The cohesion-tension theory is widely accepted as the best explanation for how water moves in plants. According to this theory, substantial negative pressures often exist in the xylem, the hydraulic systems of plants. However dissolved atmospheric gas has been shown to exist in xylem sap, and at high concentrations this could pose a threat to water transport. With diurnal temperature fluctuations, gas supersaturation of sap may occur at mid-day, potentially causing bubble formation and embolisms, and ultimately hydraulic failure. Two Southern California Chaparral species were studied; Malosma laurina and Heteromeles arbutifolia. Both are evergreen shrubs, but they differ in their vulnerability to the spread of embolism. Malosma is deeply rooted with long, wide xylem vessels, making it much more vulnerable than Heteromeles which has shallower roots and shorter, narrower vessels. To investigate the relationship between dissolved atmospheric gas and daily temperature ranges, xylem sap samples were collected at 6 hour intervals within 24 hour periods, and analyzed using membrane inlet mass spectrometry for concentrations of dissolved argon, which was used as a proxy for atmospheric gas concentrations. Diurnal stem hydraulic conductance measurements were also performed, to determine whether embolism formation and/or repair occur diurnally.
For both species, gas supersaturation of the xylem sap occurred during periods of sharp temperature increase and at maximum daily temperatures. This relationship was independent of species and season, and may be a common occurrence in all plants. Stem hydraulic conductance showed little change over the course of a day, indicating that no diurnal embolism formation or repair occurred in either of the two species. These results provide some of the first insight into the relationship between dissolved atmospheric gas and sap supersaturation, and spark further questions as to how plants are able to transport water under tension when sap is saturated or super-saturated with dissolved gas. Logically it would seem that when sap is at levels of supersaturation and gas comes out of solution, embolisms should form within the negative pressure system. The findings suggest that there may be mechanisms within the xylem that help prevent the formation of embolisms from dissolved gas, especially if gas supersaturation of sap is as prevalent in all species as it is in the species that have been examined. Analyses supporting this research were performed at LLNL under Contract DE-AC52-07NA27344.