Contrasting adaptations to drought stress in field-grown Ziziphus mauritiana and Prunus persica trees: water relations, osmotic adjustment and carbon isotope composition

Drought resistance strategies of Ziziphus mauritiana Lamk. and peach (Prunus persica L.) were studied, focusing on changes in leaf water potential, carbon isotope composition, and solute and stress metabolite contents during an annual cycle under natural rainfed conditions at a field site in Zimbabwe. After a 100-d drought period, leaf water potential (yleaf) of peach trees decreased to –2.0 MPa, whereas yleaf of Z. mauritiana remained constant at –0.7 MPa. Values for the natural abundance of 13 C (d13 C) of bulk peach leaves as well as of total water-soluble compounds and soluble sugars of leaves increased gradually, resulting in significantly higher values as drought stress developed, indicative of increased water use efficiency (WUE). By the end of the dry season, both leaves and roots of peach exhibited osmotic adjustment, with significant accumulation of monosaccharide sugars, anions and cations in the leaves. Sorbitol and oxalate accounted for the greatest proportion of solute increases during drought, while foliar sucrose content decreased. In roots, soluble sugars such as sorbitol, glucose and fructose all increased, whereas root starch content decreased. For Z. mauritiana leaves, neither d13 C values nor soluble sugar concentrations changed markedly during the study period, and Z. mauritiana plants showed no osmotic adjustment during the dry season. Data indicate that the two species exhibited different strategies for coping with soil moisture deficits under field conditions. Although Z. mauritiana exhibited the capacity for osmotic adjustment in glasshouse experiments, the trees avoided drought stress in this investigation, which is an indication of a root system that has access to deeper moist soil layers. In contrast, the increased WUE in peach is likely due to stomatal control of water loss with onset of drought stress. The observed active osmotic adjustment to maintain turgor is in contrast to glasshouse studies, where no osmotic adjustment was found, and emphasizes the importance of field studies where stress develops more slowly. Keywords: drought avoidance, drought stress, drought tolerance, malate, osmotic adjustment, peach, Prunus persica L., root to shoot ratio, sorbitol, stable isotopes, Ziziphus mauritiana Lamk.