Abstract
We have studied the mechanism of retardation of growth in UV-B irradiated plants. Cucumber seedlings were cultivated under artificial visible light, and irradiated with supplemental UV-B using fluorescent lamps for 12 hr a day. The growth of cucumber seedling first leaves was promoted by fertilization. However, the growth of first leaves of fertilized seedlings was suppressed by UV-B irradiation to a level similar to that of unfertilized seedlings. The growth of first leaves of unfertilized plants did not decline significantly by UV-B irradiation. We found that the photosynthetic activity of first leaves was decreased by UV-B irradiation, especially, in the case of fertilized plants. The activity of ascorbate peroxidase, an enzyme involved in detoxication of active oxygen, in first leaves was increased by UV-B irradiation, suggesting that active oxygen was generated by UV-B irradiation in first leaves. Active oxygen was probably the cause of the observed decrease in photosynthetic activity.
Exogenous benzyladenine, a synthetic cytokinin, promoted the growth of disks excised from the first leaves of plants without exposure to UV-B, especially of those supplied with nutrients. However, the cytokinin did not promote the growth of leaf disks from UV-B irradiated plants. The growth-promoting activity of extracts from first leaves was examined. The activity of the extracts from fertilized, unirradiated plants was high, but the activity of extracts from UV-B irradiated plants was not significant, irrespective of fertilization. These results suggest that grwoth-promoting substances in first leaves were increased by fertilization, but inactivated by UV-B irradiation. Therefore, UV-B induced decrease in the growth of first leaves could be related to decrease in sensitivity to plant hormones and the growth-promoting activity in first leaves.
