The Earth's climate has always been changing. Recent observations indicate that the changes are occurring faster and greater than those in the past. Consequently, the main scientific concern is with the ability of organisms to cope with these changes, especially terrestrial plants and crops that are sessile.
Stratospheric ozone depletion, resulting in increased intensity of UV-B radiation at the Earth's surface, has been one of the most evident changes since the 1980s. The effects of enhanced UV-B radiation on plants have been widely studied, showing that UV-B radiation can have deleterious effect on plants i.e. damage to DNA, alterations in transpiration, photosynthesis and respiration potential, growth, development and morphology (Rozema et al. 1997; Bjorn 1999; Gaberscik et al. 2002; Jansen 2002). The intensity of the effects depends on the species and on the balance between potential damage and the induction of protective mechanisms (Stephanou and Manetas 1998; Gaberscik et al. 2001). Consequently, natural systems can be affected by changes in plant phenology, biomass, seed production, plant consumption by herbivores, diseases and changes in species composition (Caldwell et al. 1998). Crops have been shown to be even more sensitive, because they have been subjected to long-term breeding (Breznik et al. 2005).
New agricultural practices are being developed to adapt food production to recent climate changes, since UV-B radiation could eliminate crops that are less tolerant. Thus, it is important to estimate the tolerance of certain species and to improve their tolerance by plant breeding and by application of protective substances on the field, such as soaking of seeds or foliar spraying by selenium (Se) compounds.
National Institute of Biology, Vecna pot 111, SI-1000 Ljubljana, Slovenia e-mail: [email protected]
S.N. Singh (ed.), Climate Change and Crops, Environmental Science and Engineering, DOI 10.1007/978-3-540-88246-6.12. © Springer-Verlag Berlin Heidelberg 2009
12.2 Changes of UV-B Radiation at the Earth's Surface
Depletion of the stratospheric ozone layer is the consequence mainly of huge emissions of chlorofluorocarbons (CFCs), methyl bromide (CH3Br) and nitrogen oxides (NOx) arising from human activities. Continuous observation since the 1980s has shown this layer to have decreased by 3-6%, resulting in 6-14% increase of UV-B radiation at Earth's surface (WMO 2003). The recovery of the ozone layer has been predicted either to speed up (Dyominov and Zadorozhny 2005) or to slow down, the uncertainty arising from the fact that the possible interactions with global warming are poorly understood. The recent, generally accepted forecast is that the stratospheric ozone layer will recover by 2050, but other possible scenarios are also being studied. In any case, the enhanced UV-B radiation will still be an issue and could significantly affect biocenosis (McKenzie et al. 2003). Climate change can also affect UV radiation due to changes in cloud cover and reflectivity, since temperature changes over the 21st century are likely to be about 5 times greater than in the previous century. Ozone is expected to increase slowly over the coming decades, but it is not known whether it will return to levels that are higher or lower than those present prior to the onset of ozone depletion in the 1970s.
The intensity of UV-B radiation at the Earth's surface depends also on other atmospheric conditions, i.e. clouds, aerosols, tropospheric ozone and other gases, season and location (Nielsen 1996). The stratospheric ozone depletion in high latitudes of the Northern Hemisphere (Jones and Shanklin 1995) is less pronounced and more erratic than in the Southern Hemisphere (Dahlback 2002).
Knowledge of the level of biologically effective UV-B (UV-BBE) radiation is essential for interpreting the effects of UV-B radiation on organisms. Different action spectra have been obtained for different processes in organisms (Bjorn 1999). The increasing shorter wavelength UV-B requires that action spectra are determined for interpretation of UV-B effects. The Thimijan curve of biological effectiveness is the basis for developing action spectra that express the relative influence of a particular wavelength on the biological response (Thimijan et al. 1978).
