Increase in resistance to low temperature photoinhibition following ascorbate feeding is attributable to an enhanced xanthophyll cycle activity in rice (Oryza sativa L.) leaves

The mechanistic basis for protection of exogenous ascorbate against photoin hibition at low temperature was examined in leaves of rice (Oryza sativa L. ). Exposure of intact leaves to chilling temperature resulted in a drastic decrease in the speed of development of non-photochemical fluorescence quen ching (NPQ). This was related to the low temperature-imposed restriction on the formation of the fast relaxing component of NPQ (q(f)). Feeding with 2 0 mM ascorbate markedly increased the rate of of development at chilling te mperature due primarily to the enhanced rate of zeaxanthin (Z) formation. O n the other hand, ascorbate feeding had no influence on photosystem 2 (PS2) -driven electron flow. The reduced state of the PS2 primary electron accept or QA decreased in ascorbate-fed leaves exposed to high irradiance at chill ing temperature owing to the increased Z-associated thermal energy dissipat ion in the light-harvesting antenna system of PS2. Furthermore, ascorbate f eeding increased the photosynthetic apparatus of rice leaves to resist phot oinhibition at low temperature. The protective effect of exogenous ascorbat e was fully accounted for by the enhanced xanthophyll cycle activity.