Depletion of stratospheric ozone (O3) caused by anthropogenic chlorofluorocarbons has increased the amount of ultraviolet radiation, especially ultraviolet-B (280-320,UV-B) radiation reaching the Earth’s surface. For every percentage decreases in stratospheric ozone, the amount of biologically effective UV-B radiation (UV-BBE) is predicated to increase by approximately double that percentage. Enhanced UV-B radiation causes damage to the growing development and physiobiochemical processes of plants. e.g. photosynthesis, which would induce the change of morphological characteristics and alter normal metabolic processes.
This study was conducted to determine the effect of UV-B radiation on the photosynthetic physiology and the ultrastructure of leaves in rice (Oryza sativa L.) . Three-leaf-aged seedlings of rice Lemont (tolerant) and Dular (sensitive) were subjected to UV-B radiation 18.6kJ m^-2 d^-1 for treatment in a network for 3 weeks, and natural light for control.. Under 3-weeks of UV-B treatment, photosynthetic pigment, fluorescence induction kinetics parameters of chlorophyll a, photosynthetic efficiency, ultrastructure of leaf surface and mesophyll were investigated. The results showed as follows: (1) UV-B radiation stress significantly decreased chlorophyll content, and the ratio of chlorophyll a to chlorophyll b (Chla/Chlb), changed fluorescence induction kinetics parameters of chlorophyll a, and in turn reduced photosynthetic efficiency. The tested rice cultivar, Dular, was inhibited more seriously than Lemont. (2) Further studies with transmission electron microscopic (TEM) observation and scanning electron microscope (SEM) observation, revealed that the stomatal apparatus was damaged, the chloroplast structure was distorted, the arrangement in the lamellae of the grana and stroma was loose and disordered. The effect of UV-B radiation stress on the ultrastructure of leaves in the two rice cultivars tested was consistent with their changes in photosynthetic physiology. (3)The adaxial surface was damaged more seriously than the abaxial surface in the two rice cultivars. This might contribute to its higher density of silicic papillae. (4) The amount of silicic papillae on the adaxial surface in the two rice cultivars and their responses to UV-B radiation stress were significantly different, i.e. the amount of papilla in Lemont was higher than Dular, and was elevated by UV-B radiation. The reverse was true in the case of Dular. The findings suggested that the characteristics of silicic cumulation might be one of the mechanisms for rice adaptive to enhanced UV-B radiation stress.