Acclimation to future atmospheric CO2 levels increases photochemical efficiency and mitigates photochemistry inhibition by warm temperatures in wheat under field chambers

A study was conducted over 2 years to determine whether growth under elevated CO2 (700 mu mol mol(-1)) and temperature (ambient + 4 degrees C) conditions modifies photochemical efficiency or only the use of electron transport products in spring wheat grown in field chambers. Elevated atmospheric CO2 concentrations increased crop dry matter at maturity by 12-17%, while above-ambient temperatures did not significantly affect dry matter yield. In measurements with ambient CO2 at ear emergence and after anthesis, growth at elevated CO2 concentrations decreased flag leaf light-saturated carbon assimilation. The quantum yield of electron transport ((PSII)) measured at ambient CO2 and higher irradiances increased at ear emergence and decreased after anthesis in plants grown at elevated CO2. At higher light intensities, but not in low light, photochemical quenching (qP) decreased after growth in elevated CO2 conditions. Growth under CO2 enrichment increased dark- (Fv:Fm) and light-adapted (Fv':Fm') photochemical efficiencies, and decreased the chlorophyll a:b ratio, suggesting an increase in light-harvesting complexes relative to PSII reaction centres. A relatively higher decrease in carbon assimilation than the decrease in (PSII) pointed to a sink other than CO2 assimilation for electron transport products at defined growth stages. With higher light intensities, warmer temperatures increased (PSII) and Fv':Fm' at ear emergence and decreased (PSII) after anthesis; in ambient-but not elevated-CO2, warmer temperatures also decreased qP after anthesis. CO2 fixation increased or did not change with temperature, depending on the growth stage and year. We conclude that elevated CO2 decreases the carbon assimilation capacity, but increases photochemistry and resource allocation to light harvesting, and that elevated levels of CO2 can mitigate photochemistry inhibition as a result of warm temperatures.