responses to environmental changes and guides methods to improve photosynthetic efficiency in crops.

Photosynthetic rate depends on a number of external and internal factors.

Light intensity and quality: Light drives the light-dependent reactions. As light intensity increases from darkness, the rate of photosynthesis rises rapidly at first and then levels off to a plateau where another factor becomes limiting (e.g., CO2 or enzyme capacity). Different wavelengths are absorbed differently by pigments; red and blue wavelengths are most effective for photosynthesis while green is reflected. Quality of light thus affects efficiency.

Carbon dioxide concentration: CO2 is the carbon source for sugar formation. At low CO2 levels, photosynthesis is limited even if light is abundant. As CO2 concentration rises, rate increases until it saturates when RuBisCO or other biochemical steps cannot use the additional substrate. In controlled environments like greenhouses, increasing CO2 can enhance growth until other factors become limiting.

Temperature: Temperature influences enzyme activity in the Calvin cycle and other metabolic steps. There is an optimum temperature range for photosynthesis; below this range enzyme reactions are slow and above it enzymes may denature, reducing rate. Temperature also affects stomatal opening: very high temperatures may close stomata to conserve water, reducing CO2 uptake.

Water availability and humidity: Water stress causes stomata to close, limiting CO2 entry and lowering photosynthesis. High vapour pressure deficit (low humidity) increases transpiration and can lead to stomatal closure, while saturated humidity reduces transpiration but can increase disease risk. Waterlogged soils restrict root respiration and nutrient uptake, indirectly affecting photosynthesis.

Mineral nutrition and leaf health: Deficiencies of essential nutrients (e.g., nitrogen, magnesium) reduce chlorophyll content and enzyme availability, lowering photosynthetic capacity. Leaf age matters: young and mature fully expanded leaves are most photosynthetically active; very old leaves have declining capacity. Pests and diseases damage tissue and impair photosynthesis.

Limiting factor concept and interactions: The limiting factor principle states that if several factors influence a process, the one closest to its minimum relative requirement will limit the rate. For example, on a cloudy day light may be limiting; on a bright day CO2 or temperature may be limiting. In practice, multiple factors interact: increasing CO2 without sufficient nitrogen may not raise yield.

Practical implications: Knowledge of these factors informs crop management: providing adequate irrigation, balanced fertilisation, and appropriate greenhouse control of light, temperature and CO2 can optimise photosynthesis and yields. In experiments, students learn to vary one factor while keeping others constant to determine its effect on photosynthetic rate.

Respiration is the process by which organisms obtain energy from organic compounds.

Stages of respiration: The major stages are glycolysis, the Krebs cycle (citric acid cycle) and oxidative phosphorylation via the electron transport chain. Glycolysis occurs in the cytoplasm and converts glucose into two molecules of pyruvate, producing a small net yield of ATP and NADH. Pyruvate enters mitochondria where it is decarboxylated to acetyl-CoA, which enters the Krebs cycle producing more reduced cofactors (NADH and FADH2) and small amounts of ATP. The reduced cofactors donate electrons to the electron transport chain on the inner mitochondrial membrane; the flow of electrons is coupled to proton pumping that generates a proton gradient. ATP synthase uses this gradient to produce the bulk of ATP in oxidative phosphorylation.

Aerobic versus anaerobic respiration: Aerobic respiration uses oxygen as the final electron acceptor and yields up to about 36–38 ATP per glucose under ideal conditions. Anaerobic respiration or fermentation occurs when oxygen is limited; in plant tissues and many microorganisms pyruvate is converted to ethanol and CO2 (alcoholic fermentation), releasing much less energy (about 2 ATP per glucose from glycolysis only). Anaerobic pathways are important in waterlogged soils or during intense exercise in roots and tubers where oxygen diffusion is restricted.

Physiological roles: Energy from respiration supports growth, nutrient uptake by roots, phloem loading, cell maintenance and responses to environmental stimuli. Seeds germinating in the dark rely on stored food and respiration to power radicle emergence; developing fruits and actively growing meristems have high respiratory rates. Maintenance respiration consumes a large part of assimilated carbon, especially under stress conditions where repair and defence responses are active.

