Summer Drought and Water Balance of Plants Growing in the Near East

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Abstract
The author surveys 20 yrs. of research on the water ecology of plants, done mainly in Palestine, distinguishing water spendthrifts and savers, according to their avg. transpiration intensity. Deciduous woody plants belong mainly to the former, evergreens to the latter group. During the rainless summer, plants use either ground water or rain water stored in soils and even in porous rocks, like soft limestones. Death by desiccation is avoided by: preponderance of deeply penetrating or far-extending roots over shoots; relative high osmotic values increasing absorption; successive reduction of leaf size and number; xeromorphic structure; leaf cutinization and highly effective stomata, which reduce water losses to very low levels; the capacity to endure large water deficits for a long time. In deciduous trees and certain desert plants, transpiration reaches very high levels, if soil moisture is ample, e.g. in the almond 3446 mg./g. fresh weight/hr. (Poljakoff, Jerusalem). Xeromorphic types transpire relatively little per yr. in spite of high losses under favorable conditions. Their turn-over should be calculated per unit of soil surface. Saturation deficits (% of max. storage capacity) may reach approx. 70% in Rosmarinus. citrus stocks, desert plants and osmotic values 50-60 atm. in Phillyrea media, with-out disastrous consequences for the affected leaves, while other spp. (carob, Retama Roetam) have a uniform water balance even after prolonged drought, as a consequence of earlier and more effective reduction of water losses.

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