Plant Physiology

All autotrophic organisms need carbon dioxide and water, which supply carbon, oxygen and hydrogen — the predominant elements plants use to build organic molecules. Plant nutrients are divided into two groups.

Macronutrients (9) are needed in relatively larger amounts: carbon, hydrogen, oxygen, nitrogen, potassium, calcium, phosphorus, magnesium and sulphur.

• Carbon, oxygen, hydrogen are required for making organic compounds.
• Nitrogen is necessary for growth, energy metabolism and the production of proteins. Its deficiency causes leaf loss and stunted growth.
• Phosphorus is a part of ATP, promotes root growth and favours flowering. Its deficiency delays flowering and causes browning and wrinkling of leaves.
• Potassium is involved in water regulation and the transport of reserve substances, enhances photosynthesis, reinforces cellular tissue and stimulates the uptake of nitrates. Its shortage causes dark patches on leaves.
• Calcium gives stability to the cell wall and helps in cell proliferation, maturation and the development of seeds. Its shortage causes yellow and brown patches on leaves.
• Magnesium is the core of the chlorophyll molecule, so it is essential for photosynthesis; it also promotes the absorption and transport of phosphorus and the storage of sugars. Its deficiency causes weak stalks, loss of greenness in the oldest leaves and yellow and brown spots.
• Sulfur is fundamental in the metabolism of nitrogen. Its shortage makes the plant lighter in colour.

Micronutrients (7) are needed in very small amounts: iron, manganese, zinc, molybdenum, copper, chlorine and boron.

• Iron is essential for the synthesis of chlorophyll and acts as a cofactor for several enzymes of energy metabolism.
• Manganese is important for antioxidant enzymes such as superoxide dismutase (SOD).
• Zinc helps in chlorophyll synthesis, root development and nutrient uptake; its deficiency causes stunted growth.
• Molybdenum helps in nitrogen fixation and reduction, sulfur metabolism, phosphorus metabolism and iron utilization; its deficiency causes chlorosis of older leaves and stunted growth.
• Copper helps in lignin synthesis, strength and rigidity, nitrogen metabolism and reproduction; its deficiency causes chlorosis, twisted leaves and stunted growth.
• Chlorine helps in stomatal regulation, osmotic adjustment and the transport of nutrients.

📌 Note: Fertilizers are added to the soil to provide macro and micronutrients to the crops.

Insectivorous plants supplement their organic molecules by capturing and breaking down insects and tiny animals. They are true autotrophs, but their development accelerates when they get prey, because the nitrogenous compounds of the animal body benefit them. Enzymes released by their leaves break down the captured insects and the products are absorbed.

• Pitcher plant: its leaves are modified into a sac or pitcher partly filled with water; the terminal portion is altered into a hood that partly covers the opening, and stiff hairs prevent insects from crawling out.
• Venus-fly trap: its trap is made of two lobes hinged at the end of a leaf. The inner surfaces contain trichomes (hair-like projections) which trigger rapid closure on contact; the hinged traps are lined with fine bristles that interlock on closure, and the trapped insect is digested by enzymes secreted from glands on the leaf surface.
• Sundew: it catches its prey with shiny drops of "dew" — that is where its common name comes from. Its leaves are covered with tiny tentacle-like hairs, each with a gland and a single drop of dew at the tip. Insects attracted by the plant's odour are trapped by the tentacles, digested by enzymes and absorbed.

Stomata (singular = stoma) are the tiny openings or pores present within plant tissues which are necessary for gaseous exchange. They are typically found in leaves but can even be present in some stems. Stomata are surrounded by specialized cells called guard cells that facilitate the opening and closing of the stomatal pores. Guard cells are bean-shaped and contain chloroplasts. They can open and close depending on environmental conditions, and the opening and closing of stomata control the transpiration rate in plants.

During daylight, stomata open so that carbon dioxide can enter for photosynthesis. At night they typically close to conserve water, although plants still respire (oxygen in, carbon dioxide out); closing at night minimizes water loss through transpiration.