Measurement and indices: Respiration can be measured by O2 consumption or CO2 production using respirometers or gas probes. The respiratory quotient (RQ = CO2 produced / O2 consumed) indicates the type of substrate being respired: RQ of 1 suggests carbohydrate metabolism, RQ < 1 indicates fats or proteins. Temperature strongly influences respiration: rates increase with temperature up to an optimum, beyond which enzymes denature and rates fall. Q10, the factor by which rate changes for a 10°C temperature rise, is used to express temperature sensitivity.

Practical examples and implications: Waterlogged soils reduce oxygen availability and aerobic respiration in roots, causing root damage. Storage of fruits and vegetables at low temperatures slows respiration and prolongs shelf life. Understanding respiration aids post-harvest storage practices, seed storage and management of crops under stress conditions.

Transpiration is the continuous loss of water vapour from aerial parts, primarily leaves. It occurs mainly through stomata but also via the cuticle and, less frequently, through lenticels. While often seen as wasteful, transpiration is essential to plant function: it creates a transpirational pull that helps draw water and dissolved minerals up the xylem from roots to leaves, supplies water for photosynthesis and cooling, and maintains tissue turgor required for cell expansion and growth.

Pathway of water movement: Water absorbed by root hairs moves across root cortex via the apoplast (cell walls), symplast (through cytoplasm and plasmodesmata) or transmembrane pathways until it enters xylem vessels in the stele. From xylem it rises in a continuous column to stems and leaves, ending on mesophyll cell walls where it evaporates into intercellular air spaces and diffuses out through stomata to the atmosphere.

Cohesion-tension mechanism: The cohesion of water molecules (hydrogen bonding) and their adhesion to the walls of xylem conduits create a continuous column of water under tension. Transpiration from the leaf surface generates negative pressure which is transmitted down the column, pulling water upward — this is the cohesion-tension theory of ascent of sap. Root pressure, created by osmotic influx of water into xylem at night, can also push water upward but is generally a minor contributor except under low transpiration conditions.

Types and regulation: Stomatal transpiration is the dominant form and is regulated by guard cell turgor. Guard cells respond to light, CO2 concentration, humidity, soil water status and hormones (notably abscisic acid) to open or close stomata. Cuticular transpiration occurs across the cuticle and is usually small unless the cuticle is damaged. Lenticular transpiration from lenticels in stems is minor in most plants.

Factors affecting transpiration: Environmental factors include light (increases stomatal opening), temperature (increases evaporative demand), humidity (low humidity increases transpiration), wind (removes boundary layer and increases transpiration) and soil water availability (drought causes stomatal closure). Plant factors include stomatal density and distribution, leaf area, thickness of cuticle, presence of hairs and leaf orientation.

Measurement and practical significance: Transpiration can be measured with potometers that estimate water uptake by cut shoots, or by weighing potted plants. Understanding transpiration helps in irrigation scheduling, greenhouse climate control and breeding for drought-tolerant crops. Excessive transpiration can lead to wilting and reduced growth, while some transpiration is necessary for nutrient transport and cooling.

Stress responses and adaptations: Xerophytes show adaptations to reduce water loss: sunken stomata, thick cuticles, reduced leaf area and CAM or C4 photosynthetic pathways. Hydrophytes have large air spaces and reduced cuticles. Knowledge of transpiration and water relations is essential for managing crops in varied climates and for conserving water in agriculture.

Water absorption and ascent of sap are central to plant water relations. Roots absorb water and dissolved minerals from the soil, and these are transported to leaves where water is used in photosynthesis, lost by transpiration and provides turgor for cells. The process combines physical forces, cellular transport and anatomical specialisations to move water against gravity to considerable heights in tall trees.

Absorption at root hairs: Root hairs are tubular extensions of epidermal cells that greatly increase the surface area for absorption. They extend into soil pores where soil water contains dissolved mineral ions. Water moves into root hairs by osmosis because the cell sap has a higher solute concentration than the surrounding soil water. Mineral ions are often taken up by active transport via membrane proteins that use cellular energy to accumulate ions against gradients.

Pathways across root cortex: Water travels toward the central stele by three main routes: apoplast (through cell walls and intercellular spaces), symplast (through cytoplasm connected by plasmodesmata) and transmembrane (repeated crossing of cell membranes). The apoplast route is rapid until the endodermis, where the Casparian strip (a band of suberin in cell walls) blocks apoplastic flow; this forces selective uptake via the symplast so the plant controls which ions enter the xylem.

