Abstract: [Objective]Stem photosynthesis plays an important role in growth, defense, reproduction and survival for woody plant, especially for deciduous trees. In this study, we investigate dradical heterogeneity and tissue specificity of chloroplast photochemical characteristics in branches, which can provide a basis for quantifying the contribution of different tissue to stem photosynthesis, and alsolay a foundation for revealing the adaptation mechanism of chloroplast to the heterogeneous light environment in twigs.[Methods]The CO₂ release rates of current-year twigs of Salix matsudana clones were measured under light and dark conditions, and the contribution of stem photosynthesis to carbon recycle was evaluated. Besides, the photosynthetic pigment content, light absorption coefficient, photochemical efficiency and electron transport rate of bark, xylem and pith were determined, and the differences inphotochemical characteristics of chloroplasts between distinct tissues were analyzed.[Results]When exposed to the saturation light intensity, the stem photosynthesis rate of twig was 1.27 μmolCO₂·m^-2s^-1, which was able to recover and fix 77% of the carbon consumed by respiration. In the internal of twig, the total chlorophyll content of bark was highest, and was15.90 and 1.83 times higher than that of xylem and pith, respectively.The ratio of chlorophyll b to chlorophylla increased from bark to pith with the increase of radial depth. The carotenoid content and the ratio of carotenoid to chlorophyll in bark were significantly higher than those in xylem.The absorptivity coefficient of different tissues of the current-year-old branches significantly decreased along the radial direction.The maximum photochemical efficiency, effectively photochemical efficiency and relative electrontransport rate of PSⅡ of bark were significantly higher than xylem and pith.[Conclusion]Stem photosynthesis in current-year-old twigs of Salix matsudana was able to recover and fix 77% of the carbon consumed byrespiration. The photosynthetic pigment content and photochemical efficiency of chloroplasts decreased significantly with the increase of radial depth.Barkchlorenchyma was the major contributor to stem photosynthesis,and had a stronger capacity for quenching excess excitation energy. Xylem and pith chloroplasts maximized the limited light energy in the tissue by regulating the proportion of photosynthetic pigments.

Key words: stem photosynthesis, chlorophyll, photochemical efficiency, heterogeneity