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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 iso orientin-2″- O - β - d -glucopyranoside, iso orientin-2″- O - β -[6- O - E - p -coumaroylglucopyranoside] and iso orientin-2″- O - β -[6- O - E -feruloylglucopyranoside], the last two being new natural products. Enhanced to a lesser extent were the iso vitexin analogues of the last two , together with iso scoparin-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. |
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