Canny's compensating pressure theory for water transport (American Journal
of Botany 85: 897-909) has evolved from the premise that cavitation pressur
es are only a few tenths of a megapascal negative (approximately -0.3 MPa).
In contradiction. "vulnerability curves" indicate that xylem pressures can
drop below -3 MPa in some species without causing a loss of hydraulic cond
uctivity. Canny claims these curves do not measure the limits to negative p
ressure by cavitation, but rather the limits to the compensating tissue pre
ssure that otherwise quickly refills cavitated conduits. Compensating press
ure is derived from the turgor pressure of the living cells in the tissue.
To test this claim, we compared vulnerability curves of Betula nigra stems
given three treatments: (1) living control, (2) killed in a microwave oven,
and (3) perfused with a -1.5 MPa (10% w/w) mannitol solution. According to
Canny's theory, the microwaved and mannitol curves should show cavitation
and loss of conductance beginning at approximately -0.3 MPa because in both
cases, the turgor pressure would be eliminated or substantially reduced co
mpared to controls. We also tested the refilling capability of nonstressed
stems where compensating pressure would be in full operation and compared t
his with dead stems with no compensating pressure. According to Canny's int
erpretation of vulnerability curves, the living stems should refill within
5 min. Results failed to support the compensating tissue theory because (a)
all vulnerability curves were identical, reaching a -1.5 MPa threshold bef
ore substantial loss of conductance occurred, and (b) killed or living stem
s had equally slow refilling rates showing no significant increase in condu
ctivity after 30 min. In consequence, the cohesion theory remains the most
parsimonious explanation of xylem sap ascent in plants.