CONSIDERATIONS OF DROUGHT TOLERANCE IN IRRIGATION MANAGEMENT OF VEGETABLE CROPS

Therefore, growth chamber studies were conducted to investigate the ability of the leaves and roots of a range of vegetable species and cultivars to osmotically adjust in response to water deficit. A technique was developed to measure osmotic adjustment whereby pot-grown plants were water stressed until stomatal closure and, upon relief of the stress and complete rehydration, the subsequent adjustment in solute potential monitored with thermocouple psychrometers. The ability of these crops to osmotically adjust was species dependent and also varied for cultivars within a species. Leaves generally exhibited a larger absolute osmotic adjustment than roots, but the roots exhibited a greater percentage adjustment. Lycopersicon esculentum Mill. and Phaseolus vulgaris L. exhibited significant differences in the magnitude of both leaf and root osmotic adjustment, whereas Pisum sativum L. and Abelmoschus esculentus Moench only showed changes in the roots and Piper nigrum L. only in the leaves. Beta vulgaris L. and Spinacia oleracea L. did not show any significant osmotic adjustment. Significant differences in osmotic adjustment were found among the tomato cultivars tested. Osmotic adjustment of leaves and roots was associated with increased cellular turgor. The ability of the horticultural plants used in this study to osmotically adjust in response to water stress was low compared to sorghum (Sorghum bicolor L.) which has been shown to exhibit considerable drought tolerance. The ability of a vegetable crop to withstand periods of water shortages should be taken into consideration in the selection of cultivars for drought conditions and in certain irrigation management decisions.

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