Introduction

One of the most interesting aspects of desert-plant ecophysiology is the complex nature of adaptations that allow them not only to survive but also to reproduce and maintain their populations under the extreme conditions of environmental stress present in these ecosystems. Of course, deserts are defined by having limited rainfall (typically <250 mm annually) and thus produce significant climatic drought stress, but the seasonality of rainfall patterns and thus drought varies across desert regions, as do the plant growth forms that dominate the landscape (Fig. 28.1).

Terrestrial vascular plants, in any environment where water is limiting for growth, face a dilemma. Their uptake of CO2 from the atmosphere can only occur by opening stomata on their exposed organs regulated by guard cells. However, at the same time that stomata are open and CO2 diffuses into green tissues for photosynthesis, water vapour diffuses out of the plant into the surrounding, drier air. A plant’s strategy can be to reduce transpirational loss by keeping its stomata closed and thereby lose no water vapour, but during that time it also fixes no atmospheric CO2 and thus cannot produce new sugars necessary for growth and respiration. Obviously then, stomata must be opened for photosynthesis to occur but at a significant cost to water levels. The hotter and drier the outside air, the more rapid will be the rate of water-vapour loss from plant tissues.