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 H2O2, O2
, 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.