Interactive effects of high irradiance and moderate heat on photosynthesis, pigments, and tocopherol in the tree-fern Dicksonia antarctica

Effects of high irradiance and moderate heat on photosynthesis of the tree-fern Dicksonia antarctica (Labill., Dicksoniaceae) were examined in a climate chamber under two contrasting irradiance regimes (900 and 170 mu mol photons m(-2) s(-1)) and three sequential temperature treatments (15 degrees C; 35 degrees C; back to 15 degrees C). High irradiance led to decline in predawn quantum yield of photochemistry, F-v/F-m (0.73), maximal Rubisco activity (V-cmax; from 37 to 29 mu mol m(-2)s(-1)), and electron transport capacity (J(max); from 115 to 67 mu mol m(-2) s(-1)). Temperature increase to 35 degrees C resulted in further decreases in F-v/F-m (0.45) and in chlorophyll bleaching of high irradiance plants, while V-cmax and J(max) were not affected. Critical temperature for thylakoid stability (T-c) of D. antarctica was comparable with other higher plants (c. 47 degrees C), and increases of T-c with air temperature were greater in high irradiance plants. Increased T-c was not associated with accumulation of osmotica or zeaxanthin formation. High irradiance increased the xanthophyll cycle pigment pool (V+A+Z, 91 v. 48 mmol mol(-1) chlorophyll(-1)), de-epoxidation state (56% v. 4%), and alpha-tocopherol. Temperature increase to 35 degrees C had no effect on V+A+Z and de-epoxidation state in both light regimes, while lutein, beta-carotene and alpha-tocopherols increased, potentially contributing to increased membrane stability under high irradiance.