Enhanced photochemical light utilization and decreased chilling-induced photoinhibition of photosystem II in cotton overexpressing genes encoding chloroplast-targeted antioxidant enzymes

The aim of this study was to determine whether increases in stromal superox ide dismutase (SOD; EC 1.15.1.1), ascorbate peroxidase (APX; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) via transformation could reduce photosystem (PS) II photoinhibition at low temperature for cotton (Gossypiu m hirsutum L.) plants and to determine by what mechanism this protection ma y be realized. During 3-h exposures of lincomycin-treated leaf discs to 10 degreesC and a photon flux density of 500 pmol m(-2) s(-1), all transgenic plants exhibited significantly greater PSII activity and O-2 evolution than did wild-type plants. Also, the rate constant of PSII photoinactivation wa s significantly lower for all transgenic plants than for wild-type plants. No significant differences existed between genotypes in non-photochemical q uenching of chlorophyll a fluorescence and the regulated component of the t hermal dissipation of excitation energy. The relationship between changes i n variable to maximum chlorophyll fluorescence (F-v/F-m) and the time-depen dent averaged excessive light exposure was similar for all genotypes. This observation excluded the possibility that differences in PSII photodamage w ere due to improvements in the direct protection of PSII from active oxygen by antioxidant enzyme overproduction. Similar decreases in F-v/F-m during the stress treatment for all genotypes when leaves were pre-treated with 3- (3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the effect of overproduction involved events downstream of PSH in the electron transfer pathway. Since all transgenic plants exhibited a significantly higher photo chemical quenching of chlorophyll fluorescence during the chilling treatmen t, we concluded that, under the conditions used in this study, the enhancem ent of the protection of PSII from photodamage by increasing the stromal an tioxidant enzyme activity in cotton leaves was due to the maintenance of a higher rate of electron transport and, consequently, a lower reduction stat e of QA.