Our results also suggest that moderate water stress
has a specific effect on root hydraulic conductivity because PEG
favoured water transport without affecting the solute concentra-
tion gradient between the root xylem and the growth medium, i.e.,
PEG did not affect the osmotic gradient, which is the main driving
force for root-water transport in non-transpiring plants (Kramer,1983). The water flow across roots (JV) is dependent on the osmotic pressure gradient between the xylem sap and the external medium(D
p
) and the root hydraulic conductivity (L
P
)(J
V
=
s
L
P
D
p
;
s
is the
overall re
flection coefficient of the root, the value of which is
assumed to be unity) (BassiriRab et al.,1991; Quintero et al.,1999).
Because the water-stressed Olimpia cultivar exhibited greater
water flow after re-watering, without modi fication of the osmotic
gradient, we suggest that moderate water stress enhances root
hydraulic conductivity. This effect has also been observed in