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

The investigation reported here defines the structures of the flavonoids en hanced 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 Phys iology, 1997, 24, 261], as isoorientin-2"-O-beta-D-glucopyranoside, isoorie ntin-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 compou nds and no enhancement resulted from increased UV-B levels. Since the two c lasses of flavonoid differ very little in their ability to absorb radiation in the 290-320 nm range, these findings support recently expressed views t hat flavonoids may play a more subtle role in plant UV-B protection than si mple UV-B screening. The presence of an ortho-dihydroxyl grouping in the st ructure is seen as of pivotal importance in that role, which may involve fr ee radical scavenging or more efficient dissipation of UV energy. (C) 1998 Elsevier Science Ltd. All rights reserved.