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 170mmol photonsm s) and three sequential temperature treatments (15 C; 35 C; back to 15 C). High irradiance led to decline in predawn quantum yield of photochemistry, Fv/Fm (0.73), maximal Rubisco activity (Vcmax; from 37 to 29mmolms), and electron transport capacity (Jmax; from 115 to 67mmolm s). Temperature increase to 35 C resulted in further decreases in Fv/Fm (0.45) and in chlorophyll bleaching of high irradiance plants, while Vcmax and Jmax were not affected. Critical temperature for thylakoid stability (Tc) ofD. antarcticawas comparable with other higher plants (c. 47 C), and increases of Tc with air temperature were greater in high irradiance plants. Increased Tc was not associated with accumulation of osmotica or zeaxanthin formation. High irradiance increased the xanthophyll cycle pigment pool (V+A+Z, 91 v. 48mmolmol chlorophyll), de-epoxidation state (56% v. 4%), anda-tocopherol. Temperature increase to 35 Chad no effect on V+A+Z and de-epoxidation state in both light regimes, while lutein, b-carotene and a-tocopherols increased, potentially contributing to increased membrane stability under high irradiance. Additional keywords: a-tocopherols, b-carotene, critical temperature, de-epoxidation state, Jmax, light regime, lutein, osmolality, photoinhibition, Vcmax, temperature, xanthophyll pigments.