Woody tissue photosynthesis reduces stem CO 2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

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