Photosynthetic acclimation to warming in tropical forest tree seedlings

Tropical forests have a mitigating effect on man-made climate change by acting as a carbon sink. For that effect to continue, tropical trees will have to acclimate to rising temperatures, but it is currently unknown whether they have this capacity. We grew seedlings of three tropical tree species over a range of temperature regimes (T(Growth) = 25, 30, 35 °C) and measured the temperature response of photosynthetic CO(2) uptake. All species showed signs of acclimation: the temperature-response curves shifted, such that the temperature at which photosynthesis peaked (T(Opt)) increased with increasing T(Growth). However, although T(Opt) shifted, it did not reach T(Growth) at high temperature, and this difference between T(Opt) and T(Growth) increased with increasing T(Growth), indicating that plants were operating at supra-optimal temperatures for photosynthesis when grown at high temperatures. The high-temperature CO(2) compensation point did not increase with T(Growth). Hence, temperature-response curves narrowed with increasing T(Growth). T(Opt) correlated with the ratio of the RuBP regeneration capacity over the RuBP carboxylation capacity, suggesting that at high T(Growth) photosynthetic electron transport rate associated with RuBP regeneration had greater control over net photosynthesis. The results show that although photosynthesis of tropical trees can acclimate to moderate warming, carbon gain decreases with more severe warming.