Figure 1.

Change in xylem vessel functionality (a), water potential, and incident irradiance (b) in the petioles of a mature walnut tree during 2 consecutive d. Vessel functionality was estimated indirectly via the PLC (▪) or directly as the percentage of vessels seen air-filled on a cross-section in a cryo-microscope. Samples observed in the cryo-SEM were either frozen intact on the tree with LN₂ (○) or frozen after the xylem pressure was released to zero (●). Each circle represents one sample and the lines are through the mean values. Error bars represent one sd (n = 10). Only when the xylem pressure was released prior to freezing was a good agreement found between the direct cryo-SEM and the indirect hydraulic methods.