Opening and closing of stomata. Guard cells function as multisensory hydraulic valves. Two hypotheses explain their working:

• Starch-sugar hypothesis (1856, H. Van Mohl): guard cells in the leaf epidermis alone carry out photosynthesis, producing sugars during the day. The sugar concentration rises, so the water potential drops, water moves into the guard cells, they become turgid and the stoma opens. At night photosynthesis ceases, sugars are converted to insoluble starch or used in respiration, free sugars decline, water moves out, the cells become flaccid and the stoma closes. This hypothesis does not fully explain the rapid turgor changes.
• Influx of K⁺ ions: opening is facilitated by the active transport of potassium ions into the guard cells. This reduces the osmotic potential, water enters by osmosis, the cells become turgid and the stoma opens. Blue light enhances this by acidifying the surrounding environment, promoting K⁺ uptake and water absorption. At night K⁺ passively diffuses out, water is lost, the cells become flaccid and the stoma closes.

📌 Note: At high temperature, when leaf cells start wilting, abscisic acid released from mesophyll cells stops the active transport of K⁺ into guard cells — overriding light and CO₂ concentration — so K⁺ pumping stops and stomata close.

Palisade tissue lies just beneath the upper epidermis. Its cells are elongated and tightly packed and rich in chloroplasts, an arrangement that maximizes light absorption for converting light energy into chemical energy. Carbon dioxide from the atmosphere diffuses into the leaf through stomata, passes through the air spaces of the spongy mesophyll into the palisade mesophyll cells, and is used in photosynthesis; the oxygen produced diffuses back through the spongy mesophyll and exits the leaf through the stomata.

Supporting tissues play an important role in maintaining the structural integrity, support and flexibility of plants. They consist of parenchyma, collenchyma, sclerenchyma, xylem and phloem.

1. Parenchyma. This tissue provides support to herbaceous plants and to parts of larger plants. The parenchyma cells of the epidermis, cortex and pith absorb water. This water creates an internal hydrostatic pressure known as turgor pressure that maintains the rigidity of the cells. Turgor pressure arises from the elevated osmotic pressure within the cell vacuole. The membrane that surrounds the vacuole is called the tonoplast; it has many active transport mechanisms that move ions into the vacuole even when the concentration within is higher than that of the surrounding fluid. Due to the elevated ionic concentration, water is drawn into the vacuole, resulting in turgidity and providing mechanical support to the plant's soft tissues.

2. Collenchyma. These specialized cells are grouped in the form of strands or cylinders. They are found beneath the epidermis of young stems, leaf stalks and along veins in leaves. Collenchyma cells lack secondary walls; their primary walls are thickened at the corners due to extra deposition of cellulose. They elongate when the stem or leaf grows lengthwise, and they provide support to the young parts of the plant in which secondary growth has not taken place.

3. Sclerenchyma. This tissue also provides structural support. Typically its cells possess thick secondary cell walls saturated with lignin, an organic compound that confers strength and rigidity to the walls. The majority of sclerenchyma cells are non-living, and their main function is to support the various components of the plant. There are three types: fibres (tracheids) — elongated and cylindrical, found either as compact bundles inside the xylem or as bundle caps; sclereids — smaller than fibres, present in the seed coats and shells of nuts, whose function is to offer protection; and vessels (tracheae) — long tubular structures joined end to end to form a long water-conducting pipe in the xylem.

Water molecules possess kinetic energy, which means that in liquid or gaseous form they move about rapidly and randomly from one place to another. So, the greater the concentration of water molecules in a system, the greater is the total kinetic energy of water molecules. This is called water potential (symbolized by the Greek letter psi = Ψ_w). In plant cells, two factors determine water potential: solute potential (Ψ_s) and pressure potential (Ψ_p).

