Mechanisms of air removal during xylem vessel refilling in plants

Background/Question/Methods

120 years after the cohesion tension theory was first proposed, there are still many questions about how plants manage to transport water under negative pressure without constantly forming gas emboli in their hydraulic systems. Even more mysterious is their apparent ability to remove such emboli while the xylem is under tension. There is currently no universal agreement among scientists about whether xylem embolisms are formed and repaired naturally on a daily basis or whether they are irreparable and only occur under severe drought stress. We addressed this question by using a new method to test for embolism repair in functioning wood. Embolism repair in xylem under tension would involve not only the refilling of embolized conduits but also the removal of air. We tested for air removal by drilling small holes in functioning sapwood and thereby causing artificial embolisms. The disappearance of air from these embolisms was measured as air flow into the holes through attached tubing connected to a flow gage. Xylem water potentials, wood temperature, and sap flow were logged simultaneously. We hypothesized that air flow would increase with increasing sap flow and decreasing temperature, as both factors would increase gas dissolution into the transpiration stream.

Results/Conclusions

Experiments were conducted on two California shrubs, the desert shrub Encelia farinosa (Asteraceae) and the chaparral shrub Malosma laurina (Anacardiaceae). Air flow mostly moved into xylem rather than out of it, but, contrary to predictions, it did not correlate positively with sap flow or inversely with temperature. Instead, it tended to peak after the noontime peak observed for sap flow, when temperatures were highest.  Decreasing xylem water potentials were most closely correlated with increasing air flow into xylem, suggesting that the flow was not caused by dissolution into the transpiration stream. Xylem water potentials during the experiments were not low enough to cause substantial xylem cavitation, therefore the observed air flow was unlikely to be caused by runaway cavitation. We propose that negative xylem pressure pulls air through non-aspirated xylem pit membranes as nanobubbles that are smaller than the Blake threshold, which characterizes the maximum size of a stable gas bubble for a given negative liquid pressure. These stable bubbles then would quickly dissolve due to their high internal gas pressure.  This mechanism would contrast with air seeding through large pores of aspirated pit membranes at lower xylem water potentials, which would tend to cause embolisms.