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 µmol photons m -2  s -1 ) and three sequential temperature treatments (15°C; 35°C; back to 15°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 µmol m -2 s -1 ), and electron transport capacity (J max ; from 115 to 67 µmol m -2 s -1 ). Temperature increase to 35°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°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 α-tocopherol. Temperature increase to 35°C had no effect on V+A+Z and de-epoxidation state in both light regimes, while lutein, β-carotene and α-tocopherols increased, potentially contributing to increased membrane stability under high irradiance.