Physiological response and resistance of three cultivars of Acer rubrum L. to continuous drought stress

Abstract

Red maple (Acer rubrum L.) is a fast-growing tree of medium height that is indigenous to the eastern United States and southern Canada. The tree furnishes reasonably strong, close-grained wood that is used extensively for inexpensive furniture, in the manufacture of baskets and crates, for mine props, railroad ties, and fuel wood. Many cultivars have been selected, of which almost all are selections for leaf color characteristics. In this study, three cultivars of red maple (Acer rubrum ‘October Glory’, Acer × freemanii ‘Jeffersred’, and Acer rubrum ‘Brandywine’) were selected to compare their physiological response and resistance to continuous drought stress. Before the experiment, two-year-old seedlings were grown at respective day/night temperatures of 25/18°C and 70%/80% RH without water limitation. The drought treatment was initiated by ceasing water supply until the seedlings showed visible signs of wilting. Drought is a common environmental stress that severely limits plant growth and development. Plants have evolved many mechanisms to adapt to environmental stresses via changes at the physiological, morphological, and molecular levels. In the present study, nine physiological indices, namely concentration of proline, soluble protein, and soluble sugar, activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), malondialdehyde (MDA) concentration, relative chlorophyll concentration, and relative conductivity of cell membranes, were measured to investigate the tolerance mechanism of red maple during the whole drought stress. Water deficit caused a progressive spread of tissue damage, as indicated by the physiological indices monitored, which was always observed first in the young leaves. October Glory experienced severe damage, whereas injury to Brandywine developed more slowly and was less severe, and Jeffersred showed an intermediate level of injury. A significant increase in the levels of osmoregulatory substances was observed in seedlings of all cultivars. Soluble protein and soluble sugar contents showed the same trends, namely a decrease after a rapid transient increase, whereas proline contents showed no significant changes during the drought-stress treatment. Osmoregulatory responses to drought stress of Jeffersred and Brandywine were initiated from the 6th day of treatment, whereas in October Glory responses were observed from the 13th day of treatment. October Glory showed the best osmoregulatory capability of the three cultivars. The activities of SOD and CAT initially increased, and thereafter rapidly decreased, in response to drought stress, whereas POD activity showed the opposite trend. Brandywine showed the most active antioxidant system in which activities of SOD, CAT, and POD were higher than those of the other cultivars throughout the experiment. Antioxidant activity of Jeffersred was higher than that of October Glory, in which only POD activity was maintained at a high level. Continuous drought damaged the cell membrane systems of seedlings, with gradual increases in relative membrane conductivity and MDA concentrations of cells closely associated with the duration of drought stress. Relative membrane conductivity and MDA concentration did not differ significantly between Jeffersred and Brandywine, whereas in October Glory both indices increased significantly at advanced stages of drought stress (P<0.05). Among the three cultivars, damage to the cell membrane system was most severe in October Glory over the duration of the experiment. Chlorophyll concentration decreased gradually with increasing duration of drought stress, but significant variation in response among the cultivars was observed; chlorophyll concentration was most stable in ‘Jeffersred’, whereas October Glory showed the most dramatic loss of chlorophyll and Brandywine showed an intermediate response. Chlorophyll loss with increasing duration of drought stress was consistent with the damage observed to the cell membrane system. Combining the membership function with the physiological observations for assessment of the drought resistance of the three red maple cultivars, we concluded that the relative drought stress resistance of the cultivars was Brandywine>Jeffersred>October Glory.