species of Acacia). Or they may change the nature of their leaves. Some desert shrubs in Israel (e.g. Teucrium polium) bear finely divided, thin‐cuticled leaves during the season when soil water is freely available. These are then replaced by undivided, small, thick‐cuticled leaves in more drought‐prone seasons, which in turn fall and may leave only green spines or thorns (Orshan, 1963): a sequential polymorphism through the season, with each leaf morph being replaced in turn by a less photosynthetically active but more watertight structure.
Next, leaves may be produced that are long lived, transpire only slowly and tolerate a water deficit, but which are unable to photosynthesise rapidly even when water is abundant (e.g. evergreen desert shrubs). Structural features such as hairs, sunken stomata and the restriction of stomata to specialised areas on the lower surface of a leaf slow down water loss. But these same morphological features reduce the rate of entry of CO2. Waxy and hairy leaf surfaces may, however, reflect a greater proportion of radiation that is not in the PAR range and so keep the leaf temperature down and reduce water loss.
physiological strategies
Finally, some groups of plants have evolved particular physiologies: C4 and Crassulacean acid metabolism (CAM). We consider these in more detail in Sections 3.4.1–3.4.3. Here, we simply note that plants with ‘normal’ (i.e. C3) photosynthesis are wasteful of water compared with plants that possess the modified C4 and CAM physiologies. The water‐use efficiency of C4 plants (the amount of carbon fixed per unit of water transpired) may be double that of C3 plants.
tactical changes in stomatal conductance
The major tactical control of the rates of both photosynthesis and water loss is through changes in stomatal ‘conductance’. These may occur rapidly during the course of a day and allow a very rapid response to immediate water shortages, such that rhythms of stomatal opening may ensure that the above‐ground parts of the plant remain more or less watertight except during controlled periods of active photosynthesis. Stomatal movement may even be triggered directly by conditions at the leaf surface itself – the plant then responds to desiccating conditions at the very site, and at the same time, as the conditions are first sensed.
coexisting alternative strategies in Australian savannas
The viability of alternative strategies to solve a common problem is nicely illustrated by the trees of seasonally dry tropical forests and woodlands (Eamus, 1999). Communities of this type are found naturally in Africa, the Americas, Australia and India, and as a result of human interference elsewhere in Asia. But whereas, for example, the savannas of Africa and India are dominated by deciduous species, and the Llanos of South America are dominated by evergreens, the savannas of Australia are occupied by roughly equal numbers of species from four groups (Figure 3.11a): evergreens (a full canopy all year), deciduous species (losing all leaves for at least one and usually two to four months each year), semideciduous species (losing around 50% or more of their leaves each year) and brevideciduous species (losing only about 20% of their leaves). At the ends of this continuum, the deciduous species avoid drought in the dry season (April–November in Australia) as a result of their vastly reduced rates of transpiration (Figure 3.11b), but make no net photosynthate at all for around three months, whereas the evergreens maintain a positive carbon balance throughout the year (Figure 3.11c). The alternative, contrasting strategies are clearly sufficiently viable for them to coexist in Australia. Why this is not equally true elsewhere is not known.
Figure 3.11 Alternative strategies for combining photosynthesis and water conservation among trees in Australian savannas. (a) Percentage canopy fullness for deciduous (red), semideciduous (yellow), brevideciduous (purple) and evergreen (blue) trees in Australian savannas throughout the year. (Note that the southern hemisphere dry season runs from around April to November.) (b) Susceptibility to drought as measured by increasingly negative values of ‘predawn water potential’ for deciduous and evergreen trees. (c) Net photosynthesis as measured by the carbon assimilation rate for deciduous and evergreen trees.
Source: After Eamus (1999).
3.3.2 Roots as water foragers
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.
