Response of photosynthetic apparatus of spring barley (Hordeum vulgare L.) to combined effect of elevated CO2 concentration and different growth irradiance

The response of barley (Hordeum vulgare L. cv. Akcent) to various photosynthetic photon flux densities (PPFDs) and elevated [CO2] [700 mumol(CO2) mol(-1); EC] was studied by gas exchange, chlorophyll (Chl) a fluorescence, and pigment analysis. In comparison with barley grown under ambient [CO2] [350 mumol(CO2) mol(-1); AC] the EC acclimation resulted in a decrease in photosynthetic capacity, reduced stomatal conductance, and decreased total Chl content. The extent of acclimation depression of photosynthesis, the most pronounced for the plants grown at 730 mumol m(-2) s(-1) (PPFD730), may be related to the degree of sink-limitation. The increased non-radiative dissipation of absorbed photon energy for all EC plants corresponded to the higher de-epoxidation state of xanthophylls only for PPFD730 barley. Further, a pronounced decrease in photosystem 2 (PS2) photochemical efficiency (given as P-V/F-M) for EC plants grown at 730 and 1200 mumol m(-2) s(-1) in comparison with AC barley was related to the reduced epoxidation of antheraxanthin and zeaxanthin back to violaxanthin in darkness. Thus the EC conditions sensitise the photosynthetic apparatus of high-irradiance acclimated barley plants (particularly PPFD730) to the photoinactivation of PS2.