Introduction: The Plant Body Plan

Plants are multicellular, photoautotrophic eukaryotic organisms encompassing algae, bryophyta, pteridophyta, gymnosperms, and angiosperms; bryophyta, pteridophyta, gymnosperms, and angiosperms are collectively referred to as land plants. Angiosperms (flowering plants) are the most numerous, diverse, and successful terrestrial plant group, representing over 90% of living land-plant species, ranging from microscopic Wolfia to towering Eucalyptus (over 100 metres).

Angiosperms are vascular plants containing two conducting tissues: xylem (conducts water and dissolved minerals upward from the roots) and phloem (conducts food throughout the plant). Angiosperms are also classified as spermatophytes (phanerogams) because they produce seeds — an embryo packaged with a nutrient supply inside a protective coat, a crucial adaptation protecting the embryo from desiccation on land.

A flowering plant body divides into the root system (underground) and the shoot system (stems, leaves, flowers, and fruits above ground).

• Root: typically non-green. The primary root develops from direct elongation of the embryonic radicle; any root arising from other plant organs is an adventitious root. The primary root's branches (lateral/secondary/tertiary roots) form the tap root system; stem-borne adventitious roots and their laterals form the fibrous root system (typical of monocots such as wheat). Roots absorb water and minerals, anchor the plant, store reserve food, and synthesise growth regulators.
• Stem: supports leaves and reproductive organs and conducts water, minerals, and photoassimilates. Characterised by nodes (leaf attachment points) and internodes (regions between nodes). Stems bear buds (embryonic shoots), classified by position (apical/terminal, axillary/lateral, adventitious) and function (vegetative, floral, or mixed).
• Leaf: the principal photosynthetic organ and major site of transpiration, derived from leaf primordia, consisting of leaf base, petiole, and lamina (blade). A simple leaf bears a single continuous blade; a compound leaf divides into discrete leaflets. Veins (vascular bundles) conduct water, minerals, and sugars between leaf and stem; their arrangement is the venation pattern.

3.1 Plant–Water Relationship

Water is essential for life and is the most abundant substance in the living cell, accounting for ~70% of cell weight. It is essential for all physiological activities and provides the medium in which most substances remain dissolved. Water is an electrically neutral polar molecule and an excellent solvent, with a high dielectric constant, high specific heat, and high heat of vaporisation. Extensive hydrogen bonding among water molecules gives rise to cohesion — high tensile strength, the ability to resist stretching (tension) without breaking — and also accounts for high surface tension.

3.1.1 Diffusion and Osmosis

Diffusion is the random movement of molecules along a concentration gradient (high to low concentration) driven by their own kinetic energy — a spontaneous, passive process. The rate of diffusion depends on concentration difference, molecular size, and temperature, and is described by Fick's Law:

where J is the flux per unit area, D is the diffusion coefficient (cm²/sec), and ΔC is the concentration difference between two regions separated by distance Δx. The negative sign reflects net movement toward the lower concentration.

Osmosis is a specialised case of diffusion involving the passive transport of solvent (water) only, through a semipermeable membrane, from a region of higher water concentration to a region of lower water concentration. Plant plasma membranes are selectively permeable, not strictly semipermeable, because they allow movement of both solvent and specific solutes via the lipid bilayer and specific transport proteins.

In osmosis, selective diffusion of solvent is driven by the internal energy of the solvent molecules; the available energy per unit volume is conveniently expressed as osmotic pressure — the minimum pressure required to completely stop the entry of water into an osmotically active solution across a semipermeable membrane (measured in atmospheres or bars; 1 bar = 100 kPa). Osmotic pressure is directly proportional to the difference in total solute concentration on each side of the membrane.

Tonicity, Turgor, and Plasmolysis

Tonicity is the measure of the osmotic pressure gradient of two solutions separated by a semipermeable membrane. A solution with lower solute content than another is hypotonic; one with higher solute content is hypertonic; equal solute content is isotonic.

When a plant cell is placed in a hypotonic solution, water enters the protoplast and the cell becomes turgid. In a hypertonic solution, water exits, the protoplast shrinks, and the cell becomes flaccid. In an isotonic solution, there is no net water flow.

3.1.2 Chemical Potential of Water and Water Potential

The chemical potential of water is the free energy associated with one mole of water. In plant physiology we use the related parameter water potential (Ψ), introduced by Slatyer and Taylor, defined as the chemical potential of water divided by its partial molal volume, expressed in pressure units (Pascals). By convention, the water potential of pure water at standard temperature and pressure is 0 MPa; dissolving solute reduces free water and lowers water potential, so an aqueous solution at atmospheric pressure has Ψ < 0. Water moves from higher to lower water potential.

