RESPONSES OF DETACHED RICE LEAVES (ORYZA-SATIVA L) TO MODERATE SUPPLEMENTARY ULTRAVIOLET-B RADIATION ALLOW EARLY SCREENING FOR RELATIVE SENSITIVITY TO ULTRAVIOLET-B IRRADIATION

Responses to short-term supplementary ultraviolet-B (UV-B) radiation were studied in detached ]eaves of two indica rice cultivars (Er Bai Ai and Lemont) to evaluate whether this might be an initial method for screening for UV-B susceptibility. Leaf tissue from plants grown in a greenhouse (28-degrees-C day/20-degrees-C night, with a maximum irradiance of 800-1000 mumol photons M-2 S-1) Was placed under moderate supplementary UV-B radiation for 20 h. The effects of this short-term treatment were measured by determining the ratio of variable to maximum chlorophyll fluorescence (F(v)/F(m)), quantum yield of photosynthetic O2 exchange, chlorophyll content, maximum Rubisco activity as well as the concentrations of total soluble protein and Rubisco protein. All the above parameters showed considerable declines, which were always greater in cv. Er Bai Ai than in cv. Lemont. The in vivo activation of Rubisco was markedly increased in detached leaves treated with supplementary UV-B compared with control leaves; the increase was greater in cv. Er Bai Ai than in cv. Lemont. The photosynthetic responses invoked in the detached rice leaves are remarkably similar to those observed previously [He et al. (1993). Aust. J. Plant Physiol. 20, 129-421 in intact rice plants which had a longer-term supplementary UV-B exposure of comparable cumulative biologically effective UV-B dosage. We conclude that rapid short-term responses of detached leaves allow early screening of relative sensitivity of rice cultivars to UV-B.