Time course of embolism repair during rehydration of droughted grapevines

We rehydrated potted grapevines at 07.00 h after a stress period of ten days when pots had not been watered, to test whether hydraulic conductance recovers by repairing embolism while plant is transpiring or not. During subsequent 24 hours we measured stomatal conductance, leaf water potential, and hydraulic conductance of leaves, petioles, and shoot and root segments. Degree of embolism was determined by comparing the initial hydraulic conductance of plant organs to the maximum hydraulic conductance after removal of air emboli after transient flushings of degassed water by HPFM technique. Stomatal conductance increased from 10 to 100 mmol*m-2*s-1 from 07.00 to 11.00 h, 4 hours after rehydration; thereafter it remained constant till 15.00 h, and slightly decreased till 21.30 h. It was null during the night, while the day after rehydration the maximum reached was about double. Leaf water potential was -1.10 MPa at 07.00 h; it was -0.40 after the first 3 hours, and -0.15 either at the and of the day or the subsequent morning. Recover in hydraulic conductance caused by rehydration (i.e. embolism repair) was about 80% in the plant during the day (from 07.00 to 21.00 h) and a 10% more during the following night. Embolism repair occurred during about two hours in the late morning for root segments, in the early afternoon for the shoot and later for petioles and leaves. We conclude that leaf water potential quickly recovers after rehydration, while embolism repair is a delayed phenomenon, also occurring during leaf transpiration starting from roots toward leaves