Plant–Water Relations

Abstract

Abstract Plant–water relations concern how plants control the hydration of their cells, including the collection of water from the soil, its transport within the plant and its loss by evaporation from the leaves. The water status of plants is usually expressed as ‘water potential’, which has units of pressure, is always negative, and in simple form is the algebraic sum of the hydrostatic pressure and the osmotic pressure of water. Flow of water through plant and soil over macroscopic distances is driven by gradients in hydrostatic pressure. Over microscopic distances (e.g. across semipermeable membranes) it is driven by gradients in water potential. Evaporation of water from leaves is primarily controlled by stomata, and if not made good by the flow of water from soil through the plant to the leaves, results in the plants wilting. Resistances to this flow are still not well understood. Key concepts: Plants perform best when they are turgid, that is when the water within their cells has a positive hydrostatic pressure. Leaves often transpire several times their own volume of water each day, but the net loss is usually small owing to the inflow of water drawn up the plant from the soil, this flow being known as the ‘transpiration stream’. The water status of a plant is expressed as ‘water potential’, the chemical potential of water divided by the volume of 1 mole of water to give units of pressure. Water potential (ψ) comprises two main components, hydrostatic pressure ( P ) and osmotic pressure (π), such that ψ= P −π. The flows of water through plant and soil are driven by gradients in hydrostatic pressure over macroscopic distances, by differences in water potential across semipermeable membranes or by diffusion as water vapour from the leaves to the atmosphere. Resistance to these flows, and the factors influencing them, vary markedly as the transpiration stream moves from soil, across the roots, longitudinally in the xylem and eventually through the tissue of the leaves to the evaporating surfaces within the leaf.