Agriculture is the human activity with the world's highest consumption of water, the main environmental resource limiting crop yield. Citrus plants are subject to seasonal variation in water availability in many areas of cultivation and an effective strategy to mitigate the impacts of water scarcity in the citrus industry is the use of rootstocks tolerant to drought. Studies suggest that a significant amount of water can be transported from a wet portion to a dry portion of the root system, with the dry fraction of the root system becoming an additional water sink. The osmotic adjustment is a process by which the cell water potential can be reduced by the active accumulation of osmotic substances, being an important physiological adaptation to drought. As plants are able to uptake water of dry surrounding environment, the osmotic adjustment favors the cell turgor, gas exchange and growth in dry conditions. This study aims to test the hypothesis that drought tolerance in citrus species is associated with the ability of redistribute water within root system, revealing which morpho-physiological characteristics are associated with such hydraulic redistribution. To test this hypothesis will carry out an experiment with Valencia orange trees grafted on three different rootstocks, using a split-root system. One part of the root system will be subjected to water withholding while the other part will be well-watered. Measurements of sap flow in both rootstocks and in scion, leaf turgor, leaf water potential and leaf gas exchange will be taken under growth chamber conditions. Osmotic adjustment and plant biometry will also be evaluated. We expected that the water redistribution varies with the rootstock used and that the osmotic adjustment is an important physiological strategy associated with root water redistribution.
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