Entry into xylem and ascent: Once in the stele, water enters xylem vessels and tracheids — hollow, lignified conduits that provide a continuous low-resistance path. The ascent of sap is explained primarily by the cohesion-tension mechanism: evaporation of water from mesophyll cell walls creates tension transmitted through the continuous water column in xylem. Cohesive forces between water molecules and adhesion to the vessel walls prevent the column from breaking as it is pulled upward. Root pressure, generated by osmotic movement of water into xylem at night when transpiration is low, can produce a positive push, occasionally visible as guttation droplets at leaf margins.

Anatomical features and resistance: Vessel diameter, presence of pits, and the degree of lignification affect resistance to flow and susceptibility to cavitation (formation of air bubbles) which can break the water column and interrupt flow. Many plants have mechanisms to refill embolised vessels or route flow through alternative conduits. Seasonal and environmental factors influence vulnerability to cavitation; drought and freezing conditions increase risk.

Evidence and experiments: Demonstrations using coloured water show dye movement from roots to leaves. Measurement of xylem pressure with pressure chambers (pressure bomb) detects negative pressures consistent with transpiration pull. Observations of guttation and root pressure at night further support multiple contributing forces. Understanding absorption and ascent of sap guides irrigation, informs tree physiology and explains symptoms like wilting, leaf scorch and reduced water transport under drought.

Mineral transport and translocation of food

Uptake and transport of minerals: Mineral ions dissolved in soil water are absorbed by root hairs using passive diffusion and active transport. Some ions enter passively with the transpiration stream; others are actively accumulated in root cells using membrane transporters. Once across the endodermis into the xylem, minerals travel primarily in the transpiration stream driven by transpiration pull. Distribution depends on demand: some minerals accumulate in leaves, others preferentially move to growing roots or developing seeds. Plants can regulate uptake via changes in root membrane transporters and by mycorrhizal associations that enhance access to phosphate and other poorly mobile nutrients.

Phloem translocation — the pressure-flow mechanism: Phloem translocates organic solutes, mainly sucrose, from source tissues where sugars are produced (mature leaves) to sink tissues that consume or store them (growing roots, buds, developing seeds and fruits). The widely accepted pressure-flow (mass flow) model explains this movement: at the source, sucrose is actively loaded into sieve tube elements, reducing water potential and causing water to enter from adjacent xylem by osmosis. The increased turgor pressure drives bulk flow of the phloem sap toward sinks. At the sink, sucrose is actively or passively unloaded and either metabolised or stored, raising water potential so water leaves the phloem and returns to xylem, maintaining the pressure gradient.

Phloem structure and function: Phloem sieve tube elements are living cells joined end to end with sieve plates allowing sap flow. Companion cells, connected by plasmodesmata, support sieve tubes metabolically and assist in loading and unloading solutes. The phloem is capable of bidirectional flow; a single plant can have multiple source–sink pathways simultaneously depending on developmental stage and environmental conditions.

Experimental evidence: Classic experiments include girdling (ringing) which removes phloem but leaves xylem intact; sugars accumulate above the ring and tissues below starve and die, demonstrating downward translocation. Use of radiolabelled carbon (C14) shows rapid movement of labelled sugars from leaves to roots and developing tissues. Aphid stylet experiments allow sampling of phloem sap to analyse composition and pressure, confirming high sucrose concentration in source phloem and lower in sink phloem.

Practical implications: Understanding source–sink relationships helps manage pruning, fertilisation and harvest timing to maximise yields. For example, removing competing sinks (thinning fruits) can increase size of remaining fruits. Storage organ formation (tubers, bulbs) requires coordinated photosynthate supply and hormonal signals. Disorders like phloem blockage by pathogens or pests can severely reduce crop productivity; hence phloem health is critical in crop protection and breeding.

Plant hormones, or phytohormones, are organic compounds produced by plants at low concentrations that regulate growth, development and responses to environmental stimuli. Unlike animal hormones that are often produced in specific glands, plant hormones are synthesised in various tissues and transported to target sites where they influence processes such as cell division, elongation, differentiation, dormancy, senescence and stress responses.

