Jeffrey Kirby ez043438 at
Sun Mar 8 00:30:49 EST 1998

Bernard Brennan (brennanb at wrote:
: Is it true that guttation is triggered by a fall in temperature of the
: leaf, if so what is the mechanism/s responsible and what is the benefit
: derived by the plant from this response.

At night stomata are closed and transpiration is shut off. Ion
transporters are still active and transporting ions from the surrounding
soils (in solution) into the xylem in the roots. Without the transpiration
stream to pull the water in the xylem up to the leaves and dilute out the
root xylem (movement of ions by bulk flow), osmostic water potential
builds in the root xylem. Water moves down the potential into the root
xylem and results in root pressure. Since xylem is lignified it doesn't
stretch much and the root pressure actually pushes water up the xylem
column, overcoming any cavitations in the xylem elements and then pushing
water out the leaves via hydathodes (specialized pores). That's guttation. 
Not all plants have hydathodes and not all plants guttate. The classic
example of guttation is grass in the morning - it's not dew from the air
on the grass but water from guttation.

Benefits? I suppose it prevents the plant from going boom while allowing
the plant to collect nutrients in non-transpirational circumstances.


-----------------------------                                      -----
        Jeffrey A. Kirby                         jakirby at


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