Freeze-induced fluctuations in xylem sap pressure in Acer pseudoplatanus

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Abstract
Xylem sap pressures in response to temperature changes were investigated in Acer pseudoplatanus L., the British sycamore. Mature trees were used for field experiments; excised seedling stem segments were used in the laboratory. A general correlation was observed between changes in pressure potential (ψp) of the xylem sap of mature trees and ambient temperatures above zero (freezing point). More outstanding, however, were the characteristic decreases in pressure potential observed in response to subzero air temperatures. Investigation of this phenomenon under controlled conditions using excised stem segments revealed that rapid decreases in sap pressure occurred simultaneously with freezing. Freezing caused the stem wood to absorb water. This is a surprising response because water expands on freezing. Subsequently exudation of sap was triggered by thawing. Freezing and thawing responses appeared to be similar whether stem segments were collected during summer, when the xylem sap contained no detectable sugars, or during winter, when sugars were present in the sap. Apparently water uptake or replenishment (termed "conditioning") during cooling is of paramount importance in determining the capability of maple wood to exude sap. The freeze-induced uptake of sap under negative pressures described here is considered to be the process fundamental to conditioning.