ABSTRACT
Many studies have confirmed that cavitation in xylem is caused by air bubbles. Recently Shen et al. (Tree Physiol. 22 (2002) R655), analysed the expansion of a pre-existent bubble in xylem and one formed by air seeding. The present paper makes a further analysis of bubble expansion by the equilibrium criterion of the Helmholtz function. It has been proved that when xylem pressure P'l decreases to a special value P'l* from a value higher than, or equal to, or lower than -Po (Po is atmospheric pressure), an air bubble in xylem can grow up steadily, corresponding to minimums of the Helmholtz function F(r). As soon as P'l is lower than P'l*, since F(r) will be a decreasing function when P'l < P'l*, resulting in non-equilibrium of the bubble, it will break inducing a cavitation event. The analysis is consistent with the results of mechanism. Given P'l > or = -3Po, if an air bubble could enter a conduit, it would be in a stable equilibrium. When P'l < -3Po an air bubble entering a conduit will be in an unstable equilibrium. As the water further vaporizes, it will break at once. This is the case to which the former published formula P'l = -2sigma/rp is applicable.
