The Effect of Low Temperature on Ascent of Sap in Trees

Abstract

Although the so-called cohesion theory is widely accepted as the most reasonable explanation of how water is lifted up to the leaves of tall trees, there are investigators who question or even reject it. Among the objections to the cohesion theory, Handley's experiments (5) have been cited repeatedly during recent years (e.g. 4, 8). In 1939 Handley published a paper describing experiments in which he chilled the stems of 4-foot-tall saplings of Acer Pseudo-Platanus L. and Fraxinus excelsior L. He reported that if the stem was chilled to between 0 and 20, the leaves wilted; if the temperature was raised again to slightly above 20, the leaves recovered, provided the low-temperature treatment had not lasted too long. This observation is rather remarkable since no known property of water changes drastically enough at a temperature above freezing so that the wilting could be explained in terms of a physical interference with water movement. Handley concluded therefore that metabolic forces are somehow involved in sap ascent. Handley's experiments are by no means the only ones in the literature dealing with the effect of temperature on the ascent of sap, but since instrumentation of the older experiments was rather crude when compared to modern standards, it is difficult to evaluate them today. It may suffice here to refer to Handley's own review of these older papers (5). During the summer of 1959 a study was begun on the effect of locally applied temperature on phloem transport (11). In the course of these experiments the question arose as to the temperature at which xylem sap freezes in the tree.