Possible protective role for 3′,4′-dihydroxyflavones induced by enhanced UV-B in a UV-tolenant 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-beta-D-glucopyranoside, isoorientin-2"-O-beta[6-O-E-p-coumaroylglucopyranoside]] and isoorientin-2"-O-beta-[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-beta-[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. (C) 1998 Elsevier Science Ltd. All rights reserved.