UV-B IRRADIATION INDUCES DIFFERENTIAL LEAF DAMAGE, ULTRASTRUCTURAL-CHANGES AND ACCUMULATION OF SPECIFIC PHENOLIC-COMPOUNDS IN RICE CULTIVARS

Enhanced UV-B exposure of rice, Oryza sativa L., induced ultrastructur al changes and visible symptoms including the formation of necrotic sp ots, accumulation of dark pigments, and finally desiccation of damaged leaves. Thirty-four genetically diverse cultivars were classified as either tolerant, intermediate or susceptible to W-B irradiation on the basis of the observed damage symptoms. The development of visible sym ptoms was significantly reduced and delayed in highly tolerant cultiva rs. Highly susceptible cultivars were severely damaged by UV-B irradia tion in a shorter period of time. Light and electron microscopy of the se cultivars revealed intracellular and cell wall disruption in UV-B t reated leaves. However, the level of disruption was significantly less in tolerant cultivars. There were significant differences in the rela tive levels of phenolic compounds produced in extracts of rice after U V-B exposure of the plants. Tolerant cultivars accumulated relatively higher levels of phenolics. Two prominent HPLC peaks (peak I and peak II) accumulated in response to an applied UV-B stress. There was a sig nificant relationship between the UV-B tolerance category and the rela tive mean level of phenolics, and the mean area of HPLC peaks I and II .