Abstract
This study examined the influence of high light levels on antioxidant metabolism and the photosynthetic properties of Begonia x erythrophylla leaves. The pigment composition of shaded leaves and those developing in full sunlight was typical of shade- and sun-leaves, respectively. After 28 d in full sunlight, the preformed leaves of shade plants transferred to full sunlight (transferred-leaves) showed photo-bleaching with lower Chl (a + b) content and Chl a : Chl b ratios than shade-leaves, with Chl (a + b) : carotenoid ratios not significantly different. The variable/maximal fluorescence (Fv/Fm) of sun-leaves was not significantly different from that of shade-leaves, but transferred-leaves had reduced Fv : Fm ratios. Light response curves for the electron transport rate (ETR), the oxidation state of photosystem II (qP) and non-photochemical quenching (NPQ) showed significant differences between the three leaf types, with transferred-leaves not able to acclimate completely to full sunlight, having lower ETR, qP and NPQ values at high light levels than sun-leaves. Transfer to full sunlight caused a rapid increase in H2O2 and lipid hyperoxides, and a slight increase in protein oxidation. Ascorbate and glutathione levels decreased rapidly, as did the size of the total glutathione pool and, in addition to the general oxidation of proteins, rapid decreases in both the initial and total activities of chloroplastic fructose-1,6-bisphosphatase and glyceraldehyde-3-phosphate dehydrogenase were observed. These results suggest that a more oxidizing cellular environment is the likely cause of the photo-bleaching observed upon transfer of shade-leaves to full sunlight. Acclimation of transferred-leaves to full sunlight involved gradual increases in the activities of enzymes involved in antioxidant metabolism, including superoxide dismutase, catalase, glutathione reductase, ascorbate peroxidase, dehydroascorbate reductase and monodehydroascorbate reductase, but the levels of these enzymes still remained at levels lower than those found in sun-leaves.