This paper reviews growth chamber, greenhouse, and field studies on the effects of ultraviolet-B (UV-B. between 280 and 320 nm) radiation on agricultural crop plants. Our understanding of the physiological effects of UV-B radiation comes primarily from growth chamber studies, where UV-B is artificially supplied via filtered lamps. Both photosystems I and II, as well as carboxylating enzymes, are sensitive to UV-B radiation. Ultraviolet-B radiation also affects stomatal resistance, chlorophyll concentration, soluble leaf proteins, lipids, and carbohydrate pools. In general, the effects of UV-B radiation are accentuated by the low levels of visible radiation typically found inside growth chambers. Ultraviolet-B radiation has also been shown to affect anatomical and morphological plant characteristics. Commonly observed UV-B induced changes include plant stunting, reductions in leaf area and total biomass, and alterations in the pattern of biomass partitioning into various plant organs. In sensitive plants, evidence of cell and tissue damage often appears on the upper leaf epidermis as bronzing, glazing, and chlorosis. Epidermal transmission in the UV region decreases in irradiated leaves. This decrease is primarily associated with a stimulation in flavonoid biosynthesis and is thought to be a protective, screening response to the deleterious effects of UV-B. A considerable degree of variability exists in sensitivity to UV-B radiation between different species. Approximately 30% of the species tested were resistant, another 20% were extremely sensitive, and the remainder were of intermediate sensitivity, in terms of reductions in total dry weight. In addition to this sizable interspecific variability, there appears to be a similarly wide intraspecific variability in UV-B response. The effects of UV-B radiation on crop yield have only been examined in a limited number of field studies, with ambient levels of UV-B radiation being supplemented with fluorescent sun lamps. Due to various deficiencies, all these field experiments to date have only limited utility for assessing the potential impact of enhanced levels of UV-B on crop productivity.
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