Elevated chloroplastic glutathione reductase activities decrease chilling-induced photoinhibition by increasing rates of photochemistry, but not thermal energy dissipation, in transgenic cotton

The effect of the overproduction of glutathione reductase (GR+) in cotton (Gossypium hirsutum L. cv. Coker 312) chloroplasts on the response of photosynthetic parameters to chilling in the light was examined. After 180 min at 10degreesC and 500 mumol photons m(-2) s(-1) in the chamber of an oxygen electrode, leaf discs of GR+ plants exhibited lower levels of sustained PSII photoinhibition than leaf discs of wild-type plants. No genotypic differences in thermal energy dissipation, leaf pigment composition, or the dynamics of xanthophyll cycle de-epoxidation were observed. The rate of induction and steady-state levels of photochemistry were greater for GR+ in comparison to wild-type plants. Enhanced photochemistry in GR+ plants could not be attributed to higher rates of CO2 assimilation at 10degreesC. Although GR overproduction afforded some increased protection against PSI photoinactivation, suggesting improved scavenging of reactive oxygen species, higher PSI activities could not completely explain the greater rates of photochemistry. Pools of glutathione and ascorbate were significantly more reduced in GR+ plants. Increased demand for reducing power to maintain these constituents in the reduced state may contribute to the higher rates of photochemistry observed in GR+ plants.