Abstract
To know interspecific difference in protection mechanisms against high ultraviolet-B radiation in plants, cucumber, a sensitive species, and rice, a less sensitive species, were grown in sun-lit glass chambers under enhanced UV-B with 11.3 kJ m-2day-1 biologically effective ultraviolet-B (UV-BBE) for cucumber and 11.7 kJm-2day-1(UV-BBE) for rice, while UV-B was almost nonexistent in the No UV-B treatment. Foliar damage occurred only in cucumber. The growth depression by the UV-B enhancement was also severer in cucumber than in rice. In rice, the content of ascorbic acid in the leaves was much higher than dehydroascorbic acid, whereas, in cucumber, the dehydroascorbic acid content was higher than ascorbic acid. The foliar damage in cucumber may be due to the shortage of ascorbic acid. The enhanced UV-B radiation increased the ascorbic acid and glutathione contents as well as the dehydroascorbate/ascorbate ratios in the leaves of both species. All the enzyme activities investigated (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehy droascorbate reductase, glutathione reductase and guaiacol peroxidase) in both species were increased by the UV-B enhancement with greater increases in monodehydroas corbate reductase and guaiacol peroxidase in cucumber leaves than in rice leaves.
