Thermal optimum of photosynthesis is controlled by stomatal conductance and does not acclimate across an urban thermal gradient in six subtropical tree species

Abstract

AbstractModelling the response of plants to climate change is limited by our incomplete understanding of the component processes of photosynthesis and their temperature responses within and among species. For ≥20 individuals, each of six common subtropical tree species occurring across steep urban thermal gradients in Miami, Florida, USA, we determined rates of net photosynthesis (Anet), maximum RuBP carboxylation, maximum RuBP regeneration and stomatal conductance, and modelled the optimum temperature (Topt) and process rate of each parameter to address two questions: (1) Do theToptofAnet(ToptA) and the maximumAnet(Aopt) of subtropical trees reflect acclimation to elevated growth temperatures? And (2) What limitsAnetin subtropical trees? Against expectations, we did not find significant acclimation ofToptA,Aopt or theToptof any of the underlying photosynthetic parameters to growth temperature in any of the focal species. Model selection for the single best predictor ofAnetboth across leaf temperatures and atToptArevealed that theAnetof most trees was best predicted by stomatal conductance. Our findings are in accord with those of previous studies, especially in the tropics, that have identified stomatal conductance to be the most important factor limitingAnet, rather than biochemical thermal responses.