Cavitation Fatigue. Embolism and Refilling Cycles Can Weaken the Cavitation Resistance of Xylem^{<a href="#FN1" rid="FN1" class=" fn">1</a>}

Abstract

There have been a number of reports suggesting that cavitation and emptying of xylem conduits can be followed at close intervals by their refilling (Tyree et al., 1986; Salleo et al., 1996; McCully et al., 1998; Holbrook and Zwieniecki, 1999; Tyree et al., 1999). The implication is that xylem conduits of at least some species undergo frequent (daily) cycles of cavitation and refilling. This possibility leads to a question: What effect does this cycling have on the cavitation resistance of the xylem? Can vessels be recycled without compromising their ability to withstand negative pressures? Or does the act of cavitation fatigue the vessels in some way to make them more prone to cavitate in the future? If refilling is to be effective in restoring xylem conductance, the xylem should not become weakened in the process.

There are reasons to suspect that refilled vessels might not be as resistant to cavitation as when fresh from the vascular cambium. There is substantial evidence that cavitation by water stress occurs by the “air seeding” mechanism (Zimmermann, 1983) operating at pits between cavitated and functional conduits (Crombie et al., 1985; Jarbeau et al., 1995; Sperry et al., 1996). Although it has usually been thought that the air seeding occurs through pre-existing pores in the pit membranes, it is possible that in some cases the pit membrane ruptures before admitting the air (Sperry and Tyree, 1990) or becomes damaged as a result of the rapid energy release upon cavitation. Even if membranes do not irreversibly rupture, the meshwork of microfibrils could become stretched as a result of the great pressure difference (several MPa for many species) separating a water-filled vessel from an already embolized one. If a vessel with pre-stressed and more porous pit membranes would be refilled, it should be more susceptible to cavitation. Even if stress and cavitation were to have no effect on pit membrane function, refilling may leave small microbubbles behind that could nucleate cavitation prematurely during subsequent stress.

There is only one instance that we know where the cavitation resistance of refilled vessels has been tested. In this experiment, stems of Betula occidentalis were used to generate a “vulnerability curve” showing the loss of xylem conductivity from cavitation (percentage loss of conductivity [PLC]) with decreasing xylem pressure. The stems were then refilled in the laboratory and the curve repeated. The second curve was identical to the first, demonstrating that cavitation did not weaken the vessels (Alder et al., 1997; Fig. ​Fig.1C). 1C).

Figure 1

Native (▪) versus stressed (□) vulnerability curves showing the resilient response of stem xylem in Acer negundo, Alnus incana, and B. occidentalis. Curves show the PLC with decreasing xylem pressure. Native and stressed curves were similar (t test, P > 0.05). Means and se, n = 6. B. occidentalis curves are from Alder et al. (1997).

In this paper we surveyed six species to determine the effect of a cavitation and refilling cycle on cavitation resistance. We used stem or petiole segments that had not been significantly water stressed prior to collection. These segments were subjected to water stress and refilling cycles under controlled laboratory conditions. Additional experiments were conducted in selected species to compare laboratory results with controlled soil drought in intact plants and to investigate the mechanism for the surprising differences that we observed in the response of xylem to the cavitation and refilling cycle.

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LITERATURE CITED