Woody tissue photosynthesis reduces stem CO₂ efflux by half and remains unaffected by drought stress in young <scp><i>Populus tremula</i></scp> trees

Abstract

Abstract A substantial portion of locally respired CO 2 in stems can be assimilated by chloroplast‐containing tissues. Woody tissue photosynthesis (P wt ) therefore plays a major role in the stem carbon balance. To study the impact of P wt on stem carbon cycling along a gradient of water availability, stem CO 2 efflux (E A ), xylem CO 2 concentration ([CO 2 ]), and xylem water potential (Ψ xylem ) were measured in 4‐year‐old Populus tremula L. trees exposed to drought stress and different regimes of light exclusion of woody tissues. Under well‐watered conditions, local P wt decreased E A up to 30%. Axial CO 2 diffusion (D ax ) induced by distant P wt caused an additional decrease in E A of up to 25% and limited xylem [CO 2 ] build‐up. Under drought stress, absolute decreases in E A driven by P wt remained stable, denoting that P wt was not affected by drought. At the end of the dry period, when transpiration was low, local P wt and D ax offset 20% and 10% of stem respiration on a daily basis, respectively. These results highlight (a) the importance of P wt for an adequate interpretation of E A measurements and (b) homeostatic P wt along a drought stress gradient, which might play a crucial role to fuel stem metabolism when leaf carbon uptake and phloem transport are limited.