Antioxidants and xanthophyll cycle-dependent energy dissipation in Cucurbita pepo L. and Vinca major L. upon a sudden increase in growth PPFD in the field

Portions of shade-acclimated shoots of Cucurbita pepo L. (pumpkin) and Vinca major L, were rapidly transferred to full sunlight exposure and chlorophyll fluorescence emission, pigment composition, antioxidant enzyme activities, ascorbate contents, and the content of thiobarbituric acid-reactive substances (TBARS) were measured at regular intervals for 17 d. The most notable response of leaf pigment composition was a pronounced increase in the pool size of xanthophyll cycle carotenoids that occurred over a period of 4 d (pumpkin) or 11-18 d (V. major). On day 1 after the transfer midday efficiencies of open PSII units decreased to levels similar to or below those observed in full sun-acclimated leaves. Efficiencies of open PSII units were inversely correlated with xanthophyll cycle conversion states in both species on all dates of sampling. A rapid increase in thermal energy dissipation on day 1 may thus have contributed to the absence of either an increase in the content of TEARS (a measure of lipid damage) or pronounced depressions in predawn F-v/F-m. Activities of the antioxidant enzymes ascorbate peroxidase, superoxide dismutase, glutathione reductase, and catalase as well as ascorbate content increased in both species upon transfer, with superoxide dismutase exhibiting the most dramatic increase, Pumpkin, but not V, major, developed new leaves during the study which possessed the attributes of sun-acclimated leaves.