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 (P-x) 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. A direct relationship was found between percentage of HSC-cells and P-x, with HSC-cells between 65 and 75% of the total at P-x >=-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 P-x 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 P-x threshold for embolism repair was >=-0.6 MPa, and (ii) impeded phloem loading limits starch content in wood parenchyma and embolism repair. We conclude that starch depolymerisation acts as a signal to phloem unloading sugars to embolised conduits thus generating the necessary osmotic gradients driving refilling.
