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

Drought resistance strategies of Ziziphus mauritiana Lamk. and peach (Prunu s persica L.) were studied, focusing on changes in leaf water potential, ca rbon isotope composition, and solute and stress metabolite contents during an annual cycle under natural rainfed conditions at a field site in Zimbabw e. After a 100-d drought period, leaf water potential (psi (lea)f) of peach trees decreased to -2.0 MPa, whereas psi (leaf) of Z. mauritiana remained constant at -0.7 MPa. Values for the natural abundance of C-13 (delta C-13) of bulk peach leaves as well as of total water-soluble compounds and solub le sugars of leaves increased gradually, resulting in significantly higher values as drought stress developed, indicative of increased water use effic iency (WUE). By the end of the dry season, both leaves and roots of peach e xhibited osmotic adjustment, with significant accumulation of monosaccharid e sugars, anions and cations in the leaves. Sorbitol and oxalate accounted for the greatest proportion of solute increases during drought, while folia r sucrose content decreased. In roots, soluble sugars such as sorbitol, glu cose and fructose all increased, whereas root starch content decreased. For Z. mauritiana leaves, neither delta C-13 values nor soluble sugar concentr ations changed markedly during the study period, and Z. mauritiana plants s howed no osmotic adjustment during the dry season. Data indicate that the t wo species exhibited different strategies for coping with soil moisture def icits under field conditions. Although Z. mauritiana exhibited the capacity for osmotic adjustment in glasshouse experiments, the trees avoided drough t stress in this investigation, which is an indication of a root system tha t 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 drough t stress. The observed active osmotic adjustment to maintain turgor is in c ontrast to glasshouse studies, where no osmotic adjustment was found, and e mphasizes the importance of field studies where stress develops more slowly .