Pure water has maximum water potential which by definition is zero. Water moves from a region of higher Ψ_w to lower Ψ_w. All solutions have lower Ψ_w than pure water and so have a negative value of Ψ_w (at atmospheric pressure and at a defined temperature). So, osmosis can be defined as the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Solute potential (Ψ_s), or osmotic potential, is a measure of the change in water potential due to the presence of solute molecules. Ψ_s is always negative, so if more solute molecules are present, lower (more negative) is the Ψ_s.

Pressure potential (Ψ_p) is the part of water potential which is due to the pressure exerted by water. If pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. When water enters plant cells by osmosis, pressure may be built up inside the cell, making the cell turgid and increasing the pressure potential.

Thus, the total water potential (Ψ_w) is the sum of solute potential (Ψ_s) and pressure potential (Ψ_p): Ψ_w = Ψ_s + Ψ_p. If we use the term water potential, the tendency for water to move between any two systems can be measured — not just from cell to cell in a plant but also from soil to root, from leaf to air and from soil to air. The steeper the potential gradient, the faster is the flow of water along it.

Uptake of water by roots. Roots provide a large surface area for absorption by their extensive branching systems. Roots have tiny root hairs, which are extensions of epidermal cells of roots, and most of the uptake of water and minerals in roots takes place through root hairs. From the soil, water and minerals enter the root epidermal cells by active and passive transport. From the root epidermis, they move to the cortex, and then into the xylem tissue in the centre of the root. Inside roots, water and minerals move in three different pathways to reach the xylem:

• 1. The apoplast pathway: a continuous pathway that involves a system of adjacent cell walls in the plant roots. It becomes discontinuous in the endodermis in the roots due to the presence of Casparian segments.
• 2. The symplast pathway: water and minerals move through the interconnected protoplasts of root cells. The protoplasts of neighbouring cells are interconnected through plasmodesmata, which are cytoplasmic strands that extend through pores in adjacent cell walls. The symplast pathway is less important, except for minerals in the region of the endodermis.
• 3. The vacuolar pathway: water and minerals move through cell membranes, cytoplasm and tonoplast (membranes of vacuoles) and vacuoles. They move from vacuole to vacuole and bypass the symplast and apoplast pathways. Movement in the vacuolar pathway is negligible.

Structure of xylem tissue. Xylem is the vascular tissue in plants that carries water and dissolved minerals from the roots to the stem and leaves. It is also a key structural component which provides mechanical support to the plant body. Xylem comprises tracheids, vessels, xylem fibres and xylem parenchyma.

• Tracheids are elongate and thin cells that have thick walls made of lignin. The ends of the cells are tapered and they are linked to each other by bordered pits, which enable the lateral movement of water between cells.
• Vessels are shorter and broader compared to tracheids. They are arranged in a linear fashion, forming continuous channels. Perforation plates are present at the outer edges of these structures, enabling efficient movement of water.
• Xylem fibres are elongated cells with thickened lignified walls. At maturity, they are dead and enhance the structural integrity of the xylem, offering additional structural support to the plant.
• Xylem parenchyma are living cells with thin walls that have the ability to retain and hold nutrients and water. They participate in the lateral translocation of water and nutrients and can also contribute to the healing and regeneration of xylem tissue.

The movement of water through xylem — the TACT mechanism. The movement of water within plants, from roots to leaves, occurs primarily through the specialized vascular tissue known as xylem. The TACT (Transpiration, Adhesion, Cohesion, Tension) mechanism is a widely accepted model explaining how water moves against gravity through the xylem to reach all parts of the plant. This mechanism depends on both physical and chemical properties of water and the plant's interaction with its environment.

