7 - A proposed mechanism of freezing and thawing in conifer xylem  pp. 75-85

A proposed mechanism of freezing and thawing in conifer xylem

By D. J. Robson and J. A. Petty

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ABSTRACT

From the evidence of recent work on freezing in the conifer xylem, previous theories of how cavitation is avoided during freezing and thawing must be rejected. What may happen during freezing is that two pressure rises occur in the tracheid lumen. The first pressure rise occurs as the water expands on crystallising. The second pressure rise is caused by water being drawn out of freezing and unfrozen tracheid lumens, through cell wall capillaries and into the already frozen lumens.

Despite migration of water out of the lumen during freezing, enough water may subsequently be drawn back into the frozen lumen to provide positive pressures during thawing. If positive pressures occur, any bubbles nucleated during freezing quickly redissolve. If negative pressures occur in the xylem before the ice has fully thawed then transitory pit aspiration may occur. Pits may deaspirate when pressures in the thawed and thawing tracheids equalise.

INTRODUCTION

The cohesion-tension theory (Dixon & Joly, 1894) is widely accepted as the mechanism of the ascent of sap in tall trees. However, some aspects of this mechanism remain incompletely explained. One of these aspects is the effect of freezing and thawing on the conductivity of conifer xylem.

During freezing gases dissolved in water come out of solution and, at ice/water interface velocities above 2.5 mm s−1, gas bubbles nucleate (Bari & Hallet, 1974). Lybeck (1959), Sucoff (1969) and Robson, McHardy and Petty (1988) observed gas bubbles in frozen xylem tracheids. If bubbles are present in the tracheid lumen when the xylem sap is in tension (Le. at pressures below absolute vacuum) they may expand and cavitate individual tracheids.