High CO₂ concentration increases relative leaf carbon gain under dynamic light in Dipterocarpus sublamellatus seedlings in a tropical rain forest, Malaysia

Understory plants in tropical forests often experience a low-light environment combined with high CO₂ concentration. We hypothesized that the high CO₂concentration may compensate for leaf carbon loss caused by the low light, through increasing light-use efficiency of both steady-state and dynamic photosynthetic properties. To test the hypothesis, we examined CO₂gas exchange in response to an artificial lightfleck in Dipterocarpus sublamellatus Foxw. seedlings under contrasting CO₂ conditions: 350 and 700μmol CO₂ mol^-1air in a tropical rain forest, Pasoh, Malaysia. Total photosynthetic carbon gain from the lightfleck was about double when subjected to the high CO₂when compared with the low CO₂concentration. The increase of light-use efficiency in dynamic photosynthesis contributed 7% of the increased carbon gain, most of which was due to reduction of photosynthetic induction to light increase under the high CO₂. The light compensation point of photosynthesis decreased by 58% and the apparent quantum yield increased by 26% at the high CO₂compared with those at the low CO₂. The study suggests that high CO₂increases photosynthetic light-use efficiency under both steady-state and fluctuating light conditions, which should be considered in assessing the leaf carbon gain of understory plants in low-light environments.