Protecting cotton photosynthesis during moderate chilling at high light intensity by increasing chloroplastic antioxidant enzyme activity

This study examined the effect of increasing chloroplastic superoxide dismu tase (SOD), ascorbate peroxidase (APX), or glutathione reductase (GR) activ ity via plant transformation of cotton on the initial recovery of photosynt hesis following exposures to 10 degreesC and high photon flux density (PFD) . Growing wild-type or non-expressing segregate plants (controls) and trans formants at two PFDs (600 mu mol m(-2) s(-1) and full sun) resulted in a ra nge of total antioxidant enzyme activities. Total SOD activities above that for control leaves grown in full sun did not substantially improve the rec overies of CO2-saturated photosynthesis, especially for stress treatments l asting more than 1 h, while elevated APX or GR activity did improve recover ies after 1-3 h of the chilling treatment. No synergistic effects were note d when the activities of more than one antioxidant enzyme were elevated in transgenic hybrids. Although these results suggest that the protection of p hotosynthesis can be realized by reducing either superoxide or H2O2 levels, thereby reducing the possibility of hydroxyl radical formation, the situat ion is complicated, since elevated APX or GR activity can improve recoverie s even when additional SOD activity has no effect. In conclusion, to enhanc e the protection of photosynthesis using stroma-targeted antioxidant enzyme s, enhancing metabolism associated with H2O2 is more effective than enhanci ng the capacity for superoxide scavenging. Although small, the improvement in the protection of photosynthetic capacity may be sufficient to improve c otton yield in temperate regions with large diurnal temperature fluctuation s.