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Counteracting loss is, of course, only one side of the balance sheet. For most terrestrial plants, the main source of water is the soil and they gain access to it through a root system. We proceed here (and in Section 3.5, on plant nutrient resources) on the basis of plants simply having ‘roots’. In fact, most plants do not have simple, plant‐only roots – they have mycorrhizae: associations of fungal and root tissue in which both partners are crucial to the resource‐gathering properties of the whole. Mycorrhizae, and the respective roles of the plants and the fungi, are discussed in Chapter 13.

field capacity and the permanent wilting point

Water enters the soil as rain or melting snow and forms a reservoir in the pores between soil particles. What happens to it then depends on the size of the pores, which may hold it by capillary forces against gravity (Figure 3.12). If the pores are wide, as in a sandy soil, much of the water will drain away until it reaches some impediment and accumulates as a rising water table or finds its way into streams or rivers. The water held by soil pores against the force of gravity is called the field capacity of the soil. This is the upper limit of the water that a freely drained soil will retain. However, not all the water retained by soil is available to plants, since they must extract it from those soil pores against the surface tension holding it there, and their ability to do so depends on the structure of their root systems. Hence, there is also a lower limit to the water that can be used in plant growth, determined by the particular plant species present, known as the permanent wilting point – the soil water content at which plants wilt and are unable to recover. The permanent wilting point does not differ much between the plant species of mesic environments (those with a moderate amount of water) or between species of crop plants, but many species native to arid regions have very low permanent wilting points as a result of root systems that allow them to extract significantly more water from the soil, and of leaf morphological adaptations, discussed previously, that give them better water‐holding capacities.

Figure 3.12 Field capacity and the permanent wilting point in soil in relation to pore size and pressure. The status of water in the soil, showing the relationship between the diameter of soil pores that remain water‐filled and the pressure created by the capillary action of those pores that opposes the tendency of water to drain away under the force of gravity. Pressure values are negative because they describe the process of suction. The size of water‐filled pores may be compared in the figure with the sizes of rootlets, root hairs and bacterial cells. Note that for most species of crop plant the permanent wilting point is at approximately −15 bars, but in many other species it reaches −80 bars, depending on their ability to extract water from the narrowest pores.

roots and the dynamics of water depletion zones

As a root withdraws water from the soil pores at the root’s surface, it creates water‐depletion zones around it – another example of the RDZs described in Section 3.2.1. These determine gradients of water potential between the interconnected soil pores. Water flows along the gradient into the depleted zones, supplying further water to the root, but this simple process is made much more complex because the more the soil around the roots is depleted of water, the more resistance there is to water flow. Thus, as the root starts to withdraw water from the soil, the first water that it obtains is from the wider pores because they hold the water with weaker capillary forces. This leaves only the narrower, more tortuous pathways, and so the resistance to water flow increases. Thus, when the root draws water from the soil very rapidly, the RDZ may become very sharply defined, because water can move across its boundary only slowly. For this reason, rapidly transpiring plants may wilt in a soil that contains abundant water.

roots as foragers

Water that arrives on a soil surface does not distribute itself evenly down the soil profile. Instead, it tends to bring the surface layer to field capacity, with further rain extending this layer deeper and deeper. This means that different parts of the same plant root system may encounter water held with quite different forces. Similar variations can occur as a result of heterogeneities in soil type – clay soils with small pores can hold far more water than sandy soils with large pores. As a root passes through such heterogeneous soil (and all soils are heterogeneous seen from a ‘root’s‐eye view’), it typically responds by branching freely in zones that supply resources, and scarcely branching at all in less rewarding patches (Figure 3.13a). That it can do so depends on the individual rootlet’s ability to react on an extremely local scale to the conditions that it meets. Strategic differences in developmental programmes can be recognised between the roots of different species (Figure 3.13b), but it is the ability of root systems to override strict programmes and be opportunistic, depending both on local conditions and their overall level of resource availability, that makes them effective exploiters of the soil (de Kroon et al., 2009).

Figure 3.13 Roots as foragers. (a) The root system developed by a plant of wheat grown through a sandy soil containing a layer of clay. Note the responsiveness of root development to the localised environment that it encounters. (b–j) Profiles of root systems of plants from contrasting environments. (b–e) Northern temperate species of open ground: (b) Lolium multiflorum, an annual grass; (c) Mercurialis annua, an annual weed; and (d) Aphanes arvensis and (e) Sagina procumbens, both ephemeral weeds. (f–j) Desert shrub and semishrub species, Mid Hills, eastern Mojave Desert, California.

Source: (a) Courtesy of J.V. Lake. (b–e) From Fitter (1991). (f–j) Redrawn from a variety of sources.

The root system that a plant establishes early in its life can determine its responsiveness to future events. Where most water is received as occasional showers on a dry substrate, a seedling that puts its early energy into a deep taproot will gain little from subsequent showers, but in an environment in which heavy rains fill a soil reservoir to depth in the spring, followed by a long period of drought, that taproot may guarantee continual access to water. Indeed, it seems that the placement of roots with respect to water and especially nutrient availability is most important in the earlier stages of a plant’s life. Later there is much greater reliance on stored resources in overcoming local or temporary shortages (de Kroon et al., 2009).