3.1.3 Mass Flow

Diffusion is a slow, short-distance transport process; long-distance transport within a plant cannot occur by diffusion alone. Mass flow (bulk flow) — the concerted, en masse movement of groups of molecules from one point to another driven by pressure differences — enables long-distance translocation at a much faster rate. Pressure-driven bulk flow is the predominant mechanism in the xylem and phloem, and can be driven by a positive or a negative hydrostatic pressure gradient. Unlike diffusion, in bulk flow all dissolved substances move together regardless of individual concentration gradients.

3.2 Journey of Water in the Plant

Essentially all water used by land plants is absorbed from the soil by roots. Water crosses the epidermis, cortex, and endodermis to reach the xylem, where it moves upward (ascent of sap); it finally exits aerial parts (mainly leaves) as water vapour (transpiration). Transpiration generates the tensions that pull water through the xylem — transport from soil to atmosphere occurs purely in response to physical forces, with no direct expenditure of energy by the plant.

3.2.1 Absorption of Water

The root is the primary site of water absorption. On germination, the embryonic root (radicle) develops into the first root, with three developmental zones: meristematic zone, elongation zone, and maturation zone (also called the zone of differentiation or root-hair zone). Root hairs, epidermal extensions that increase surface area for absorption, should not be confused with lateral roots, which are multicellular, originate in the pericycle, and are endogenous. At the very tip of most roots is the root cap, protecting the underlying meristem as the root advances through soil.

Water absorption by roots is passive. On contact with the root surface, water enters root cells and extracellular spaces mainly by two processes: diffusion (water diffuses along the gradient directly through the lipid bilayer — simple diffusion, requiring no membrane proteins) and facilitated diffusion (mediated by transport channel proteins called aquaporins, AQPs). AQPs assemble as homotetramers; each monomer has six transmembrane helices with N and C termini facing the cytosol, and five loops (A–E), of which loops B and E are hydrophobic and contain a conserved NPA motif (Asparagine-Proline-Alanine) extending into the pore from each side of the membrane. Each monomer functions as an independent pore. Besides water, AQPs can also transport glycerol, urea, ammonia, and hydrogen peroxide. AQP-mediated flow is regulated by gating (opening/closing of the channel pore), controlled by phosphorylation/dephosphorylation, pH, and divalent cations.

As plants absorb water, they deplete soil water near the root surface, establishing a pressure gradient with neighbouring soil regions; because soil water-filled spaces are interconnected, water moves to the root surface predominantly by bulk flow driven by this pressure gradient, at a rate depending on the pressure gradient and the soil's hydraulic conductivity.

3.2.2 Radial Movement of Water: Root Surface to Tracheary Element

Once absorbed into root hairs or epidermal cells, water must traverse the cortex to reach the xylem, crossing the epidermis, cortex, endodermis, pericycle, xylem, and phloem in a root cross-section. Radial movement occurs via both positive and negative hydrostatic pressure. Roots generate positive hydrostatic pressure — root pressure — by absorbing ions from the dilute soil solution and transporting them into the xylem; the resulting buildup of xylem solutes lowers xylem water potential, driving radial water movement and generating positive xylem pressure. Root pressure occurs mainly when transpiration is low; when transpiration is high, water is drawn into the leaves and lost so rapidly that a positive pressure never develops, and transpiration instead generates negative pressure that draws water from the soil through the cortex.

Water moves radially through both the apoplast (the continuous system of cell walls and intercellular spaces, involving diffusion and bulk flow without crossing plasma membranes) and the symplast (the continuous system of interconnected protoplasts, connected via plasmodesmata). The apoplastic pathway extends from the root hairs to the endodermis, where the waterproof, suberin-impregnated Casparian strip on the radial walls forces water to cross the endodermal plasma membrane and continue through the symplast — making the apoplastic pathway the major route for water transport from epidermis to tracheary element, with the endodermis being the only route by which water can enter the vascular cylinder (necessarily via the symplast at that point). Water can also move via the transmembrane pathway, entering one side of a cell and exiting the other, repeatedly, across the tissue.

3.2.3 Ascent of Sap

Water and minerals from the soil enter through the root epidermis, cross the root cortex radially, and pass into the xylem, where the xylem sap moves upward — the ascent of sap, essentially unidirectional from roots to stems. Long-distance transport through the vascular system (xylem and phloem) is called translocation.

Tracheids are elongated, spindle-shaped, non-living cells with heavily lignified walls containing openings called pits (cavities in the secondary wall). A pit in one cell wall usually faces a pit in the adjoining cell; the middle lamella and two primary walls between the two pits form the pit membrane; the two opposite pits plus the membrane form a pit-pair. Water flows between tracheids through numerous pits in their lateral walls.

Vessel elements are short, cylindrical, non-living cells with perforated end walls; the perforated region is the perforation plate. Pits lack secondary walls only, whereas perforations lack both primary and secondary walls. A plate may have one large opening (simple perforation plate) or several small ones (compound perforation plate — arranged as scalariform, reticulate, or foraminate patterns). Perforated end walls allow vessel elements to join into long, interconnected tubes (vessels), providing a highly efficient, low-resistance conduit. Tracheids, lacking perforations, are less specialised; water passing tracheid-to-tracheid must cross pit membranes. Angiosperm xylem contains both tracheids and vessel elements; gymnosperms lack vessels.

