Functional significance and induction by solar radiation of ultraviolet-absorbing sunscreens in field-grown soybean crops

Colorless phenylpropanoid derivatives are known to protect plants from ultr aviolet (UV) radiation, but their photoregulation and physiological roles u nder field conditions have not been investigated in detail. Here we describ e a fast method to estimate the degree of UV penetration into photosyntheti c tissue, which is based on chlorophyll fluorescence imaging. In Arabidopsi s this technique clearly separated the UV-hypersensitive transparent testa (tt) tt5 and tt6 mutants from the wild type (WT) and tt3, tt4, and tt7 muta nts. In field-grown soybean (Glycine max), we found significant differences in UV penetration among cultivars with different levels of leaf phenolics, and between plants grown under contrasting levels of solar UV-B. The reduc tion in UV penetration induced by ambient UV-B had direct implications for DNA integrity in the underlying leaf tissue; thus, the number of cyclobutan e pyrimidine dimers caused by a short exposure to solar UV-B was much large r in leaves with high UV transmittance than in leaves pretreated with solar UV-B to increase the content phenylpropanoids. Most of the phenylpropanoid response to solar UV in field-grown soybeans was induced by the UV-B compo nent (lambda less than or equal to 315 nm). Our results indicate that pheno lic sunscreens in soybean are highly responsive to the wavelengths that are most affected by variations in ozone levels, and that they play an importa nt role in UV protection in the field.