Conclusions

In the meta-analysis of Searles et al. (2001), UV-induced changes in foliar chemistry, particularly that of production of epidermal screening compounds, were cited as one of the most common responses to UV-B radiation. In this and several related studies (Sullivan et al., 2007; Xu et al., 2008a, b), we evaluated the consequences of removal of one part of the screening system; namely, the flavonols of soybean, and found that they were important screening compounds. This attests to the role of these compounds in providing protection or at least minimizing damage. However, hypersensitivity was not found in the magenta line which suggests the existence of multiple screening pigments and repair processes that may be present in soybean and other plant species. In fact, studies have shown that although a wide range of plant responses to changes in UV-B levels may be observed, the fact remains that total plant growth is rarely severely impacted by small or moderate levels of UV-B when received under realistic field conditions (e.g., see Searles et al., 2001).

There is no question, however, that photons in this highly energetic waveband are quite biologically damaging, yet repair and protective process prevent much of the potential damage. For example, when evaluating the growth and morphology of these soybean lines (Table 15.1), there were some detrimental effects on growth and morphology, but survival and reproductive success were not compromised. Similar results were also obtained on studies with another soybean line, Harosoy, which also has a non-flavonoid isoline (Sullivan et al., unpublished data). In this case, Harosoy showed greater sensitivity which could have been related to differing flavonol composition. The Harosoy line primarily produces the flavonol kaempferol and only trace quantities of quercetin. This indicates that flavonoid composition, in addition to quantities and localization, may contribute to differing levels of UV protection. The importance of flavonoids as antioxidants, in addition to UV-screens, needs further evaluation (Xu et al., 2008b). Appreciable absorbance by the putative HCAs in the magenta lines may provide some UV protection as well. Finally, and for DNA damage in particular, the results of these and other studies indicate net dimer levels that are modulated by ongoing damage and repair processes. We still have a limited understanding of the induction of these repair processes. Clearly, if UV-screening by flavonoids was the only protection method, the results of this study would have been very different, and damage to the magenta line would have been severe at best. Therefore, the quantity and type of screening compounds, the localization and degradation of them, and other factors such as DNA repair processes, determine the sensitivity of plants to UV-B radiation. This aspect of the perception and response of plants to UV-B radiation needs further evaluation. Important topics for continued research are the evaluation of the spectral sensitivity and regulation of the induction and catalysis of these pigments and of the indirect effects of these photomorphogenic changes in leaf chemistry on plant development and ecological processes.

Table 15.1 The effects of supplemental UV-B radiation on the growth and morphology of soybean lines differing in flavonoid levels. Plants received either ambient levels of UV-B radiation (Controls or C) or ambient plus a daily maximum of 5.0 kJ of UV-B when weighted with the action spectra of Caldwell, 1971. An asterisk indicates a significant statistical difference (P < 0.05) between UV treatments as determined by the Student Newman-Keuls Multiple Range Test (SAS manual, 2000)

Table 15.1 The effects of supplemental UV-B radiation on the growth and morphology of soybean lines differing in flavonoid levels. Plants received either ambient levels of UV-B radiation (Controls or C) or ambient plus a daily maximum of 5.0 kJ of UV-B when weighted with the action spectra of Caldwell, 1971. An asterisk indicates a significant statistical difference (P < 0.05) between UV treatments as determined by the Student Newman-Keuls Multiple Range Test (SAS manual, 2000)

Cultivar

UV Treatment

Height (cm)

Plant Leaf Area (cm2)

Plant Dry Weight (g)

(root:shoot)

Clark

Control

153+15

2960 +197

34.9 + 3.5

0.241 + 0.02

Clark

+UV

130 ± 12

2400 + 210

32.0 + 3.7

0.301 + 0.03

Clarkwm

Control

139+13

2369 + 245

31.5 + 3.2

0.258 + 0.03

Clarkwm

+UV

106+11 *

1930+167

28.2 + 3.4

0.333 + 0.04

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