W. Konrad and A. Roth-Nebelsick

Institute for Geosciences, University of Tuebingen, Sigwartstrasse 10, D-72076 Tuebingen, Germany

Gas-filling of conduits decreases hydraulic conductance of the xylem vessels. Therefore, embolism formation and reversal is one of the crucial topics in plant water transport. The negative pressure (=tension) in xylem water during plant transpiration may cause embolism in two ways: (i) Homogeneous nucleation, the spontaneous formation of a water vapour bubble within the water column due to statistical fluctuations. (ii) Heterogeneous nucleation, the development of bubbles by air seeding, the drawing of gas present in already embolized conduits through pit membrane pores into functioning conduits.

This contribution deals with the behaviour of gas bubbles caused by heterogeneous nucleation. These bubbles usually contain both water vapour and air and float either freely in the xylem water (which is under tension) or attach themselves to the vessel wall (which is characterised by a constant or varying contact angle). Based on the reversible free energy of this system, we derive conditions for two kinds of equilibria: (a) Mechanical equilibrium (and its stability or instability) between the forces which try to contract and expand bubbles containing air and water-vapour. (b) Equilibrium with respect to the exchange of air particles between the bubble and the surrounding xylem water by dissolution mechanisms.

The results - given as relations between xylem water pressure, bubble radius, bubble particle content and xylem wall contact angle - allow for drawing the following conclusions:

(i) Provided that initial bubble radius and bubble particle content remain below certain limits, air seeding may produce floating small bubbles in stable mechanical equilibrium, i.e. air seeding does not necessarily lead to embolism.

(ii) A floating bubble which has established mechanical stability either loses air molecules to the surrounding xylem water (and dissolves eventually completely) or it accumulates air from the xylem water until the bubble bursts or until no more air molecules are available in the xylem water.

(iii) Bubble attachment to the xylem wall shifts the limits of initial bubble radius and bubble particle content allowing stable mechanical equilibrium to lower values. Hence, a bubble that was in stable equilibrium while floating may burst upon attachment and cause embolism.

(iv) A mechanically stable floating bubble which loses particles may remain mechanically stable upon attachment but start to accumulate particles (instead of losing them) until the bubble bursts or until no more air molecules are available in the xylem water.

(v) For any contact angle of the xylem vessel wall, bubble attachment to the vessel wall requires no energy input, i.e. it may occur spontaneously

(vi) If the contact angle varies across the vessel wall, the bubble will show a tendency to move (and enlarge) its contact circle towards regions of higher values of the contact angle.