When freshly cut segments of naturally decorticated steles of horizontal
roots of the Australian grass tree
(Xanthorrhea preissii) are subjected to gentle suction, bubbles
of gas sometimes appear, as well as liquid, in
capillaries attached to the aspirated ends of the steles. We tested the
hypothesis that this gas comes from vessels
embolized in the intact xylem stream, and that the gas volume extruded
is therefore an effective measure of the
extent of this embolism. To do this, twin samples were taken from individual
roots of X. preissii in the field, one
was fast-frozen intact for subsequent estimation of vessel embolisms in
the cryo-scanning electron microscope
(CSEM), 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
embolized 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 subsequent thawing of the ends under water. The pattern of changes
in embolisms during unstressed
conditions in early summer, shown by both CSEM and aspiration, 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 afternoon. 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 peak 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 morning peak, followed
by apparent repair of these before noon. There was a second spate of embolisms
in late afternoon, followed by
complete refilling of all xylem with liquid by an hour or so after dusk.
All vessels then remained 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 means to test earlier
findings by the other direct method of the CSEM. In a broad context, the
technique should provide new
opportunities for evaluating water relations of the xylem of whole plants.