Soil drought increases leaf and whole-plant water use of Prunus dulcis grown in the Negev Desert

Water use, both at the level of a single leaf and the whole plant, was studied for 1- to 4-year-old almond trees (Prunus dulcis) under arid conditions in the Negev Desert (Israel). By planting the trees into lysimeters of different volumes (7, 14 and 21 m3), the amount of water available to the plants was experimentally controlled. Each year, at the beginning of the growing season, the lysimeters, which had been filled with local homogenized loess, were watered to field capacity. The trees received different relative amounts of water in relation to their leaf area on the one hand and lysimeter volume on the other, which caused different rates of soil drying throughout the season. The following hypotheses were tested. (1) The amount of CO2 assimilated per transpiration and (2) biomass production per unit of water used increases with (a) decreasing amount of soil water applied and (b) increasing leaf area, which should enhance growth in spring during periods of low evaporative demand. At the leaf level, the ratio of daily CO2 assimilation (A) to daily transpiration (E) was independent of lysimeter size, leaf area and pre-dawn water potential, but decreased with increasing leaf-to-air vapour pressure difference (Dl). Consequently, during the course of the season, A/E decreased from spring to summer in accordance with rising Dl. However, when measured at a constant Dl, the seasonal course in A/E disappeared. At the whole plant level, the ratio of total lifetime biomass production (B) to the amount of water transpired (W) increased with leaf area (i.e. demand for water), the increase being stronger with increasing water supply. We conclude that almond trees did not adapt physiologically to a limited water supply, but maximized their carbon gain for a given amount of water available by phenological processes such as high growth rate during periods of low evaporative demand of the atmosphere.