We compared two apple rootstocks -Malus prunifolia and Malus hupehensis - that differ in their tolerance to this abiotic stress. The former is considered drought-tolerant, the latter, sensitive. We monitored changes in their leaf ultrastructure and responses by their antioxidant defense systems. Irrigation was withheld for 12 d from two-year-old potted plants. Compared with the control, this treatment led to considerable ultrastructural alterations in organelles. Plants of M. prunifolia maintained their structural cell integrity longer than did M. hupehensis. M. hupehensis was more vulnerable to drought than was M. prunifolia, resulting in larger increases in the levels of H(2)O(2), O(2)(-), and MDA from the former. Except for catalase (CAT) and monodehydroascorbate reductase (MDHAR), the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), and dehydroascorbate reductase (DHAR) analyzed here were enhanced to a greater extent in M. prunifolia than in M. hupehensis in response to drought. This was also true for levels of ascorbic acid (AsA) and glutathione (GSH). Under well-watered conditions, changes in lipid peroxidation and relevant antioxidant parameters were not significantly different between the two species throughout the experimental period. These results demonstrate that, in order to minimize oxidative damage, both the activities of antioxidant enzymes and antioxidant concentrations are increased in the leaves of M. prunifolia and M. hupehensis in response to water stress. Moreover, plants of M. prunifolia exhibit higher antioxidant capacity and a stronger protective mechanism, such that their cell structural integrity is better maintained during exposure to drought.