Introduction
Most of the water taken up by fruit trees is in response to evaporative loss (transpiration) through the leaf pores or stomata. This is a consequence of the need for stomata to be open to admit CO2 entry for photosynthesis.
Over any given period the fruit tree usually takes up more than 100 times as much water as it produces dry matter (Lenz, 1986). A cropping orchard in Washington State, USA requires up to 1 m of irrigation per year (Evans, 1982). This is 10 000 t ha−1 as compared with about 42.5 t of water in the fruits of a 50 t ha−1 crop. The close correlation between water use and crop yield that is generally observed is not a result of water use in the actual production of the crop. It is primarily a consequence of the close correlation between CO2 assimilation and water loss, as a result of the dependence of both of these on leaf area and stomatal behaviour.
However, the ability to keep stomata open for CO2 assimilation and to avoid consequent desiccation depends on the adequacy of the supply of water as well as mechanisms for controlling water stress. To this extent water supply is a truly limiting factor to crop production.
The water flux through the soil–tree–air system is under tension, i.e. negative pressure. Tree tissues and cells equilibrate with this tension and different aspects of growth, development and function respond to this in different ways.
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