Figure 2.

Representative cryo-SEM observation of a walnut petiole collected at midday on a field grown tree. The petioles was frozen intact on the tree during active leaf transpiration and while the xylem water potential was around −0.7 MPa. The cross-section was observed uncoated at −150°C and 5 kV. Vessels were either entirely water filled (vessels on the left side of the picture) or one-half-filled with sap (right side). When xylem pressures were relaxed shortly before freezing all the vessels entirely filled with sap.