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 w ere studied in detached ]eaves of two indica rice cultivars (Er Bai Ai and Lemont) to evaluate whether this might be an initial method for s creening for UV-B susceptibility. Leaf tissue from plants grown in a g reenhouse (28-degrees-C day/20-degrees-C night, with a maximum irradia nce of 800-1000 mumol photons M-2 S-1) Was placed under moderate suppl ementary UV-B radiation for 20 h. The effects of this short-term treat ment were measured by determining the ratio of variable to maximum chl orophyll 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 great er in cv. Er Bai Ai than in cv. Lemont. The in vivo activation of Rubi sco was markedly increased in detached leaves treated with supplementa ry 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 prev iously [He et al. (1993). Aust. J. Plant Physiol. 20, 129-421 in intac t rice plants which had a longer-term supplementary UV-B exposure of c omparable cumulative biologically effective UV-B dosage. We conclude t hat rapid short-term responses of detached leaves allow early screenin g of relative sensitivity of rice cultivars to UV-B.