© 2001 Heron Publishing—Victoria, Canada
Physiological and morphological adaptations of the fruit tree Ziziphus rotundifolia in response to progressive drought stress
S. K. Arndt (1, 4), S. C. Clifford (2), W. Wanek (1), H. G. Jones (3) and M. Popp (1)
1. Institute of Ecology and Conservation Biology, Althanstrasse 14, A-1090 Vienna, Austria / 2. Horticulture Research International, Wellesbourne, Warwick, Warwickshire, CV35 9EF, U.K. / 3. Department of Biological Sciences, University of Dundee, Dundee, DD1 4HN, U.K. / 4. Botany Department, The University of Western Australia, Nedlands, WA 6009, Australia / Received January 2, 2001
Summary
The physiological basis of drought resistance in Ziziphus rotundifolia Lamk., which is an important, multipurpose fruit tree of the northwest Indian arid zone, was investigated in a greenhouse
experiment. Three irrigation regimes were imposed over a 34-day period: an irrigation treatment, a gradual drought stress
treatment (50% of water supplied in the irrigation treatment) and a rapid drought stress treatment (no irrigation). Changes
in gas exchange, 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 different physiological
responses, but merely affected the time at which they were expressed. The initial response to decreasing soil water content
was reduced stomatal conductance, effectively maintaining predawn leaf water potential (Ψleaf), controlling water loss and increasing intrinsic water-use efficiency, while optimizing carbon gain during drought. Carbon
isotope composition (δ13C) of leaf tissue sap provided a more sensitive indicator of changes in short-term water-use efficiency than δ13C of bulk leaf tissue. As drought developed, osmotic potential at full turgor decreased and total solute concentrations increased
in leaves, indicating osmotic adjustment. Decreases in leaf starch concentrations and concomitant increases in hexose sugars
and sucrose suggested a shift in carbon partitioning in favor of soluble carbohydrates. In severely drought-stressed leaves,
high leaf nitrate reductase activities were paralleled by increases in proline concentration, suggesting an osmoprotective
role for proline. As water deficit increased, carbon was remobilized from leaves and preferentially redistributed to stems
and roots, and leaves were shed, resulting in reduced whole-plant transpiration and enforced dormancy. Thus, Z. rotundifolia showed a range of responses to different drought intensities indicating a high degree of plasticity in response to water
deficits.
Keywords:
carbon allocation, carbon isotope discrimination, carbon partitioning, compatible solutes, osmotic adjustment, proline, stable
isotopes, stomata, water-use efficiency.
