Starch-to-sugar conversion in wood parenchyma of field-growing Laurus nobilis plants: a component of the signal pathway for embolism repair?

The ability of stems of Laurus nobilis (L.) to refill embolised xylem conduits was studied in plants both at optimal water supply (W) and under conditions of soil drought inducing xylem pressures (Px) of –1.54 (S1) and –2.35 MPa (S2). Starch depolymerisation in wood parenchyma was measured as percentage of cells ‘with high starch content’ (HSC-cells) counted under a microscope. HSC-cells decreased during embolism and increased again in refilled stems. Adirect relationship was found between percentage of HSC-cells and Px, with HSC-cells between 65 and 75% of the total at Px–0.6 MPa, at which recovery from PLC was recorded. At low transpiration, starch re-appeared in wood parenchyma cells but only in plants that showed diurnal stomatal opening (W- and S1-plants). In S2-plants showing diurnal stomatal closure and nocturnal opening with Px between –1.2 to –2.4 MPa, HSC-cells were only 25% and plants did not recover from PLC. This finding suggests that (i) the Px threshold for embolism repair was–0.6 MPa, and (ii) impeded phloem loading limits starch content in wood parenchyma and embolism repair. Weconclude that starch depolymerisation acts as a signal to phloem unloading sugars to embolised conduits thus generating the necessary osmotic gradients driving refilling.