5 - Refilling of embolized xylem

Summary

SUMMARY

The water content of xylem has often been shown to undergo seasonal and even diurnal fluctuations, apparently in response to changes in trans pi rational flux. When the water content declines, it may be the result of cavitation in water in tracheids or simply indicative of water menisci receding into the tapered ends of already-cavitated tracheids. Similarly, an increase in water content may be caused by movement of menisci and/or complete refilling of tracheids. Recent experiments suggest that parenchyma has no role in the refilling process. Rather, refilling is brought about by the dissolving of trapped gas in the tracheids as the menisci adjust their position in response to the water potential.

INTRODUCTION

It has long been known that the water content of the woody tissues of trees undergo seasonal fluctuations, but the fact was overlooked in most discussion of the theory of water transport and little attention was given to the underlying processes. Some of the most convincing early data illustrating the phenomenon were provided by Gibbs, working on Canadian forest trees in the 1930s (Gibbs, 1958). He showed that the wood of young trees of Betula populiferaunderwent an annual cycle of water content, varying from 100 per cent of dry weight in the early spring to as little as 60 per cent in the late summer (Fig. 1). Fluctuations were most extreme, and earlier in the season, in the upper part of the stem; and the rise in the spring was immediately after the thawing of the soil.