Figure 1.

Schematic representation of putative roles of stem NSCs in the hydraulic regulation in woody plants. (a) During drought, root-to-leaf water transport is reduced and NSC reserves (manly from roots ad stems) are mobilized. Stomata are closed and photosynthesis is reduced, and hence sugar export from leaves and transport through phloem is limited. Decreased xylem water potential (Ψ_x) induces embolism formation in xylem conduits and translocation of sugars to embolized vessels, putatively involving NSC metabolism in stem parenchyma cells and phloem unloading. (b) In the post-drought phase, restored water availability in the soil induces an increase in Ψ_x and a cascade of events leading to the refilling of previously embolized vessels. In the short term upon rehydration, stomata are still closed, transpiration is limited, sugars would still be loaded to the refilling vessels, and water would start to move in the direction of the refilling vessels, washing away the sugars accumulated in xylem sap. In the long term, stomata open and “normal” NSC metabolism and water relations are reestablished, and NSCs are invested for reactivation of cambial growth. Blue and red arrows indicate the direction of water and sugar fluxes, respectively, and their size indicates their magnitude. Question marks indicate putative processes. v = xylem vessel; p = wood parenchyma cells. < indicates low and > indicates high.