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DepartmentAbstract-The potential impacts of an increase in solar UV-B radiation reaching the Earth's surface due to stratospheric ozone depletion have been investigated by several research groups during the last 15 years. Much of this research has centered on the effects of plant growth and physiology undcr artificial UV-B irradiation supplied to plants in growth chambers or greenhouses. Since these artiticial sources do not precisely match the solar spectrum and due to the wavelength dependency of photobiological processes, weighting functions, based on action spectra for specific responses. have been developed to assess the biological effectiveness o f the irradiation sources and of predicted ozone depletion. Recent experiments have also utilized artificially produced ozonc cuvettes to filter natural solar radiation and simulate an environment of reduced UV-B for comparative purposes.Overall, the effectiveness of UV-B varies both among species and among cultivars of a given species. Sensitive plants often exhibit reduced growth (plant height, dry weight, leaf area, etc.), photosynthetic activity and flowering. Competitive interactions may also be altered indirectly by differential growth responses. Photosynthetic activity may be reduced by direct effects on photosynthetic enzymes, metabolic pathways or indirectly through effects on photosynthetic pigments or stomata1 function. The fluence response of these changes has yet to be clearly demonstrated in most cases. Plants sensitive to UV-B may also respond by accumulating UV-absorbing compounds in their outer tissue layers, which presumably protect sensitive targets from UV damage. Several key enzymes in the biosynthetic pathways of these compounds have been shown to be specifically induced by UV-B irradiation.Few studies have documented the effects of UV-B on total plant yield under field conditions. One notable exception is a 6-yr study with soybean demonstrating harvestable yield reductions under a simulated 25% ozone depletion. These effects are further modified by prevailing microclimatic conditions. Plants tend to be less sensitive to UV-B radiation under drought or mineral deficiency, while sensitivity increases under low levels of visible light. Further studies are needed to understand the mechanisms of UV-B effects and the interactions with present stresseb and future projected changes in the environment.

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.

The increase in ultraviolet-B (UV-B; 0.290-0.320 pm) radiation received by plants due to stratospheric ozone depletion heightens the importance of understanding UV-B tolerance. Photosynthetic tissue is believed to be protected from UV-B radiation by UV-Babsorbing compounds (e.g. flavonoids). Although synthesis of flavonoids is induced by UV-B radiation, its protective role on photosynthetic pigments has not been clearly demonstrated. This results in part from the design of UV-B experiments in which experimental UV-A irradiance has not been carefully controlled, since blue/UV-A radiation is involved in the biosynthesis of the photosynthetic pigments. The relationship of flavonoids to photosynthetic performance, photosynthetic pigments, and growth measures was examined in an experiment where UV-A control groups were included at two biologically effective daily UV-B irradiances, 14.1 and 10.7 kJ m-' . Normal, chlorophyll-deficient, and flavonoiddeficient pigment isolines of two soybean (Glycine max) cultivars that produced different flavonol glycosides (Harosoy produced kaempferol, Clark produced quercetin and kaempferol) were examined. Plants with higher levels of total flavonoids, not specific flavonol glycosides, were more UV-B tolerant as determined by growth, pigment, and gas-exchange variables. Regression analyses indicated no direct relationship between photosynthesis and leaf levels of UV-8-absorbing compounds. UV-B radiation increased photosynthetic pigment content, along with UV-B-absorbing compounds, but only the former (especially carotenoids) was related to total biomass (9 = 0.61, linear) and to photosynthetic efficiency (negative, exponential relationship, rZ = 0.82). A reduction in photosynthesis was associated primarily with a stomatal limitation rather than photosystem II damage. This study suggests that both carotenoids and flavonoids may be involved in plant 'UV-B photoprotection, but only carotenoids are directly linked to photoprotection of photosynthetic function. These results additionally show the importance of UV-A control in UV-B experiments conducted using artificial lamps and filters.

The photosynthetic apparatus of some plant species appears to be well-protected from direct damage from UV-B radiation. Leaf optical properties of these species apparently minimizes exposure of sensitive targets to UV-B radiation. However, damage by UV-B radiation to Photosystem II and Rubisco has also been reported. Secondary effects of this damage may include reductions in photosynthetic capacity, RuBP regeneration and quantum yield. Furthermore, UV-B radiation may decrease the penetration of PAR, reduce photosynthetic and accessory pigments, impair stomatal function and alter canopy morphology, and thus indirectly retard photosynthetic carbon assimilation. Subsequently, UV-B radiation may limit productivity in many plant species. In addition to variability in sensitivity to UV-B radiation, the effects of UV-B radiation are further confounded by other environmental factors such as CO2, temperature, light and water or nutrient availability. Therefore, we need a better understanding of the mechanisms of tolerance to UV-B radiation and of the interaction between UV-B and other environmental factors in order to adequately assess the probable consequences of a change in solar radiation.