Embolism Repair and Xylem Tension: Do We Need a Miracle?1

There is widespread agreement that negative hydrostatic pressure makes water transport in the xylem intrinsically vulnerable to cavitation (Pickard, 1981; Zimmermann and Milburn, 1982; Tyree and Sperry, 1989). Xylem hydraulic conductivity is often substantially reduced by cavitation and the subsequent formation of embolized (gas-filled) conduits (Milburn, 1993). To maintain hydraulic capacity, plants must replace embolized vessels, maintain a highly redundant transport system, or repair embolized conduits. The idea that embolized vessels might be restored to their functional state is not new (Pickard, 1989) but has generally been thought to be limited to those plants and/or conditions in which the entire vascular system could be pressurized because of active solute transport by the roots (Tyree et al., 1986; Cochard et al., 1994; Fisher et al., 1997). Recent studies, however, indicate that embolism removal may be concurrent with transpiration (Salleo et al., 1996; Canny, 1997; McCully et al., 1998;Zwieniecki and Holbrook, 1998; McCully, 1999; Tyree et al., 1999), raising the question of how embolized vessels can be refilled while the majority of the water in the xylem remains under tension. Some researchers have viewed the difficulty in reconciling this conflict as evidence that the basic ideas of water transport need to be re-examined (Canny, 1997). We argue that cavitation repair and the presence of xylem tension are not mutually exclusive.