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UV-A a.c. - UV-A absorbing compounds; UV-B a.c. - UV-B absorbing compounds; TR - transpiration rate ; Fv/Fm - potential quantum yield of PSII; Y - effective quantum yield of PSII.

UV-A a.c. - UV-A absorbing compounds; UV-B a.c. - UV-B absorbing compounds; TR - transpiration rate ; Fv/Fm - potential quantum yield of PSII; Y - effective quantum yield of PSII.

even an increase in broad bean and one cultivar of wheat (Al-Oudat et al. 1998). Al-Oudat et al. (1998) hypothesised that species and cultivars, that originate from locations with high total solar radiation, exhibit efficient protection, even respond positively to enhanced UV-B radiation. For example, higher tolerance to UV-B was detected in plants in Mediterranean areas (Stephanou and Manetas 1998).

Successful reproduction is very important for agriculture. Literature data on crop yield are contradictory, but the studies reporting negative effects are more frequent. Lower seed biomass has been detected in common and tartary buckwheat (Gaberscik et al. 2002; Breznik 2007, unpublished), rice, pea and mustard (Caldwell et al. 1998). Although the crop yield decreased under enhanced UV-B radiation, the harvest index (economic yield) was not affected in all crops, for example, it was not affected in tartary buckwheat (Yao et al. 2006b) and winter maize (Zheng et al. 2003). Responses of common and tartary buckwheat to enhanced UV-B radiation studied by Breznik (2007) have been reflected in Table 12.1.

12.4 UV-B Radiation in Combination with Other Environmental Parameters

12.4.1 Effects of Carbon Dioxide and Ultraviolet-B Radiation on Plants

The UV-B radiation effect can be increased or mitigated by other environmental factors. Koti et al. (2007) showed that elevated CO2 levels compensated the damaging effects caused by negative stressors, such as high temperature and high UV-B radiation on growth and physiological parameters of Glycine max. Authors suggest that there is a need to develop breeding strategies to develop genotypes that will be able to cope with future climates at both vegetative and reproductive stages. Qaderi et al. (2007) studied the combined effects of UV-B radiation and CO2 concentration on reproductive organs of canola. UV-B radiation exerts a detrimental effect on canola siliquas and seeds, and some of the negative effects of UV-B on these reproductive parts can be partially mitigated by CO2. Additionally, elevated CO2 induced increased assimilation and water use efficiency, but lowered transpiration under UV-B conditions.

12.4.2 Ultraviolet-B Radiation in Combination with Low Temperatures

UV-B radiation increased freezing tolerance in winter wheat seedlings (Triticum aestivum L.). This response probably involves the scavenging enzymes and compounds in the antioxidant defence systems, particularly the ascorbate-glutathione cycle (Yang et al. 2007).

12.4.3 Drought Mitigates UV-B Radiation Effects

Evidence of both synergistic and antagonistic interactions between enhanced UV-B radiation and drought in plants has emerged in recent years (Kakani et al. 2003), but the mechanisms involved have received little attention and remained quite unknown (Alexieva et al. 2001).

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