Elevated CO2 and sunflecks interact to increase photosynthesis and growth in the tropical rainforest tree seedling Shorea leprosula

Abstract

To maintain a positive carbon balance and survive in the deeply shaded understorey of S.E. Asian rain forests, dipterocarp seedlings must utilize sunflecks for photosynthetic carbon gain. Despite the potentially important ecological consequences, very little is known about the response of dynamic photosynthesis to elevated CO2. How, therefore, do photosynthesis and growth in Shorea leprosula respond to elevated CO2 under sunflecks, compared with uniform irradiance? Steady state photosynthesis acclimated to elevated CO2, with no difference between plants grown under flecked and continuous irradiance. There was a reduction in the ratio of Vcmax/Jmax and greater light-saturated photosynthetic rates, under the elevated CO2 treatment. Light response curves indicated greater apparent quantum yield and a lower light compensation point, as well as greater photosynthetic rates at all PPFDs, in leaves grown and measured at elevated CO2. The responses to elevated CO2 of the dynamic components of photosynthesis during sunflecks included: faster photosynthetic induction gain, greater post-irradiance CO2 fixation and lower post-irradiance CO2 burst. When these effects are added to changes in photosynthetic capacity, seen during steady state photosynthesis, there is an 87% increase in carbon gain.