Drought dampens the positive acclimation responses of leaf photosynthesis to elevated [CO2] by altering stomatal traits, leaf anatomy, and Rubisco gene expression in Pyrus

The responses of Chinese pear plantations to rising atmospheric CO2 concentration ([CO2]) and drought are crucial for accurately estimating pear production and forest carbon sink of China under future climate change. Here, we investigated the effect of CO2 enrichment on plant growth and leaf photosynthesis of Chinese pear trees grown under full irrigation and different levels of drought. We identified that leaf photosynthetic capacity and stomatal conductance increased by high [CO2] acclimation. However, such positive effects disappeared when drought occurred, especially under moderate and severe drought. This down-regulation of high [CO2] acclimation responses in leaf photosynthesis by drought was mainly caused by the decline in stomatal diffusional efficiency, thinner mesophyll tissues and smaller mesophyll cells. The decrease of stomatal diffusional efficiency was further caused by the reduced stomatal density and decreased regularity of stomatal distribution pattern under drought. Moreover, the down-regulation of those positive acclimation responses was relevant with the declines of carboxylation and photochemical efficiency of Rubisco, which was confirmed by the down-regulated expression levels of Rubisco coding genes. Overall, our results demonstrated that drought dampened the positive acclimation responses of leaf photosynthesis to elevated [CO2] by modifying stomatal morphology and distribution, leaf anatomy, and Rubisco gene expression. Therefore, under future climate change where CO2 enrichment is coupled with higher frequency of drought events, drought limitations should be incorporated into the current coupled climate-carbon models to avoid overestimation of the positive responses of Chinese pear plantations to elevated [CO2] and to accurately predict forest carbon sequestration.