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stomatal opening

For plants, in terrestrial habitats especially, it is not sensible to consider radiation as a resource independently of water. Intercepted radiation does not result in photosynthesis unless there is CO₂ available, and the prime route of entry of CO₂ is through open stomata (see Section 3.4). But if the stomata are open to the air, water will evaporate through them. Indeed, the volume of water that becomes incorporated in higher plants during growth is infinitesimal in comparison to the volume that flows through the plant in the transpiration stream (in through the roots, out through the stomata). If water is lost faster than it can be gained, the leaf (and the plant) will sooner or later wilt and eventually die. In most terrestrial communities, water is, at least sometimes, in short supply. The question therefore arises: should a plant conserve water at the expense of present photosynthesis, or maximise photosynthesis at the risk of running out of water? Once again, we meet the problem of whether the optimal solution involves a strict strategy or the ability to make tactical responses. There are good examples of both solutions and also compromises.

short active interludes in a dormant life

Perhaps the most obvious strategy that plants may adopt is to have a short life and high photosynthetic activity during periods when water is abundant, but remain dormant as seeds during the rest of the year, neither photosynthesising nor transpiring. Many desert annuals do this, as do annual weeds and most annual crop plants.

leaf appearance and structure

Second, plants with long lives may produce leaves during periods when water is abundant and shed them during droughts (e.g. many 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).