• Transpiration: the process by which water evaporates from the surface of plant leaves, specifically through stomata. As water vapour exits the leaf, a negative pressure is generated within the leaf tissue. This negative pressure creates a pulling force, drawing water upward from the roots through the stem and toward the leaves. Transpiration, therefore, acts as the primary driving force behind water transport in the xylem.
• Adhesion: the attraction between water molecules and the walls of the xylem vessels. Due to this attraction, water molecules stick to the walls of xylem vessels as they move upward. This property prevents any break in the water column within xylem, and plays a crucial role in maintaining the continuity of the water column, especially in tall plants where gravity exerts a significant downward force on the water column.
• Cohesion: the attractive force between water molecules themselves, caused by hydrogen bonding. Water molecules within the xylem stick together, forming an unbroken column from the roots to the leaves. This cohesive property ensures that the "pull" initiated by transpiration at the leaf level extends down through the entire water column.
• Tension: the negative pressure created by the pulling force of the transpiration at the leaf level. As water evaporates from the leaf surface, it creates a low-pressure area that extends through the xylem. This tension pulls the cohesive water column upwards, and is therefore vital for the continuous ascent of water within the xylem.

📌 Water potential gradient (root to leaf): root cells −0.2 MPa → stem −0.6 MPa → leaf air space −1.5 MPa → atmosphere −100 MPa. Water moves from higher (less negative) to lower (more negative) potential — that is why the pull works all the way up a tall tree.

Structure of phloem. Phloem is a vascular tissue in plants responsible for the transport of organic nutrients, particularly the products of photosynthesis, from the leaves to other parts of the plant where they are needed or stored. Phloem is generally found on the outer side of both primary and secondary vascular tissue in plants with secondary growth, and it constitutes the inner bark. Phloem comprises sieve elements, companion cells, phloem fibres and phloem parenchyma.

• Sieve tube elements (cells) are the cells of phloem that transport sugars and other organic material throughout the plant. They have "sieve areas", which are the portions of the cell wall where pores interconnect the sieve tube elements. Some of the sieve areas are generally formed in the end walls of sieve tube elements where the individual cells are joined together to form a longitudinal series called a sieve tube. Each sieve tube element is associated with one or more companion cells, and sieve tube elements and companion cells are in communication with each other by plasmodesmata. Companion cells supply ATP and proteins to sieve tube elements.
• Phloem parenchyma stores substances such as sugars, resins, latex and mucilage, which are important for plant defence and moisture retention.

Mechanism of translocation. The transport of sugars in plants takes place through phloem tissue. Passive theories of phloem transport include diffusion — but diffusion is far too slow to account for the velocities of sugar movement in phloem, which on the average is 1 meter per hour, while the rate of diffusion is 1 meter per eight years.

Pressure flow theory (mass-flow hypothesis) is the most widely accepted explanation for the transport of sugars in plants through the phloem. This process of translocation moves sugar from the source (where they are synthesized) to the sink (where they are consumed or stored). This theory was proposed by Ernst Münch in 1930 and relies on the principle of osmotic pressure differences between source and sink regions. The following steps explain the pressure-flow theory:

1. The glucose formed during photosynthesis in mesophyll cells is used in respiration. The excess glucose is converted into non-reducing sugar, i.e., sucrose.
2. Sucrose is actively transported from mesophyll cells to the companion cells of phloem. From here, sucrose diffuses to sieve tubes, through plasmodesmata. So, the concentration of sucrose in sieve tubes increases.
3. Due to higher sucrose (solute) concentration in sieve tubes, water moves into them by osmosis from the nearby xylem of leaf. It results in an increase in the water potential at the source end of sieve tubes.
4. At the sink end, sugar is actively unloaded from sieve tubes and water also follows by osmosis. The exit of water lowers the water potential at the sink end. So, there is a higher water potential at the source end while a lower water potential at the sink end.
5. The difference in water potential causes water to flow from source to sink. Since sucrose is dissolved in water, it is carried along from source to sink along with water.

Growth in plants refers to a permanent increase in size, which can occur in various dimensions such as height, width and mass. Throughout life, the plant adds organs such as branches, leaves and roots. Its organs increase in size from the tips but the rate of growth is not uniform throughout the body. In lower plants, the entire plant body is capable of growing, but in higher plants, growth is limited to certain regions known as growing points. These growing points consist of groups of cells, called meristems, that are capable of continuous cell division.

