Woody Tissue Photosynthesis Reduces Stem Co2 Efflux By Half And Remains Unaffected By Drought Stress In Young Populus Tremula Trees

A substantial portion of locally respired CO2 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 CO2 efflux (E-A), xylem CO2 concentration ([CO2]), and xylem water potential (psi(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 CO2 diffusion (D-ax) induced by distant P-wt caused an additional decrease in E-A of up to 25% and limited xylem [CO2] 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.