Kr. Markham et al., Possible protective role for 3 ',4 '-dihydroxyflavones induced by enhancedUV-B in a UV-tolenant rice cultivar, PHYTOCHEM, 49(7), 1998, pp. 1913-1919
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
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