Types of meristems. There are three types of meristems in plants: apical meristems, intercalary meristems and lateral meristems.

• Apical meristems are found at the tips of roots and shoots. They are primarily responsible for the extension of the plant body, are perpetual growth zones, and are responsible for the increase in the number of cells at the tips of roots and stems. They play an important role in primary growth.
• Intercalary meristems are separated from the apex by permanent tissues. They are situated at the bases of internodes in many plants such as grasses and play an important role in the production of leaves and flowers. These are temporary.
• Lateral meristems are cylinders of dividing cells present along the peripheral regions. They are responsible for growth in thickness of stems and roots, are found in woody plants and are crucial for secondary growth. There are two main forms of lateral meristems: vascular cambium and cork cambium. Vascular cambium is located between the xylem and phloem and is responsible for the production of secondary xylem and secondary phloem. Cork cambium is formed in the outer layer of stems and roots; it produces cork cells which replace the epidermis and form the outer protective bark.

Types of growth. In plants, there are two types of growth: primary growth and secondary growth.

Primary growth is responsible for an increase in the length of the plants and is facilitated by the activity of apical meristems. Herbaceous plants generally display primary growth with little secondary growth as compared to woody plants. The process of primary growth in plants occurs in three phases:

1. Cell division: the cells of apical meristems undergo divisions and the number of cells is increased. It happens at the tips of apical meristems of root and shoot. The area of apical meristem where cell division occurs is called the zone of cell division. In this zone, cells are non-vacuolated and small, with spherical nuclei in the centre of cytoplasm.
2. Cell elongation: after the formation of new cells, their volume increases due to uptake of water. Plasticity of the cell wall increases and wall pressure is reduced. It happens at a little distance from the tips of apical meristems, in the zone of cell elongation. In this zone, cells are vacuolated and large, with nuclei in the peripheries of cytoplasm. During this phase, different cells elongate in different dimensions and the final size of cells is attained — for example, the cells which are determined to develop into pith and cortex do not elongate much length-wise, while the cells which are determined to develop into xylem tissue elongate more length-wise.
3. Cell differentiation: after the cells have got their final size and shape, elongation stops and cells are specialized to perform specific functions. Their cell walls become thicker and many new structural features develop. It happens in the area next to the zone of elongation, called the zone of differentiation, where cells are fully differentiated and each type of cell performs a specific function.

Secondary growth refers to the increase in thickness or girth of stems and roots. It is due to the activity of lateral meristems, specifically the vascular cambium and cork cambium. It is more prominent in woody perennial plants, while herbaceous plants show only primary growth. The cells of vascular cambium divide and produce new cells on both of its outer and inner margins: cells produced on the outer margins make secondary phloem, while cells produced on the inner margins make secondary xylem. Secondary tissues (particularly secondary xylem) cause an increase in the plant's thickness. Division in cork cambium produces cells on both outer and inner sides, which make new cork. The region of a mature stem outside the vascular cambium, which contains secondary phloem, cork cambium and cork, is collectively called bark.

Annual rings are formed due to the seasonal activity of the cambium layer in trees. This process is influenced by environmental factors and results in the production of two distinct types of wood each year: spring wood (early wood) and autumn wood (late wood). These rings provide valuable information about the age of the tree and the environmental conditions experienced during each growing season. The cambium is a meristematic tissue that generates new vascular tissues, i.e., xylem and phloem. In spring, when conditions are favourable, the cambium is highly active, producing a large volume of xylem with wider vessels, resulting in lighter-coloured spring wood. As the season changes to autumn, the cambium's activity decreases: it produces fewer xylem elements, which are narrower and denser, leading to darker autumn wood. The combination of spring wood and autumn wood forms a complete annual ring, and each year a new ring is added, allowing the determination of a tree's age through dendrochronology. The transition between these two types of wood is gradual from spring to autumn, but the shift from autumn back to spring in the following year is abrupt, marking a clear distinction between the growth periods. This data is valuable for studying long-term climate variability and changes.