12.3 The Effects of Enhanced UV-B Radiation on Crops 12.3.1 The Cell and Its Components
The most sensitive cell component is DNA because of its absorption in the shortwave region of solar radiation (Ravanat et al. 2001) and its importance for the organisms. The formation of cyclobutane pyrimidine and pyrimidine-(6,4)-pyrimidone dimers, that affect DNA duplication and gene transcription, has been shown in tobacco and Arabidopsis (Jansen et al. 1998; Bray and West 2005).
The structure, and hence function, of cell membranes is disrupted by radiation (Yuan et al. 2000a,b), mainly as a result of lipid peroxidation (Murphy 1990).
The effect of UV-B radiation on photosynthesis depends on species, cultivars, UV-B intensity and PAR:UV-B ratio (Kakani et al. 2003). However, enhanced UV-B radiation undoubtedly affects photosynthesis in most crops. Reduced net photosynthesis can result from disturbed PSII, reduced content and activity of Calvin cycle enzymes (Rubisco, PEPCase), down-regulated transcription of photosynthetic genes, decreased thylakoid integrity and altered chloroplast structures, but the primary target of the photosynthetic apparatus is not clear. The photochemistry of PSII has been shown to be disrupted in two buckwheat species (Breznik 2007, unpublished), in line with previous studies on pea, rice and spinach (Iwanzik et al. 1983; Renger et al. 1989; He et al. 1993). Although altered photochemistry of PSII has been ascribed to photoinhibition, it could also be a consequence of disturbed thylakoid membrane integrity caused by peroxidation of lipids and proteins (Sharma et al. 1998). On the other hand, the enhanced UV-B radiation may inhibit photosynthesis without affecting PSII photochemistry. Sharma et al. (1998) reported 66% decrease in net photosynthesis and only 16% decrease in Fv/Fm for wheat exposed to enhanced UV-B radiation, indicating a metabolic limitation of photosynthesis due to degradation/inactivation of Calvin cycle enzymes rather than disruption of PSII photochemistry. Disruption of the Calvin cycle because of altered content and inac-tivation of Rubisco and PEPCase has been confirmed in maize (Correia et al. 2005).
The effect on photosynthetic pigments, that must impact on photosynthesis, has been related to different UV bands and different species in studies on Cucumis sativus and Glycine max. Under disproportionate UV/PAR light conditions, photosynthetic pigment content was decreased by UV-A as well as by UV-B; in cucumber seedlings, the ratio of total carotenoids to total chlorophyll was increased (Yao et al. 2006a). Kakani et al. (2003) stated that a decrease in photosynthetic pigment content, following exposure to UV-B radiation in most crop species, is mainly the result of breakdown of chloroplast structure.
Responses of superhigh-yield hybrid rice Liangyoupeijiu to enhanced ultraviolet-B radiation showed that the chlorophyll content of UV-B exposed leaves was slightly reduced at the tillering stage, but recovered in new growth tissues (Xu and Qiu 2007). The light-saturated photosynthetic activity of UV-B exposed plants was, however, 45.2 and 35.3% higher than their controls.
Early studies on the effect of enhanced UV-B radiation on stomatal conductance and transpiration rate did not show any significant changes (Teramura et al. 1984). In contrast, recent findings revealed increasing or decreasing stomatal conductance as a result of UV-B radiation (Pal et al. 1999; Jansen and van-den-Noort 2000) that were in line with changes of transpiration rate (Correia et al. 1999). Nogues et al. (1998) demonstrated 65% decrease in adaxial stomatal conductance for pea. Our study on common and tartary buckwheat indicated a progressive decrease and about 50% lower transpiration rate at the end of the growth period (Breznik 2007, unpublished). A decrease was also found by Gaberscik et al. (2002) in common buckwheat, where transpiration rate was about 15% lower at the end of the season, and by Gitz III et al. (2005), 50% lower transpiration rate in soybean cultivar. Altered transpiration rate is ascribed to changes of stomatal function (Jansen and van-den-Noort 2000) and frequency (Gitz III et al. 2005). The mechanism of action of UV-B radiation on stomatal function has not been elucidated, but Yang et al. (2000) suggested that changes in ABA content and K+ outflow from guard cells could be the reason.
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