Physiological and morphological adaptations of the fruit tree Ziziphus rotundifolia in response to progressive drought stress

The physiological basis of drought resistance in Ziziphus rotundifolia Lamk ., which is an important, multipurpose fruit tree of the northwest Indian a nd zone, was investigated in a greenhouse experiment. Three irrigation regi mes were imposed over a 34-day period: an irrigation treatment, a gradual d rought stress treatment (50% of water supplied in the irrigation treatment) and a rapid drought stress treatment (no irrigation). Changes in gas excha nge, water relations, carbon isotope composition and solute concentrations of leaves, stems and roots were determined. The differential rate of stress development in the two drought treatments did not result in markedly diffe rent physiological responses, but merely affected the time at which they we re expressed. The initial response to decreasing soil water content was red uced stomatal conductance, effectively maintaining predawn leaf water poten tial (Psi,(leaf)), controlling water loss and increasing intrinsic water-us e efficiency, while optimizing carbon gain during drought. Carbon isotope c omposition (delta C-13) of leaf tissue sap provided a more sensitive indica tor of changes in short-term water-use efficiency than delta C-13 of bulk l eaf tissue. As drought developed, osmotic potential at full turgor decrease d and total solute concentrations increased in leaves, indicating osmotic a djustment. Decreases in leaf starch concentrations and concomitant increase s in hexose sugars and sucrose suggested a shift in carbon partitioning in favor of soluble carbohydrates. In severely drought-stressed leaves, high l eaf nitrate reductase activities were paralleled by increases in proline co ncentration, suggesting an osmoprotective role for proline. As water defici t increased, carbon was remobilized from leaves and preferentially redistri buted to stems and roots, and leaves were shed, resulting in reduced whole- plant transpiration and enforced dormancy. Thus, Z. rotundifolia showed a r ange of responses to different drought intensities indicating a high degree of plasticity in response to water deficits.