📌 Note: Dendrochronology is the scientific method of dating and studying tree rings to analyse past climate conditions and events.

Plant growth regulators. Plants regulate the rates of growth and the rate of metabolism in their cells. Special chemical messengers, called plant growth regulators or plant hormones, regulate their growth. There are five major groups: auxins, cytokinins, gibberellins, abscisic acid and ethylene.

• Auxins (indole acetic acid, IAA, or its variants): in the stem they promote cell enlargement in the region behind the apex; promote cell division in cambium; in the root they promote growth at very low concentrations and inhibit growth at higher concentrations (e.g., geotropism); promote growth of roots from cuttings and calluses; promote bud initiation in shoots but are sometimes antagonistic to cytokinins and inhibitory; promote apical dominance and fruit growth; can sometimes induce parthenocarpy; cause delay of leaf senescence (aging) in a few species; inhibit abscission.
• Gibberellins (produced in the apical portions of roots and shoots, transported to other parts; contain gibberellic acid; more than 110 different gibberellins): promote cell enlargement in the presence of auxins; promote cell division in apical meristem and cambium; promote "bolting" of some rosette plants; promote bud initiation in shoots of chrysanthemum callus; promote leaf growth and fruit growth; may induce parthenocarpy; in apical dominance enhance the action of auxins; break bud and seed dormancy; sometimes may substitute for red light, therefore promoting flowering in long-day plants while inhibiting it in short-day plants; cause delay in leaf senescence in a few species.
• Cytokinins (usually produced in roots, young fruits and seeds): promote cytokinesis during cell division; increase the rate of DNA replication and the rate of RNA and protein synthesis; promote stem growth by cell division in apical meristem and cambium; inhibit primary root growth; promote lateral root growth; promote bud initiation and leaf growth; promote fruit growth but can rarely induce parthenocarpy; promote lateral bud growth and break bud dormancy; cause delay in leaf senescence; promote stomatal opening.
• Abscisic acid (ABA) (synthesized mainly in mature green leaves, fruits and root caps): inhibits stem and root growth notably during physiological stress, e.g., drought and waterlogging; promotes bud and seed dormancy; promotes flowering in short-day plants and inhibits it in long-day plants (antagonistic to gibberellins); sometimes promotes leaf senescence; promotes abscission; promotes closing of stomata under conditions of water stress (wilting).
• Ethylene (a natural product of the metabolism of plants): inhibits stem growth, notably during physiological stress; inhibits root growth; breaks dormancy of bud; promotes flowering in pineapple; promotes fruit ripening.

Osmoregulation refers to the process by which an organism maintains a stable internal equilibrium of water and dissolved substances, irrespective of the surrounding environmental conditions. Many marine organisms undergo osmosis without the requirement for regulatory systems since their cells have the same osmotic pressure as that of the sea. However, some organisms must actively acquire, retain, or eliminate water or salts in order to regulate their internal water and mineral balance.

Types of solutions.

• Hypotonic solution: a solution having reduced solute concentration relative to the intracellular environment of a cell. As a result, water enters the cell by osmosis, resulting in the swelling.
• Hypertonic solution: a solution having high solute concentration relative to the intracellular environment of a cell. As a result, water moves out of the cell, which causes the cell to shrink due to loss of water, a condition called plasmolysis.
• Isotonic solution: a solution whose solute concentration resembles the intracellular environment of the cell. Net movement of water between the cell and its environment is zero in this case.

Osmotic adjustments in plants. Plants are distributed in different habitats of aquatic, moderate, severely dry terrestrial and saline nature, thus termed as hydrophytes, mesophytes, xerophytes and halophytes, respectively.

