Plant–Water Relations


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.

Keywords: plant; water potential; osmotic pressure; hydrostatic pressure; capillarity; transpiration

Figure 1.

Capillary action. Surface tension generates an upward pull on the water in the capillary tube. The water rises to a height of 1 m in a tube of radius 15 μm, or more generally to a height of 15×10−6/a m (eqns and ) for a tube of radius a (m).

Figure 2.

Instruments used to measure hydrostatic pressure of soil water. (a) Tensiometer. The porous ceramic cup allows water to move between the soil and the inside of the instrument. Eventually the pressures equalize and the pressure gauge of the instrument then gives the pressure in the soil water. (b) Pressure plate. Enough gas pressure is applied to the chamber to bring the water in the soil to atmospheric pressure, when it is on the verge of exuding from the outlet. The gas pressure is then equal and opposite to the original pressure in the soil water.

Figure 3.

Pressure chamber for measuring the water potential of a leaf. The leaf is cut from a plant and quickly placed in the chamber, with a small piece of it protruding through the pressure seal. As with the pressure plate (Figure ), enough gas pressure is applied to the chamber to bring the xylem sap to the point of bleeding from the cut surface. Because transpiration has stopped, the xylem sap is close to equilibrium with the cells of the leaf and the applied balancing pressure is equal and opposite to that of the original pressure in the equilibrated xylem sap.


Further Reading

Boyer JS (1995) Measuring the Water Status of Plants and Soils. San Diego, CA: Academic Press.

Dardanelli JL, Ritchie JT, Calmon M, Andriani JM and Collino DJ (2004) An empirical model for root water uptake. Field Crops Research 87: 59–71.

Kaldenhoff R, Ribas‐Carbo M, Flexas J et al. (2008) Aquaporins and plant water balance. Plant Cell and Environment 31: 658–666.

Kramer PJ and Boyer JS (1995) Water Relations of Plants and Soils. San Diego, CA: Academic Press.

Nobel PS (2009) Physicochemical and Environmental Plant Physiology. San Diego, CA: Academic Press.

Sack L and Holbrook NM (2006) Leaf hydraulics. Annual Review of Plant Biology 57: 361–381.

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How to Cite close
Passioura, John B(Feb 2010) Plant–Water Relations. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001288.pub2]