Elevated atmospheric CO₂ mitigated photoinhibition in a tropical tree species, Gmelina arborea

Effects of elevated CO₂ on photosynthetic CO₂ assimilation, PSII photochemistry and photoinhibition were investigated in the leaves of a fast growing tropical tree species, Gmelina arborea (Verbenaceae) during summer days of peak growth season under natural light. Elevated CO₂ had a significant effect on CO₂ assimilation rates and maximal efficiency of PSII photochemistry. Chlorophyll a fluorescence induction kinetics were measured to determine the influence of elevated CO₂ on PSII efficiency. During midday, elevated CO₂-grown Gmelina showed significantly higher net photosynthesis (p<0.001) and greater F(V)/F(M) (p<0.001) than those grown under ambient CO₂. The impact of elevated CO₂ on photosynthetic rates and Chl a fluorescence were more pronounced during midday depression where the impact of high irradiance decreased in plants grown under elevated CO₂ compared to ambient CO₂-grown plants. Our results clearly demonstrate that decreased susceptibility to photoinhibition in elevated CO₂ grown plants was associated with increased accumulation of active PSII reaction centers and efficient photochemical quenching. We conclude that elevated CO₂ treatment resulted in easy diminution of midday photosynthetic depression.