Is cohesion theory correct?

kai liu KRL121 at
Thu Nov 23 15:01:02 EST 1995

Dear netters,
Some people are challenging the cohesion theory. Canny (1995) give a new
theory for ascent of sap. Zimmermann's  pressure proble can't get negative
pressure lower than -0.7 MPa in a high tension plants (Zimmermann, 1994).
However, the pressure chamber yielded 'xylem tension', the negatiev pressure,
is between -1 to -16 MPa. In addition, water potential estimates made with
pressure chamber often do not correspond to those for same tissure made with
a thermocouple psychrometer (Ritchie and Hinchley, 1975). Smith (1994) found
that the negative pressure in bioligical system only can be maintained
at about -1 MPa.
Is the pressure chamber really measure the real negative pressure in
plant xylem? The water in the cell may be squeeze out at high pressure.
Canny (1995) also not beleive pressure chamber measure the reason tension
of xylem (see detail in the article). The pressure proble also have some
problems. When puncture the pressure proble in the vessels, it may cause
caviation in the vessel. Vapor pressure also influence the result.

Recently, I developed a method to measure the negative pressure in the
xylem vessels directly. Using this methods, we can  quantify how much negative
pressure that xylem vessels can be achieved. It can be used to test the cohesion
theory. It also can be used to test whether the pressure chamber
works or not.

I need some professor's support to do this project because I am just a
student come from China. If you have some interest in this topic and
can support a Ph.D student, we can work together. If you known some
prof. have similar interest with me, I am very appreciated getting
some information from you.  My email address is KRL121 at

Canny M.J.A new theory for ascent of sap - Cohesion supported by tissure
pressure. Annals of Botany 75: 343-357. 1995
Smith A.M. Xylem transport and the negative pressure sustaible by water.
Annals of botany 74: 647-651. 1994
Ritchie, G.A., and T.M. Hinckley. The pressure chamber as an instrument for eco
logical research. Advances Ecol. Res. 9: 165-254. 1975
Zimmermann U. et al. Xylem water transport: is the avaible evidence
consistent with the cohesion theory? Plant, cell and enviroment. 17:1169-1181.

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