Demonstration Study On Cavitation Ocurrence In Xylem Conduits Using X-ray

Cavitation effect water transport in vascular plant, and thus become a key element to plant physiology and ecology. Massive experiments have demonstrated that cavitation in xylem is caused by air-seeding. Development of an air-seed after its entering a conduit has been analyzed, but different results were derived, both the immediate and gradual processes of bubble growth have been proposed theoretically. For the lacking of appropriate method to investigate cavitation, there has not been any experiment study to support or defend their viewpoints. Recently, three types of cavitation corresponding to three ranges of xylem water pressure were proposed via mechanical analysis by director. The third type of cavitation—after an air-seed enters a conduit it will break immediately to fill the conduit with gas, leaving little water at its two ends, is deemed to be common for daily cavitation.Directed by our theory, the synchrotron X-ray phase contrast microscopy (XPCM) was employed in this study to observe cavitation process in intact leaves of rice and maize. Before XPCM experiment, paraffin section of the different species were done to know the structure of leaves, the initial xylem pressure was estimated,and the time of cavitation occurrence was detected by ultrasonic emission to narrowing the time range of XPCM experiment. Based on these former experiment results, dehydration processes of the samples were recorded to capture cavitation occurrence by acquiring consecutive XPCM images.The immediate cavitation process has been revealed in both rice and maize leaves, which coincides well with the third type of cavitation when the xylem pressure is less than - 3P0 (P0 is atmospheric pressure). The gradual cavitation process has been deduced, while the gradual process is consistent with the last phase of the first and second types when the initial xylem pressure is in the range of- 3P0≤Pl'≤0(Pl' is xylem pressure).The third type is much more than the first and second types, which also consists with our theory. The third type happens more than the first and second types, that are also consistent with the theory. By acquiring XPCM images of different regions before and after hour illumination, more information about the cavitation occurrence in the whole sample is given.