In situ temperature relationships of biochemical and stomatal controls of photosynthesis in four lowland tropical tree species

Net photosynthetic carbon uptake of Panamanian lowland tropical forest species is typically optimal at 30-32 degrees C. The processes responsible for the decrease in photosynthesis at higher temperatures are not fully understood for tropical trees. We determined temperature responses of maximum rates of RuBP-carboxylation (V-CMax) and RuBP-regeneration (J(Max)), stomatal conductance (G(s)), and respiration in the light (R-Light) in situ for 4 lowland tropical tree species in Panama. G(s) had the lowest temperature optimum (T-Opt), similar to that of net photosynthesis, and photosynthesis became increasingly limited by stomatal conductance as temperature increased. J(Max) peaked at 34-37 degrees C and V-CMax similar to 2 degrees C above that, except in the late-successional species Calophyllum longifolium, in which both peaked at similar to 33 degrees C. R-Light significantly increased with increasing temperature, but simulations with a photosynthesis model indicated that this had only a small effect on net photosynthesis. We found no evidence for Rubisco-activase limitation of photosynthesis. T-Opt of V-CMax and J(Max) fell within the observed in situ leaf temperature range, but our study nonetheless suggests that net photosynthesis of tropical trees is more strongly influenced by the indirect effects of high temperaturefor example, through elevated vapour pressure deficit and resulting decreases in stomatal conductancethan by direct temperature effects on photosynthetic biochemistry and respiration. Photosynthetic carbon uptake in tropical forests decreases at high temperature. To investigate the mechanisms underlying this decrease we analysed the temperature sensitivities of biochemical and stomatal controls over net photosynthesis for four lowland tropical tree species in Panama. While net photosynthesis and stomatal conductance peaked near current ambient temperatures, biochemical control factors V-CMax and J(Max) peaked at much higher temperatures. This, combined with model simulations, suggests that the decreased carbon uptake at high temperatures caused stomatal closure, for example, in response to increased vapour pressure deficit, and not by a direct temperature effect on the biochemical machinery of photosynthesis.