Only a few studies have been conducted on crop phenology under enhanced UV-B radiation which showed conflicting results (Santos et al. 1998). Delayed germination and flowering were found in bean (Saile-Mark and Tevini 1997; Li et al. 1998). In contrast, Al-Oudat et al. (1998) reported an approximately one week earlier flowering time for wheat and bean. Yao et al. (2006b) showed shortened flowering time for tartary buckwheat, resulting in earlier grain ripening. It has been suggested that enhanced UV-B radiation could change the morphology of reproductive organs, even though they are mostly well protected against dangerous UV-B radiation by UV-B absorbing compounds (Caldwell et al. 1998).
The habitus of crops is usually also affected by enhanced UV-B radiation. Numerous crops, such as tartary buckwheat, common buckwheat, soybean, rice and cotton, were shorter when treated by UV-B radiation (Feng et al. 2003; Gao et al. 2003; Breznik et al. 2004, 2005) as a consequence of an effect during the early developmental stage of crops (Gao et al. 2003). It is likely that a disturbed elongation process results in plant stunting (Ballare et al. 1996), but the mechanism was not worked out. According to recent findings, photooxidative destruction of the phytohormone, indole acetic acid (IAA), and thus a loss of apical dominance, could contribute to interruptions in the elongation process (Mark and Tevini 1997; Meijkamp et al. 2001). In line with loss of apical dominance, many crops, including bean (Meijkamp et al. 2001) and Lolium sp. (Deckmyn and Impens 1999), exhibited increased tillering. On the other hand, our study revealed reduced tillering for common and tartary buckwheat, despite plant being stunted (Breznik et al. 2004; Breznik 2007, unpublished).
Leaf area index (LAI) decreased because of the reduced leaf area in many crops, e.g. cotton (Gao et al. 2003), common and tartary buckwheat (Breznik et al. 2004, 2005; Breznik 2007, unpublished).
Increased leaf thickness is a common response of plants to enhanced UV-B radiation. Thicker leaf tissue successfully diminishes penetration of UV-B radiation (Ballare et al. 1996).
The numerous studies of UV-B effects on crop production have revealed no general trend, because different varieties of the same crops often react differently to elevated UV-B radiation. This might be attributed to differences in other environmental factors occurred during field experiments (Caldwell et al. 1998). Despite this, it is well known that enhanced UV-B radiation frequently affects biomass distribution and reproduction of most crops (Gao et al. 2003). Recent studies on common buckwheat, tartary buckwheat, pea plants, wheat, bush bean, rice, soybean and cotton, showed decreases in total biomass production of 10-50% (Barnes et al. 1993; Saile-Mark and Tevini 1997; Nogues et al. 1998; Correia et al. 1999; Gao et al. 2003; Breznik et al. 2004, 2005; Breznik 2007, unpublished). Lower total biomass production may reflect lower shoot or root biomass accumulation, as was found for wheat, common and tartary buckwheat (Correia et al. 1999; Breznik et al. 2004, 2005; Breznik 2007, unpublished). However, no alteration in the root-to-shoot ratio was found, which indicated that biomass allocation was not affected (Nogues et al. 1998; Correia et al. 1999; Gaberscik et al. 2002; Zheng et al. 2003; Breznik 2007, unpublished). An alteration in biomass production evidenced complex changes in morpho-genetic and physiological processes that included reduced enzyme activities, lower efficiency of PSII and stomatal conductance (Saile-Mark and Tevini 1997), disturbance in water economy and water deficiency (Larcher 1995), carbohydrate partitioning (Gaberscik et al. 2002) and decreased leaf area and lower tillering (Breznik et al. 2004; Breznik 2007, unpublished).
In contrast to the mostly reduced total biomass production, a few studies showed no effects on biomass production of barley and strawberry (Valkama et al. 2003) or
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