Major classes and their roles: Auxins (e.g., indole-3-acetic acid, IAA) promote cell elongation, root initiation and apical dominance; they are synthesised in shoot tips and young leaves and transported basipetally (downwards). Gibberellins (GAs) stimulate stem elongation, seed germination and fruit development. Cytokinins promote cell division, delay leaf senescence and influence nutrient mobilisation; they are synthesised mainly in roots and transported upward. Abscisic acid (ABA) generally inhibits growth, enforces seed dormancy and triggers stomatal closure during water stress. Ethylene, a gaseous hormone, regulates fruit ripening, leaf abscission, flower senescence and responses to mechanical stress.

Interactions and balance: Plant responses often depend on relative concentrations and interactions among hormones. Apical dominance results from a high auxin level in shoot tip suppressing lateral bud growth; removing the apex reduces auxin and lateral buds grow, especially if cytokinin levels rise. Seed germination is regulated by the balance between gibberellins (promote germination) and ABA (maintain dormancy). Tissue culture techniques exploit auxin–cytokinin ratios: high auxin relative to cytokinin promotes root formation, while high cytokinin relative to auxin promotes shoot formation.

Mechanisms of action: Hormones may act by altering gene expression, activating or inhibiting enzymes, or changing membrane properties and ion fluxes. For example, auxin promotes cell elongation by increasing proton pumping into cell walls, lowering pH and activating expansins that loosen cell walls, allowing turgor-driven extension. ABA causes ion efflux from guard cells leading to turgor loss and stomatal closure during drought.

Practical uses: Plant growth regulators are widely used in agriculture and horticulture: auxins in rooting powders to encourage cuttings to root; gibberellins to increase stem elongation or break dormancy in seeds; cytokinins to delay leaf yellowing and promote shoot proliferation in tissue culture; ethylene or ethephon to induce uniform fruit ripening. Understanding hormone actions guides crop management, post-harvest handling and biotechnological manipulations.

Experimental observations: Simple classroom experiments include applying auxin to stimulate root formation on cuttings, observing fruit ripening with ethylene exposure, and demonstrating stomatal closure with ABA treatment. These experiments illustrate how small amounts of chemical signals lead to large-scale developmental changes in plants.

Photoperiodism and vernalisation are environmental cues that regulate the timing of major developmental events in plants, especially flowering.

Photoperiodism — sensing day length: Photoperiodism is the physiological response of plants to the relative lengths of day and night. Plants are classified as short-day, long-day and day-neutral. Short-day plants flower when night length exceeds a critical duration; long-day plants flower when night length is shorter than a critical period. Day-neutral plants flower irrespective of day length. Importantly, it is uninterrupted darkness that often determines response: night-interruption experiments demonstrate that a brief light pulse during the night can prevent flowering in a short-day plant, showing that the duration of darkness is the key signal.

Phytochrome system: The phytochrome pigment system mediates light perception. Phytochrome exists in two interconvertible forms: Pr (absorbs red light ~660 nm) and Pfr (absorbs far-red ~730 nm). Red light converts Pr to Pfr while far-red converts Pfr to Pr. The Pfr form is often the physiologically active form for many photoperiodic responses. The relative amounts of these forms after night and day affect flowering: for instance, in long-day plants Pfr promotes flowering while in some short-day plants Pfr inhibits flowering, showing species-specific responses.

Florigen and systemic signalling: Leaves perceive photoperiod and generate a mobile signal, termed florigen, that travels to the shoot apical meristem to induce flowering. While molecular studies have identified specific proteins and genes associated with florigenic signals, the practical idea is that leaves integrate light signals and communicate with meristems to switch developmental programs.

Vernalisation — cold requirement: Vernalisation is the induction of flowering or breaking of seed dormancy following exposure to a prolonged period of low temperature. Many biennials and winter cereals require vernalisation so they do not flower before winter but develop in spring. Vernalisation works by altering gene expression in the meristem or seed so the plant becomes competent to flower when other cues (like day length) are favourable.

