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Grafting rootstocks are widely used to enhance plants resistance to various biologic and abiotic stresses. We determined how the rootstock genotype might influence plant responses to drought, using 2-year-old 'Gale Gala' apple trees grafted onto Malus sieversii and M. hupehensis. Under water stress, trees with the former as their rootstock had smaller reductions in rates of relative growth and photosynthesis, total biomass, leaf area, levels of leaf chlorophyll, and relative water content compared with those grafted onto the latter. They also had greater maximum photochemical efficiency and water-use efficiency. On the other hand, trees growing on M. sieversii rootstock had less production of superoxide radicals and hydrogen peroxide in both leaves and roots than those growing on M. hupehensis in response to drought stress. Furthermore, under drought conditions, leaves and roots from trees grafted onto M. sieversii had greater synthesis of ascorbic acid and glutathione, as well as higher activities of superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase. These results suggest that the choice of grafting rootstock can enhance drought resistance by improving the antioxidant system in a plant. Here, 'Gale Gala' trees grafted onto M. sieversii were more droughtresistant than those on M. hupehensis rootstock.

Plant growth, biomass allocation, carbon isotope composition (d 13 C), and water use efficiency (WUE) of 31 cultivars of apple (Malus domestica Borkh.) grown under two water regimes were measured. Drought-stressed plants showed significant declines in tree height, trunk diameter, biomass production, and total leaf area, the extent to which depended upon cultivar. Also, gas exchange rates, instantaneous and long-term efficiencies (WUE I and WUE L , respectively), and values for d 13 C differed among cultivars and watering regimes. Variations in WUE I were mainly due to changes in stomatal conductance (g s ) under drought condition. 'Qinguan' and 'Golden Delicious' had greater trunk diameter, tree height, and had higher biomass production and WUE L under drought stress, implying that they are more suitable for arid and semi-arid regions. Moreover, WUE L was significantly and positively correlated with d 13 C under two watering regimes, which suggests a potential for evaluating water use efficiency of Malus by measuring carbon isotope composition.

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