EFFECTS OF SUPPLEMENTARY ULTRAVIOLET-B RADIATION ON RICE AND PEA-PLANTS

To compare the effects of supplementary ultraviolet-B (UV-B) radiation on a tropical/subtropical and a temperate plant, two indica rice cult ivars (Er Bai Ai and Lemont) and peas were exposed to supplementary UV -B radiation for 8 days (biologically effective irradiance of 0.68 W m -2, 12 h per day). Marked decreases occurred in the ratios of variable to maximum chlorophyll fluorescence yield and in the quantum yield of photosynthetic oxygen evolution over the 8 day treatment period. The greatest decline always occurred in pea leaves, while in rice, cv. Er Bai Ai was more susceptible to UV-B radiation than cv. Lemont. Compare d with control plants, the content of soluble protein and ribulose-1,5 -bisphosphate carboxylase/oxygenase (Rubisco) protein decreased signif icantly after the UV-B treatment; the greatest decrease occurred in pe a leaves, while the rate of decrease in rice was greater in cv. Er Bai Ai than cv. Lemont. Over the 8 day UV-B treatment period, the increas e of UV-B-absorbing compounds was greater in rice leaves than in pea l eaves, and greater in cv. Er Bai Ai than in cv. Lemont, although cv. L emont was more UV-B tolerant. We suggest that the increase in these co mpounds is not the only indicator of resistance to UV-B damage; other factors, yet to be identified, may also confer UV-B tolerance. While l eaf orientation may be related to the severity of UV-B damage, natural ly near-horizontal pea leaves were still much more susceptible to UV-B damage compared with rice leaves which were held horizontally over th e 8 day treatment period. Brown spots appeared on the upper surface of leaves of cv. Er Bai Ai after 2 days of UV-B treatment and showed a c umulative increase with the duration of exposure. Our study confirms t he multiplicity of photosynthetic responses and of different protectiv e strategies that may account for the differential sensitivity of plan ts to supplementary UV-B radiation.