Quantification of vessel embolisms by direct observation: a comparison of two methods

When freshly cut segments of naturally decorticated steles of horizontal ro ots of the Australian grass tree (Xanthorrhea preissii) are subjected to ge ntle suction, bubbles of gas sometimes appear, as well as liquid, in capill aries attached to the aspirated ends of the steles. We tested the hypothesi s that this gas comes from vessels embolized in the intact xylem stream, an d that the gas volume extruded is therefore an effective measure of the ext ent of this embolism. To do this, twin samples were taken from individual r oots of X. preissii in the field, one was fast-frozen intact for subsequent estimation of vessel embolisms in the cryo-scanning electron microscope (C SEM), the other 15-cm segment rapidly assessed for volume of gas aspirated into a standard micropipette tube. The two measures mutually confirmed one another by showing a strong positive correlation between numbers of emboliz ed Vessels and extracted gas volume. Similar gas volumes were obtained from replicate root segments excised directly from a root when the ends of the segment were frozen before excision, and aspiration conducted after subsequ ent thawing of the ends under water. The pattern of changes in embolisms du ring unstressed conditions in early summer, shown by both CSEM and aspirati on, indicated almost no embolisms before dawn, followed by a rapid rise to a peak in mid morning, than a progressive loss of embolisms in late afterno on. It was also shown that the amount of embolism did not change with time after excision of the roots up to at least 30 min. A comparison of changes in leaf transpiration with gas volumes in steles during a 24-h cycle at pea k transpiration stress in mid summer showed rapid rates of transpiration in early morning and late afternoon, with an intervening period of low water loss during the rest of the day. Numbers of embolisms rose to an early morn ing peak, followed by apparent repair of these before noon. There was a sec ond spate of embolisms in late afternoon, followed by complete refilling of all xylem with liquid bq; an hour or so after dusk. All vessels then remai ned fully recharged until the following dawn. We believe that aspiration is a direct and reliable technique, which offers a simple, inexpensive means of assessing the relative extent of embolism of vessels in xylem, and a mea ns to test earlier findings bq the other direct method of the CSEM. In a br oad contest, the technique should provide new opportunities for evaluating water relations of the xylem of whole plants.