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Figure 3.1 Surface view of cleared whole mount of a wheat leaf showing large and small parallel veins (mauve) with transverse veins connecting them. Lines of stomates (shown by the orange colour of the guard cells) lie along the flanks of these veins. Water evaporates from the wet walls of mesophyll cells below the stomates, drawing water from the veins. Distance between veins is 0.15 mm; scale bar is 100 µm. (Photograph courtesy M. McCully)

Water is often the most limiting resource determining the growth and survival of plants. This can be seen in both the yield of crop species and the productivity of natural ecosystems with respect to water availability.

The natural distribution of plants over the earth’s land surface is determined chiefly by water: by rainfall (\( R \)) and by evaporative demand (potential evapotranspiration, \( PE \)) which depends on temperature and humidity. This leads to such diverse vegetation groups as the lush vegetation of tropical rainforests, the shrubby vegetation of Mediterranean climates, or stands of tall trees in temperate forests. Climates can be classified according to the Thornthwaite Index: \( (R-PE)/PE \).

Agriculture also depends on rainfall. Crop yield is water-limited in most regions in the world, and agriculture must be supplemented with irrigation if the rainfall is too low. Horticultural crops are usually irrigated.

Plants require large amounts of water just to satisfy the requirements of transpiration: a large tree may transpire hundreds of litres of water in a day. Water evaporates from leaves through stomates, which are pores whose aperture is controlled by two guard cells. Plants must keep their stomates open in order to take up CO₂ as the substrate for photosynthesis (Chapter 2). In the process, water is lost from the moist internal surfaces of the leaf through the stomatal pores (Figure 3.1). Water loss also has a benefit in maintaining the leaf temperature through evaporative cooling.

The ratio of water lost to CO₂ taken up is around 300:1 in most land plants, meaning that plants must transpire large quantities of water on a daily basis in order to take up sufficient CO₂ for normal development.

In this section we will examine plant water relations and the variables that plant physiologists use to describe the status and movement of water in plants, soil and the atmosphere.

One of the challenging aspects of understanding plant water relations is the range of pressures from positive to negative that occur within different tissues and cells. Positive pressures (turgor) occur in all living cells and depend on the semipermeable nature of the plasma membrane and the elastic nature of the cell walls. Negative pressures (tensions) occur in dead cells and depend on the cohesive strength of water coupled with the strength of heavily lignified cell walls to resist deformation. These play an important role in water transport through the xylem.