• Hydrophytes are adapted to aquatic environments, including marine and freshwater ecosystems, through specialized osmotic adjustment mechanisms. Marine hydrophytes thrive in saline (hypertonic) conditions, where water tends to leave their cells. They excrete excess salts using specialized salt glands and synthesize organic solutes like proline, glycine betaine and sugars to retain water by increasing their internal osmotic potential. They have thick cuticles which further reduce water loss, and they exhibit halophytic traits to tolerate high salinity. Freshwater hydrophytes grow in hypotonic environments, where water continuously enters their cells. These plants expel excess water through structures like hydathodes or vacuoles to avoid overhydration. They actively absorb essential ions, such as potassium and calcium, to maintain osmotic balance and compensate for the dilute surroundings. With thin or absent cuticles, these plants facilitate water exchange and often have reduced root systems, relying on direct nutrient and water absorption from their environment. Examples of hydrophytes are water lilies, lotus, seaweeds and tape grass.
• Mesophytes live in moderate environments that are neither too dry nor too wet. They prefer soil that is not waterlogged and has a moderate salt content and humidity. Mesophytes have well-developed roots and shoots with a fully formed vascular system. They do not require any special adaptations to survive. Their leaves are flat, broad and green with stomata on the surface. Examples of mesophytes include rose, tomatoes and daisies.
• Xerophytes are plants that are adapted to survive in dry conditions. They have special adaptations to minimize water loss and store water. Plants that store water are known as succulents. They possess fleshy stems that can store water and be used when needed. Other adaptations in xerophytes include waxy coatings on leaves to reduce water loss, leaf dropping during dry periods, and leaf folding or repositioning to absorb sunlight efficiently. Examples of xerophytes include thorn trees, desert marigold and blue agave.
• Halophytes inhabit saline soil with high concentrations of salts like NaCl, MgCl₂, MgSO₄, or saline water. On such a substratum, only such plants can grow which can tolerate a relatively high concentration of these salts. These plants have succulent leaves and sometimes the stem is also succulent. In certain cases, leaves are modified into spines. Examples of halophytes are sea arrowgrass and sea lavender.

📌 Note: Halophytes growing in marshy places near the seashore form a special vegetation known as the mangrove or tidal woodland. These are also called helophilous halophytes.

Thermoregulation is a homeostasis in which organisms maintain their body temperature despite variations in environmental temperature. High temperature denatures the enzymes and damages the metabolism. Plants use evaporative cooling to cope with high temperature. Hot and dry weather, however, causes water deficiency, resulting in closing of stomata, thus plants suffer in such conditions. Most plants have adapted to survive in heat stress, as the plants of temperate regions face the stress of 40°C and higher temperature. The cells of these plants synthesize large quantities of special proteins called heat-shock proteins. These proteins embrace enzymes and other proteins, thus helping to prevent their denaturation.

Low temperature, on the other hand, alters the fluidity of the cell membrane, because lipids of the membrane become locked into crystalline structures, which affects the transport of the solutes. The structure of the membrane proteins is also affected. Plants respond to cold stress by increasing the proportion of unsaturated fatty acids, which help the membrane to maintain structure at low temperature by preventing crystal formation. This adaptation requires time. Because of this reason, rapid chilling of plants is more stressful than a gradual drop in air temperature. Freezing temperature causes ice crystal formation. The confinement of ice formation around the cell wall does not affect as badly and plants survive. However, the formation of ice crystals within protoplasm perforates membranes and organelles, hence killing the cells. The plants native to cold regions, such as oaks, maples, roses and other plants, have adapted to bring changes in the solutes composition of the cells, which causes the cytosol to super cool without ice formation, although ice crystals may form in the cell walls.

Organisms react to both external and internal stimuli. Animals may exhibit locomotion in reaction to stimuli, but the plants are fixed, hence they can only alter their growth pattern.