Xylem parenchyma is living and thin-walled, storing food (starch or fat) and other substances such as tannins. Xylem fibres have highly thickened walls and provide mechanical support, occurring as septate or aseptate fibre-tracheids or libriform fibres (the latter typically longer with thicker walls). Primary xylem differentiates during primary growth from the meristematic procambium; secondary xylem develops during secondary growth from the vascular cambium.

Cohesion–Tension Theory

Plants lack a heart or circulatory system, yet water moves upward through the xylem at fairly high rates. Root pressure (typically < 0.1 MPa, and absent under high transpiration) is inadequate to move water up a tall tree. Instead, the negative hydrostatic pressure (tension) generated by transpiration from the leaves pulls water through the xylem — the cohesion–tension theory (also called transpiration pull), proposed by Dixon and Jolly.

• Cohesive force (attraction between molecules of the same type): hydrogen bonding among water molecules produces strong cohesive forces, allowing xylem water columns to sustain tension up to 30 MPa.
• Adhesive force (attraction between molecules of different types): water molecules interact attractively with the hydrophilic walls of tracheary elements.
• Surface tension: arises from cohesive interactions between liquid molecules; stronger cohesion produces stronger surface tension.

These properties give water high tensile strength (ability to resist a pulling force without breaking) and, aided by the small diameter of tracheary elements, high capillarity (ability to rise in a thin tube). Because transpiration keeps xylem water under tension, negative pressures may cause cavitation (embolism) — the appearance of a gas bubble within the liquid, more common in wide vessels than tracheids, and more likely during drought stress or when xylem sap freezes. Cavitation can block water transport and cause severe leaf water deficits, but its impact is limited structurally: the end walls of vessels and their pores prevent a bubble from spreading from tube to tube.

3.2.4 Transpiration

Transpiration is the loss of water from aerial plant parts (mainly leaves) as water vapour, driven by the difference in water vapour pressure between leaf air spaces and the external air (a greater difference increases transpiration). Transpiration is classified by route:

• Cuticular transpiration: through the waxy cuticle (mainly wax and cutin, a polymer of long-chain hydroxy/epoxy fatty acids linked by ester bonds); the cuticle is meant to check water loss but 5–10% of total transpiration still occurs this way.
• Lenticular transpiration: through lenticels (bark/root openings for gas exchange); accounts for only 1–5% of total water loss.
• Stomatal transpiration: through stomata, specialised epidermal structures that act as turgor-operated valves for gas exchange; accounts for ~90% of total transpiration.

In daylight, stomata open due to influx of water into guard cells, driven by decreased water potential resulting from increased osmotically active solute concentration. The uptake of K⁺ and Cl⁻ from the apoplast, coupled to malate biosynthesis within guard cells, is modulated by blue light: blue wavelengths activate the plasma-membrane H⁺-ATPase (a P-type transport ATPase), generating an electrochemical proton gradient (blocked by inhibitors such as orthovanadate). The carotenoid zeaxanthin acts as the blue-light photoreceptor in guard cells; its excitation starts a signal transduction cascade activating a serine/threonine kinase that phosphorylates and activates H⁺-ATPase.

The proton gradient's electrical component drives passive K⁺ uptake via voltage-gated K⁺ channels (H⁺ pumping hyperpolarises the membrane and acidifies the apoplast, opening these channels). The large positive charge from K⁺ influx is electrically balanced by chloride ions and the organic anion malate (taken up via a proton–chloride symporter, or, in Allium, balanced solely by chloride). Blue light also stimulates malate biosynthesis from starch-hydrolysis precursors, and stimulates sucrose production both from starch hydrolysis and from the photosynthetic carbon-fixation pathway in guard cell chloroplasts — unlike mesophyll chloroplasts, guard cell starch content decreases during opening and increases during closing.

Studies in broad bean indicate that stomatal opening is associated primarily with K⁺ uptake, and closing with a decrease in sucrose content.

Factors Influencing Transpiration Rate

Although stomatal opening/closing is the main determinant, other factors affect the rate of transpiration, categorised as external or internal/structural. All ultimately act by influencing the vapour pressure difference between the intercellular spaces and the leaf surface.

3.2.5 Guttation

Guttation is the exudation of xylem sap as liquid droplets from the tips or edges of leaves, driven by positive hydrostatic root pressure. It often occurs during damp, humid nights when water absorption is high but transpiration is minimal, through specialised pores called hydathodes, located at the apices of veins at leaf tips or margins. Hydathodes comprise a group of parenchyma cells (epithem cells) with numerous water-filled intercellular spaces and few or no chloroplasts. Guttation regulates the water content and turgidity of the leaves and the plant as a whole.