UV-B Irradiation Induces Differential Leaf Damage, Ultrastructural Changes and Accumulation of Specific Phenolic Compounds in Rice Cultivars

Abstract

Enhanced UV-B exposure of rice, Oryza sativa L., induced ultrastructural changes and visible symptoms including the formation of necrotic spots, accumulation of dark pigments, and finally desiccation of damaged leaves. Thirty-four genetically diverse cultivars were classified as either tolerant, intermediate or susceptible to UV-B irradiation on the basis of the observed damage symptoms. The development of visible symptoms was significantly reduced and delayed in highly tolerant cultivars. Highly susceptible cultivars were severely damaged by UV-B irradiation in a shorter period of time. Light and electron microscopy of these cultivars revealed intracellular and cell wall disruption in UV-B treated leaves. However, the level of disruption was significantly less in tolerant cultivars. There were significant differences in the relative levels of phenolic compounds produced in extracts of rice after UV-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 significant relationship between the UV-B tolerance category and the relative mean level of phenolics, and the mean area of HPLC peaks I and II.