Is green stem photosynthesis in avocado related to plant hydraulics?

Photosynthesis occurring in green stems of woody plants may play important roles in plant water and carbon balances. We evaluated the variability in stem photosynthetic re-assimilation rate (A_stem), midday stem water potential (Ѱ_{stem, midday}), sapwood-specific hydraulic conductivity (K_S), leaf-specific hydraulic conductivity (K_L), bark chlorophyll concentration (Chl_a+b), wood density (WD) and concentrations of non-structural carbohydrates (NSC) in twigs of 10 different avocado cultivars growing in a common garden. Our results demonstrate that green stems of avocado plants recycle respired CO₂ at rates that vary from 0.2 to 4.8 µmol m^‑2 s^‑1, which corresponds to re-assimilation of 12 to 96% of CO₂ that would otherwise leave the branch. Cultivars had no significant differences in A_stem, Ѱ_{stem, midday}, WD, or Chl_a+b. However, there was a significant difference in K_L, which indicates greater variability in the capacity of stems to supply water to the leaves. K_S and total NSC (bark + wood) also showed differences among cultivars. A_stem was not correlated with either K_S or K_L, but there was a significant relationship between wood and bark NSC concentrations, supporting the movement of sugars between these two plant tissues. In summary, green stems of avocado re-assimilate CO₂, contributing to the carbon economy of the plants, although this contribution may not be the primary NSC source for hydraulic functioning.