Inactivation of Cu/Zn-SOD precedes the degradation of photosystem I reaction center, PsaA/B, in cucumber leaves chilled in the light

Abstract

The selective photoinactivation of PSI under the light chilling stress was investigated with respect to reactive oxygen species (ROS) scavenging system in cucumber, a chilling-sensitive plant. The combination of low light illumination and low temperature inactivated preferentially PSI of cucumber leaves, resulting in the photoinhibition of photosynthesis. The extent of PSI photoinhibition, determined in vivo by monitoring absorption change around 820 nm was closely correlated with the redox state of the PSII electron acceptor, QA, measured as a fluorescence parameter, 1-qP. The light chilling-induced decrease in leaf absorptance is likely attributable to the decline in functional PSI as the fragmentation of the reaction center of PSI, PsaA/B, was not observed. Amongst ROS scavenging enzymes investigated such as SOD, APX, and NADH DH, Cu/Zn-SOD activities were selectively inactivated by the light-chilling treatment, with a close linear relation to the extent of PSI photoinhibition. Further, about 5-fold increases in the leaf content of H2O2, a potent inhibitor of Cu/Zn-SOD, was observed in the leaves illuminated at low temperature. From these results, we suggest that Cu/Zn-SOD is primarily inactivated by a light-chilling stress via reactive oxygen species.