Applications and management: Understanding photoperiodism and vernalisation is useful in agriculture and horticulture to manipulate flowering times. Controlled lighting in greenhouses can induce or delay flowering, while cold treatments (artificial vernalisation) can be applied to seeds or bulbs to break dormancy. Plant breeders use this knowledge to develop varieties adapted to different latitudes and climates. These mechanisms also explain seasonal behaviour of many crops and wild plants and are important for crop scheduling.

Plant movements are diverse and can be grouped into tropisms and nastic movements. Tropisms are directional growth responses oriented toward or away from a stimulus, while nastic movements are non-directional responses that depend on the type of stimulus but not its direction. Both types reveal how plants perceive their environment and alter growth or turgor to adapt.

Tropisms — growth-based directional responses: Tropic responses include phototropism (response to light), gravitropism or geotropism (response to gravity), hydrotropism (response to moisture gradients), and thigmotropism (response to touch seen in climbing plants). Tropisms usually involve differential cell elongation caused by uneven distribution of growth substances, principally auxin. In a classic phototropism example, unilateral light causes auxin to redistribute toward the shaded side of the stem; auxin promotes cell elongation in shoot tissues, so the shaded side elongates more and the shoot bends toward light. In roots, auxin inhibits elongation at higher concentrations, contributing to positive gravitropism (downward growth).

Nastic movements — turgor-driven and reversible: Nastic movements are often rapid and reversible because they are driven by changes in turgor pressure in specialised motor cells rather than by differential growth. Examples include nyctinasty (night movements of leaves or petals), seismonasty or thigmonasty (touch-induced movements such as Mimosa pudica folding) and pulvinar movements in legumes where the pulvinus, a specialised motor organ at the leaflet base, changes turgor rapidly by ion fluxes and water movement. These movements allow rapid responses to environmental cues and can protect delicate tissues or conserve water.

Mechanisms and hormones: Tropisms depend on hormone redistribution and differential growth; auxin transporters and polar auxin transport pathways are central to directional growth. Nastic movements involve ion channels, rapid efflux or influx of potassium and other solutes in motor cells, followed by water movement, resulting in loss or gain of turgor. Calcium signalling, action potentials and plant-specific electrical signals sometimes mediate rapid nastic responses.

Physiological significance: Movements enable plants to optimise light capture (phototropism), anchor roots in soil (gravitropism), climb supports (thigmotropism), close leaves to reduce herbivore damage (seismonasty) or conserve water by folding leaves. Studying movements illustrates how plants coordinate perception and response without a nervous system, using chemical and electrical signals instead.

Classroom demonstrations: Simple experiments include placing a young shoot near unilateral light to show phototropism, subjecting seedlings to reorientation to observe gravitropic curvature, and touching Mimosa leaves to see rapid folding. Recording the time course, drawing diagrams and relating observations to hormone action and turgor changes deepen understanding of plant responsiveness.

Stress physiology examines how plants perceive and respond to environmental stresses such as drought, soil salinity and extreme temperatures. These stresses disturb homeostasis, affecting water relations, nutrient uptake, photosynthesis, respiration and growth. Plants have evolved multiple strategies — physiological, biochemical and morphological — to tolerate or avoid such stresses.

Drought stress: Water deficit reduces soil water potential and limits water uptake by roots. Early responses include stomatal closure to minimise transpiration, mediated by abscisic acid (ABA) signalling. While stomatal closure conserves water, it reduces CO2 intake and hence photosynthesis. Prolonged drought leads to loss of turgor, wilting, reduced cell expansion and growth, and may trigger leaf abscission. At the cellular level, plants accumulate compatible solutes (osmolytes) like proline, glycine betaine and soluble sugars to maintain cell turgor and protect proteins and membranes.

Salinity stress: High salt concentrations in soil create osmotic stress similar to drought and cause ion toxicity when Na+ and Cl- enter tissues. Salt stress disrupts enzyme activities and nutrient balance (e.g., K+ deficiency due to Na+ competition). Plants cope by excluding salts at root level, sequestering ions in vacuoles, synthesising osmoprotectants, and activating antioxidant systems to combat reactive oxygen species. Halophytes have special salt-excreting glands or succulent tissues to store salts.

