Xylem sap pressure changes in response to imposed temperature regimes were studied using excised stems of young sycamore seedlings (Acer pseudoplatanusL). Experiments confirmed that a rapid decrease in sap pressure (inducing an uptake of water) occurred within the xylem core during freezing. The sequence was reversed by thawing. Measurement of wood diameter or bark thickness revealed no significant changes which might accommodate this sap during freezing. Indeed, investigation of the sapwood density showed an overall increase on freezing. The absorption of water in response to freezing was shown to be operative in deliberately killed tissue, although high tissue water content above about 60 disrupted the response. Experiments using twigs at suprafreezing temperatures induced sap exudation also, but it was only a small fraction of the amounts which could be expected after freeze-induced absorption. Apparently suprafreezing exudation can be explained by thermal expansion effects of gas and sap in the wood. A mechanism is proposed based on sap uptake induced by apoplastic ice formation in gas-filled spaces, such as fibre cells, which may also account for the familiar maple sap exudation.
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