Importance of stem photosynthesis in plant carbon allocation of Clusia minor

Key message Restricted access of light for stems reduced carbon acquisition there and limited the biomass growth of the roots. Light access can affect the microatmosphere within stems, creating favourable conditions for photosynthesis. We tested the hypothesis that stem photosynthesis modifies carbon allocation within plants and also can affect root growth. To verify this hypothesis, parts of Clusia minor L. stems were covered with dark material for 8 months to block light access to stems, and then, we compared morphological traits, biomass increment, photosynthetic activity and carbon isotopic signature (delta C-13) in plants with dark- and light-exposed stems. Clusia minor stems were characterized by chlorophyll presence from pith to cortex, active photosystem II and 79% re-assimilation of respired CO2. We also revealed 24-h changes in the delta C-13 of carbohydrates exported from leaves. Keeping stems in darkness led to a significant lowering in root biomass and shoot-to-root weight index (I-w). Moreover, reductions in stem CO2 efflux and the delta C-13 in the roots and stems were also observed. Our results indicate that the lack of stem photosynthesis affects photosynthate flux to heterotrophic organs, such as roots, stems and probably expanding leaves.