Regulation of photosynthetic activity during chilling in cotton plants overproducing key antioxidant enzymes

Abstract

We investigated the response to chilling of cotton genotypes that were genetically modified to overproduce superoxide dismutase, ascorbate peroxidase or glutathione reductase (GR+) in the chloroplast. Rates of photochemistry (P) and thermal dissipation (D) were assessed via chlorophyll fluorescence. After abrupt imposition of chilling temperatures (10 oC) at 500 µmol photons m-2 s-1, the rate of induction and steady-state levels of P were greater for all three transgenic genotypes in comparison to wildtype. Comparisons of the quantum yields for CO2 assimilation and P during the chilling treatment indicated that the transgenic varieties maintained greater rates of photochemistry to alternative electron sinks. We observed no differences in D between transgenic genotypes and wildtype cotton. Transgenic genotypes sustained lower levels of chilling-induced PSII and PSI photoinhibition in comparison to wildtype cotton, with GR+ plants exhibiting the greatest degree of protection. Protection against photoinhibition may be explained by the fact that elevated rates of photochemistry allowed transgenic plants to maintain lower QA reduction states. In a separate experiment, GR+ and wildtype cotton were subjected to progressively colder temperatures (from 28 to 14 oC) in a growth chamber over a two-week period. During the treatment, decreases in midday P and pre-dawn Fv/Fm were observed. However, the performance of GR+ and wildtype cotton was indistinguishable. The absence of an effect of GR overproduction during the gradual imposition of chilling stress may be explained by the more than two-fold increase in GR activity observed in wildtype cotton over the course of the treatment.