Temperature extremes: Low temperatures reduce membrane fluidity, slow enzyme kinetics and can cause chilling injury in tropical plants. High temperatures increase membrane fluidity excessively, denature proteins and disrupt photosynthesis and respiration. Heat-shock proteins (HSPs) help refold denatured proteins; antifreeze proteins inhibit ice crystal growth in freezing-tolerant species. Acclimation and hardening processes adjust membrane composition and protective molecules to improve tolerance.

Signalling and gene expression: Stress perception triggers signalling cascades involving calcium, reactive oxygen species, hormones (ABA, ethylene), kinases and transcription factors that change gene expression to produce stress-protective proteins, osmolyte biosynthetic enzymes and detoxifying enzymes. ABA is a central hormone in drought response, promoting stomatal closure and expression of stress-responsive genes.

Management and mitigation: Agricultural strategies include breeding or engineering stress-tolerant varieties, improving irrigation practices (drip irrigation, mulching), soil amendments to reduce salinity, and adjusting planting times. Use of anti-transpirants, application of osmoprotectants, and microbial inoculants (e.g., mycorrhizae, rhizobacteria) can enhance tolerance.

Practical observations: In classrooms students can compare stomatal conductance, leaf water potential and photosynthetic rates between well-watered and droughted plants, or observe leaf damage in saline-treated seedlings. These experiments demonstrate how stress alters physiology and why understanding stress responses is vital for crop security under changing climates.

C4 and CAM photosynthetic pathways are specialised adaptations

C4 pathway — spatial separation: In C4 plants, initial fixation of CO2 occurs in mesophyll cells where the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) fixes CO2 into a four-carbon acid (oxaloacetate, converted to malate or aspartate). PEP carboxylase has high affinity for CO2 and does not bind O2, so it efficiently captures CO2 even at low internal concentrations. These C4 acids travel to bundle-sheath cells that form a tight sheath around the vascular bundles (Kranz anatomy). There, CO2 is released from the four-carbon compound and concentrated around RuBisCO, which carries out the Calvin cycle in an environment with elevated CO2, minimising photorespiration. C4 plants (e.g., maize, sugarcane, sorghum) are well adapted to high light intensities, high temperatures and often drought-prone conditions; they show higher photosynthetic efficiency and productivity under such circumstances.

CAM pathway — temporal separation: CAM (Crassulacean Acid Metabolism) plants, including many succulents and epiphytes, separate CO2 uptake and fixation by time. Stomata open at night when evaporative demand is low, and CO2 enters the leaf and is fixed by PEP carboxylase into organic acids that are stored in vacuoles. During the day, stomata close to conserve water; the stored acids are decarboxylated to release CO2 internally, which is then fixed by RuBisCO in the Calvin cycle. This nocturnal CO2 uptake greatly reduces water loss and allows CAM plants to inhabit very dry sites. CAM expression can be constitutive or facultative (induced by drought in some plants).

Energetics and trade-offs: Both pathways require additional biochemical steps and energy for concentrating CO2, so in cool, CO2-rich or low-light environments C3 photosynthesis may be more efficient. The evolution of C4 and CAM represents trade-offs where investment in CO2 concentrating mechanisms pays off under high light, high temperature or water-limited conditions because they prevent carbon loss via photorespiration and reduce water loss.

Anatomical and physiological features: C4 plants show Kranz anatomy: distinct bundle sheath cells with abundant chloroplasts surrounding veins and clear separation of initial fixation (mesophyll) and Calvin cycle (bundle sheath). CAM plants often have thick succulent leaves with large vacuoles for storing organic acids and specialised stomatal rhythms. Both types often show higher water-use efficiency than C3 plants and different responses to heat and light.

Practical relevance: C4 crops like maize and sugarcane are major food and biofuel sources in warm regions. Research seeks to introduce components of C4 metabolism into C3 crops (e.g., rice) to improve productivity. Understanding CAM helps in cultivating succulents and managing plants in arid landscaping and agriculture. These pathways illustrate evolutionary solutions to environmental constraints and are important in ecology and crop science.

Seed germination

Phases of germination: Phase I involves imbibition and swelling of tissues, restoring membrane integrity and starting respiration. Phase II is a plateau phase during which metabolic repair and synthesis of enzymes (e.g., amylases, proteases) occur; stored starch in endosperm or cotyledons is hydrolysed to sugars by amylase for use as respiratory substrate. Phase III begins when the radicle (embryonic root) emerges, marking visible germination and growth driven by cell division and elongation. Hormones regulate these phases: gibberellins stimulate synthesis of hydrolytic enzymes in the endosperm, while ABA generally inhibits germination.

