Pit Membrane Degradation and Air-Embolism Formation in Ageing Xylem Vessels ofPopulus tremuloidesMichx

Air-embolism formation in xylem vessels of Populus tremuloides Michx. was quantified by its reduction of hydraulic conductivity in branch segments. Embolism was induced by increasing xylem tension in drying stems, or by inserting one end of a hydrated stem in a pressure bomb and increasing air pressure in the bomb. Both treatments produced the same response suggesting that embolism by water stress was caused by air entering water-filled vessels, presumably through inter-vessel pits. In rapidly-growing P. tremuloides branches, the vessels of the outer growth ring were functional whereas vessels in older xylem were mostly embolized. This selective embolizing of older vessels was associated with a marked increase in permeability of their inter-vessel pits to air, relative to pits of younger vessels. Air-injection pressures less than 1·0 MPa completely embolized older vessels that had been re-filled in the laboratory, whereas pressures over 4·0 MPa were required to embolize young vessels. Greater permeability of old vessels was due to degradation of their pit membranes as seen in the scanning electron microscope; large openings were present that were not seen in pit membranes of young vessels. These holes would allow air to penetrate vessel ends at low pressure differences causing embolism. Degradation of pit membranes causing the selective dysfunction of older sapwood may be a general phenomenon initiating heartwood formation in many species.