Gas Penetration of Pit Membranes in the Xylem of Rhododendron as the Cause of Acoustically Detectable Sap Cavitation

The gas pressure required to force sap from Rhododendron stems was investigated. Sap was expressed from stems, and stem permeability to gas increased, at pressures of 1.3-3.5 MPa. We interpret the changing of permeability as a removal of water films in the pores of the pit membranes which normally limit the length of xylem conduits. Similar pressure differences exist across the pit membranes separating gas and sap-filled conduits when cavitation occurs in Rhododendron. It is suggested that cavitation in detached leaves and shoots of Rhododendron occurs when gas penetrates the pit membranes. The increase in the gas permeability of xylem subjected to high gas pressures was reversed by a soaking in water. It could not therefore have been a consequence of mechanical damage, caused when xylem conduits are subjected to high gas pressures, because such structural damage would be irreversible.