Dormancy and its adaptive value: Dormancy is a temporary state in which seeds will not germinate even when environmental conditions are favourable. Dormancy ensures germination occurs at an appropriate season, enhancing seedling survival. Causes include impermeable seed coats that prevent water or gas exchange, physiological dormancy from hormone balances (high ABA), immature embryos at dispersal, and chemical inhibitors within seed coats. Some seeds require specific cues to break dormancy, such as cold stratification, scarification, light exposure or passage through an animal gut.

Breaking dormancy and promoting germination: Methods to break dormancy depend on its cause: scarification (mechanical or chemical abrasion) removes hard seed coats; stratification (exposure to chilling temperatures) simulates winter to break physiological dormancy; treating seeds with gibberellins or soaking in water may overcome hormonal inhibition; light requirements can be satisfied by exposing seeds to appropriate wavelengths. Proper storage conditions — cool and dry — preserve viability by slowing metabolic degradation and preventing fungal attack.

Practical implications: Germination under controlled conditions is critical in agriculture, forestry and horticulture. Seed testing for viability, dormancy-breaking treatments, and timing of sowing ensure good crop establishment. Understanding germination biology helps improve seed storage protocols in seed banks and supports restoration projects where native seeds are used for habitat recovery.

Experiments and observations: Classroom investigations include germinating seeds on moist filter paper under varied temperatures or light conditions, comparing treated and untreated seeds, and measuring percentage germination. Observing enzyme activity, radicle growth and seedling vigour connects biochemical processes to visible developmental stages and reinforces understanding of dormancy and germination control.

Experimental practice is central to learning plant physiology. Simple, well-designed experiments help students observe, measure and explain physiological processes. Key techniques include preparing epidermal peels to observe stomata, staining leaves for starch, using potometers to measure water uptake, germination tests on filter paper, and setting up controlled experiments to test requirements for photosynthesis (light, CO2, chlorophyll).

Designing experiments: Good experiments control variables so only the factor of interest changes. For example, to test the effect of light on starch formation, keep the same plant, same duration and same temperature; only vary light exposure on part of a leaf. Include controls (positive and negative) to validate results. Repeat trials and record data systematically in tables for analysis. Safety is important: handle chemicals like iodine with care, use heat sources safely, and dispose of plant material responsibly.

Common procedures: Iodine test for starch involves killing leaf tissue in boiling water to stop metabolism, removing chlorophyll with alcohol, and adding iodine; blue-black colour indicates starch. Potometer experiments estimate transpiration by measuring water uptake of a cut shoot; ensure airtight seals and avoid bubbles in the apparatus. CO2 requirement experiments can use soda lime to absorb CO2 in a sealed chamber with a plant to show lack of starch formation. Germination tests on moist filter paper allow measurement of germination percentage and rate under different temperatures or treatments.

Microscopy and sectioning: Preparing thin transverse sections of leaves or stems reveals internal organization. Staining with safe dyes highlights tissues; drawing labelled diagrams reinforces observational skills. Epidermal peels from leaves show stomata and guard cells, enabling direct observation of stomatal responses to light or humidity.

Quantitative measurements and calculations: Students learn simple calculations such as transpiration rate (volume of water lost per unit area per hour) and percentage germination. Recording time series and plotting graphs (e.g., water uptake vs time, germination percentage vs days) helps interpret trends and identify limiting factors. Use of simple probes (oxygen or CO2 sensors) can add quantitative depth where available.

Interpreting results and sources of error: Encourage students to relate observations to physiological explanations: if variegated leaf areas lack starch, conclude chlorophyll is needed for photosynthesis. Discuss sources of error such as leaks in potometers, uneven light, contamination, or incorrect timing, and suggest improvements. Emphasise replicates and clear lab records.

Field observations: Practical skills extend to the field: observing leaf adaptations in local plants, measuring stomatal density with clear nail polish impressions, and noting seasonal flowering patterns help link lab experiments to natural contexts. These activities develop scientific thinking and practical competence in plant physiology.