Tropic movements: the growth movements in plants that are triggered by a stimulus are collectively called tropic movements or tropisms. Such movements occur as a curvature of the whole organ towards or away from stimuli such as light, touch, chemical, water and gravity. The following are the major types of tropic movements in plants:

• 1. Phototropism: the movement of a part of a plant in response to the stimulus of light, caused by the differential growth of a part of a plant like stem or root. The tips of shoots usually show positive phototropism while roots show negative phototropism.
• 2. Geotropism: the movement of plant parts in response to gravity. Roots display positive geotropism and shoots negative geotropism.
• 3. Thigmotropism: the movement in response to the stimulus of touch, for example climbing vines. When they come in contact with some solid object, the growth on the opposite side of contact increases and the tendril coils around the support.
• 4. Chemotropism: the movement in response to some chemicals is called chemotropism. The hyphae of fungi are chemotropic.

8.11 — Photoperiodism. The response to changes in day length that enables plants to adapt to seasonal changes in their environment is termed as photoperiodism. Simply, it is the response of plants to the length of day and night.

The effect of photoperiodism was first studied in 1920 by Garner and Allard. They studied that the tobacco plant flowers only after exposure to a series of short days. The tobacco plant naturally flowers under the same conditions, in autumn, but flowering could be induced by artificially exposing it to short days. With further studies they were able to classify flowering plants into long-day plants, which require long days for flowering, short-day plants, which require short days for flowering, and day-neutral plants, which flower without being influenced by photoperiod. Later on, further studies indicated that it is really the length of the dark period which is critical. Thus, short-day plants are really long-night plants. If they are grown in short days, but the long night is interrupted by a short light period, flowering is prevented. Long-day plants will flower in short days if the long night period is interrupted.

Mechanism of flower induction — phytochrome. Phytochrome, a photoreceptor protein, exists in two forms: P₆₆₀ and P₇₃₀. P₆₆₀ is a quiescent form. It absorbs red light at a wavelength of 660 nm and is converted to active P₇₃₀, which absorbs far red light at 730 nm and is converted to P₆₆₀. In nature, the P₆₆₀ to P₇₃₀ conversion takes place in daylight and the P₇₃₀ to P₆₆₀ conversion occurs in the dark. The rate at which P₇₃₀ is converted to P₆₆₀ provides the plant with a "clock" for measuring the duration of darkness.

It has been found that red light inhibits flowering in short-day plants but promotes flowering in long-day plants, under conditions in which flowering normally takes place. This observation led to the hypothesis that the P₇₃₀-P₆₆₀ interconversion might be the plant time-regulator for flowering. According to this hypothesis, P₇₃₀, converted from P₆₆₀ by the absorption of red light, would inhibit flowering in short-day plants but promote flowering in long-day plants. Because P₇₃₀ accumulates in the day and diminishes at night, short-day plants could flower only if the night were long enough, during which a great amount of P₇₃₀ would not be completely inactivated, so that enough P₇₃₀ would remain at the end of night to promote flowering. But now it is generally agreed that the time-measuring phenomenon of flowering is not totally controlled by the interconversion of P₆₆₀ to P₇₃₀. Other factors, like the presence or absence of light and the length of dark or light period, also play an important role in flowering. The biological clock, once stimulated, causes the production of florigen hormone in leaves, which travels through phloem to the floral buds, initiating flowering.

8.12 — Vernalisation. Biennial and perennial plants are stimulated to flowering by exposure to low temperature. This is called vernalisation. The low temperature stimulus is received by the shoot apex of a mature stem or the embryo of the seed, but not by the leaves, as in photoperiodism. For some plants, vernalisation is an absolute requirement, while in some cases it simply assists in inducing flowering. The duration of low temperature (chilling) treatment required varies from four days to three months. Temperature around 4°C is found to be very effective in this regard. It stimulates the production of a hormone "vernalin" which induces vernalisation. Photoperiodism and vernalisation serve to synchronise the reproductive behaviour of plants with their environment, ensuring reproduction at favourable times of year. They also ensure that members of the same species flower at the same time, encouraging cross pollination for genetic variability.