Ultraviolet-B Radiation-induced Inhibition of Leaf Expansion and Promotion of Anthocyanin Production: Lack of Involvement of the Low Irradiance Phytochrome System.

Leaf discs from expanding leaves of Rumex patientia L. were exposed to 7 hours of visible plus different levels of ultraviolet radiation in the 290 to 315 nm waveband (UV-B) and then placed in darkness. Leaf disc expansion was reduced and anthocyanin production was increased in discs exposed to moderate or high levels of UV-B radiation when compared to control discs. The possibility that the inhibition of leaf expansion by UV-B radiation might be at least partially phytochrome-mediated was examined by giving discs brief red or far red irradiation following exposure to UV-B radiation. Brief red radiation (R) following treatment with moderate or high UV-B radiation did not alter the pattern of growth or anthocyanin production compared to discs placed in darkness following UV-B treatment. However, a posttreatment with far red radiation (FR) reduced the growth of discs subjected previously to either moderate UV-B or no UV-B irradiation to the level of growth of discs given high UV-B. FR posttreatment also decreased anthocyanin production in discs in moderate and high UV-B treatments. Effects of FR and UV-B radiation apparently do not involve the same mechanism. This was demonstrated by experiments in which FR following the UV-B treatments was in turn followed by R, which reversed the effects of the FR but did not alter the growth inhibition or increased anthocyanin production induced by moderate or high levels of UV-B radiation.