Possible protective role for 3',4'-dihydroxyflavones induced by enhanced UV-B in a UV-tolerant rice cultivar

The investigation reported here defines the structures of the flavonoids enhanced by increased UV-B levels in a UV-tolerant rice cultivar, M202 [Caasi-Lit, M., Whitecross, M. I., Nayudu, M., Tanner, G. J., Aust. J. Plant Physiology, 1997, 24, 261], as isoorientin-2_-O-β-D-glucopyranoside, isoorientin-2_-O-β-[6-O-E-p-coumaroylglucopyranoside] and isoorientin-2_-O- β-[6-O-E-feruloylglucopyranoside], the last two being new natural products. Enhanced to a lesser extent were the isovitexin analogues of the last two, together with isoscoparin-2_-O-β-[6-O-E-p-coumaroylglucopyranoside]. The UV- susceptible cultivar, Dular, accumulated virtually none of these compounds and no enhancement resulted from increased UV-B levels. Since the two classes of flavonoid differ very little in their ability to absorb radiation in the 290-320 nm range, these findings support recently expressed views that flavonoids may play a more subtle role in plant UV-B protection than simple UV-B screening. The presence of an ortho-dihydroxyl grouping in the structure is seen as of pivotal importance in that role, which may involve free radical scavenging or more efficient dissipation of UV energy.