Net O2 exchange rates under dark and light conditions across different stem compartments

• Woody plants display some photosynthetic activity in stems, but the biological role of stem photosynthesis and the specific contributions of bark and wood to carbon uptake and oxygen evolution remain poorly understood. • We aimed to elucidate the functional characteristics of chloroplasts in stems of different ages in Fraxinus ornus. Our investigation employed diverse experimental approaches, including microsensor technology to assess oxygen production rates in whole stem, bark, and wood separately. Additionally, we utilized fluorescence-lifetime imaging microscopy (FLIM) to characterize the relative abundance of photosystems I and II (PSI/PSII chlorophyll ratio) in bark and wood. • Our findings revealed light-induced increases in O2 production in whole stem, bark, and wood. We present the radial profile of O2 production in F. ornus stems, demonstrating the capability of stem chloroplasts to perform light-dependent electron transport. Younger stems exhibited higher light-induced O2 production and dark-respiration rates compared to older ones. • While bark emerged as the primary contributor to net O2 production under light conditions, our data underscored that wood chloroplasts are also photosynthetically active. FLIM analysis unveiled a lower PSI abundance in wood compared to bark, suggesting stem chloroplasts are not only active but also acclimate to the spectral composition of light reaching inner compartments.