Title : Drought-induced cavitation event in xylem
Abstract:
Many studies have confirmed that air bubbles cause drought-induced cavitation in xylem. However, the spread of embolisms throughout plant water transport systems is poorly understood. Cavitation involves air bubble activity in the xylem sap of plants. Understanding their activity, including the radii of bubbles at equilibrium, the stability of equilibrium, and bubble development, should be helpful in understanding the spread of drought induced embolisms. This study reviewed bubble activity. As a result, two types of cavitation were obtained naturally: the growth of those air bubbles, which previously existed in conduits, and air seeding, defined as the sucking of air bubbles from already gas-filled conduits. Along with the drop of the water potential, a pre-existent air bubble will gradually expand to a long-shaped bubble and extend continually, forming an embolism. This is the method: Expanding gradually. When the xylem pressure is moderate, an air bubble in xylem will expand first. Then, as soon as the decreasing pressure reaches a threshold it will blow up to form a bubble in long shape, accompanied by acoustic (or ultra acoustic) emission. It will extend further as xylem pressure decreases continually. This is the second method: Expanding——exploding becoming a long shaped bubble——lengthening by degrees. For air seeding three methods of cavitation have been proposed. After an air seed enters a conduit through a very small hole, it is possible to be a nanobubble. Therefore it activity is like the pre-existed bubble, the cavitation methods of which are the same with the first two methods of pre-existed air bubble except of air reservoirs. In the range of xylem sap pressure P'l ≤−3Po(Po is atmospheric pressure), soon after an air seed is sucked into a conduct it will explode immediately and the conduit will be full of the gas of the bubble instantly. This is the third method of air seeding: Sudden exploding and filling conduit instantly. The third method is the frequent event in daily life of plant. We did experiments so that to confirm this theory. The natural drying processes of the xylem slices of Platycladus orientalist were studied using light microscopy. Three phenomena, corresponding to the three methods, were observed in the experiment. Synchrotron X-ray microscopy was used to capture cavitation events in the intact vessel elements of corn and rice leaves. In the X-ray experiment, gas diffusion at a high rate, which could be caused by an exploding event, was more easily obtained, and it was more difficult to obtain gradual expansion of bubbles. Above experimental phenomena can be well explained by the theory. Finally, our study suggests that for some whole plants, the method of sudden exploding and filling conduit instantly, or the third method of cavitation by air seeding is a priority. Thus, the spread of embolism can be understood.
