To investigate the effects of environmental stresses on ascorbic acid content and its redox status, the effects of freezing and drought on ascorbate and dehydro-ascorbate content and activities of four enzymes involved in the ascorbate-glutathione cycle in some conifers were studied. The results showed that both freezing and drought induced the decrease in ascorbate content and the increase in dehydro-ascorbate content. The activities of ascorbate peroxidase (APX) and monodehydro-ascorbate reductase (MDAR) were decreased by freezing stress. At the beginning of exposure to air, water loss from detached needles induced the increase in the activities of APX and MDAR. Further water loss turned to decrease the APX and MDAR activities. The activities of dehydro-ascorbate reductase (DHAR) and glutathione reductase (GR) were not sensitive to changes in temperature and water content of the needles. It is concluded that moderate temperature or water stresses may induce the acclimation and increase in the ability of the H2O2 scavenging system, while strong stresses decrease the ability and induce injury of plant tissues. Correlation between ascorbate content and activities of related enzymes and cold tolerance of conifers were also reported.
