Field-grown cotton plants with elevated activity of chloroplastic glutathione reductase exhibit no significant alteration of diurnal or seasonal patterns of excitation energy partitioning and CO2 fixation

Transgenic cotton plants with elevated activity of chloroplast-targeted glutathione reductase (GR+) were grown in field plots over two seasons in order to compare their photosynthetic performance with that of wildtype plants. We hypothesised that transgenic plants would show enhanced protection of the photosynthetic apparatus against photoinhibition, primarily through an increase in electron transport associated with the role of the chloroplastic antioxidant system as an alternative electron sink. Diurnal measurements of chlorophyll a fluorescence from cotyledons, stem leaves, and leaves subtending developing fruits (bolls) were used to estimate the rate of linear electron transport (Je) and the rates of reversible, regulated (NPDREG) and photoinhibitory (NPDPI) non-photochemical energy dissipation (NPD) at several times (June, July, September, October) during each growing season. GR+ cotyledons exhibited greater Je than wildtype cotyledons during the middle of a day in early June, while NPDPI was the same for both genotypes. Throughout most days on which measurements were conducted, no genotypic differences in Je and NPD were observed for stem leaves. Only during the late morning of one day in early October did leaves subtending bolls of GR+ plants exhibit greater Je compared to that for wildtype plants. As leaves subtending bolls of both genotypes aged, Je and CO2 assimilation declined, while NPD increased. Maximum NPDPI and minimum Fv/Fm remained essentially the same for all measurement days. However, the maximum NPDREG, the greatest contributor to NPD, increased with leaf age. We conclude that the rapid rise in leaf temperature during most mornings created conditions in which elevated GR activity conferred no advantage. Also, as light energy absorption became excessive in late morning, cotton leaves exhibited a strong capacity for regulated, non-photochemical energy dissipation that may have served as the major photoprotective mechanism.