Even though UV-B represents a small fraction (0.5%) of total solar radiation, exposure to UV-B at the current and projected levels is known to elicit a variety of responses to all living organisms, including crop plants (Teramura, 1983; Runeckles and Krupa, 1994; Teramura and Sullivan, 1994; Caldwell et al., 1998; Kakani et al., 2003a). Changes in CO2 and temperature accompanied by emission of ozone-depleting compounds, such as CFCs, CH4, and N2O caused by anthropogenic activities, reduces the thickness and affects distribution of the stratospheric ozone column (IPCC, 2007). The increase in UV-B radiation is closely associated with stratospheric ozone depletion as ozone absorbs the UV-B radiation portion of the solar spectrum (Long, 1991). Relative to the 1970s, the midlatitudes' O3 column losses for the period of 2002 - 2005 were approximately 3% in the northern and 6% in the southern hemisphere (WMO, 2007). Current global distribution of mean daily doses of UV-B radiation during summer in most crop growing regions ranges from 2 to 9 kJ m (McKenzie et al., 2007).
Previous reviews and published studies clearly demonstrate the extent of damage caused by both ambient (Teramura, 1983; Caldwell et al., 1989; Teramura and Sullivan, 1994) and elevated UV-B radiation (Teramura, 1983; Rozema et al., 1997; Krupa, 1998; Searles et al., 2001; Kakani et al., 2003a) on crop growth and yield. Damage varies widely among species and among cultivars of the same species. Teramura (1983) reported that more than 70% of 130 species were significantly affected by elevated UV-B in terms of total biomass production. In a statistical analysis of 77 crop species, mostly based on vegetative growth and a few yield parameters, Krupa (1998) reported sensitivity of more than 50% crop species, including several agriculturally important crops. In a recent review of 129 reports of 35 crop species, including cereals, legumes, oil, sugar, fiber, and tuber crops, enhanced UV-B radiation was shown to directly affect most of the crop growth (Kakani et al., 2003a). Affects included photosynthesis, production of defensive compounds (UV-B absorbing compounds and wax contents), and decreased vegetative growth which led to a myriad of secondary and tertiary effects, including altered crop growth and development. This, in turn, affected light interception which lowered canopy photosynthesis, reduced fruit production and retention, and finally yield.
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