Agriculture Reference
In-Depth Information
and its value is always negative.
ψ
p
represents forces resulting from external
ψ
t
) and other local pressures, and can be positive or
negative depending on whether the pressure is above or below atmospheric.
Water moves by bulk flow or by diffusion. Within xylem cell walls and
within protoplasts, where there is a continuous liquid phase, bulk flow pre-
dominates. It also seems to be the major mechanism of water movement
through membranes, bulk flow having been shown to be the major mecha-
nism of water movement through artificial membranes having conductivities
similar to higher plant membranes (Boyer,
pressures, i.e. turgor (
).
During daylight hours the main driving force for water uptake is the tension
(negative pressure) developed in the xylem following evaporation of water in
the leaves as a result of the vapour pressure gradient between the internal leaf
surfaces, which are assumed to be fully wet, and the atmosphere. The water
potentials, expressed in bars, are typically
−
to
−
in the external
◦
C
◦
C
atmosphere (at
%RHand
% RH respectively),
−
to
−
in leaves,
−
to
−
in root cells and
−
.
in soil water (Kennedy and Fujii,
). Water, together with dissolved solutes moves up through the tree in a
cohesive stream. The fluxof water through any part of a transpiring plant is
a function of the drop in water potential across that part and the resistance to
flow, i.e.
ψ
−
ψ
)
F
=
(
/
r
(
.
)
ψ
−
ψ
the drop in potential and
r
the resis-
tance to flow in the pathway. For the soil to leaf water fluxof the whole plant
this can be written as
where
F
is transpirational flux,
E
l
=
(
ψ
soil
−
ψ
l
)
/
R
sp
(
.
)
ψ
l
is the
leaf water potential and
R
sp
is the total hydraulic resistance in the soil plant
pathway. Resistance is the reciprocal of conductance.
Anadditionaldrivingforceforwateruptakeandfluxcomesfromtheenergy-
requiring accumulation of solutes, especially ions, in root tissues and their
transfer to the xylem. The xylem sap has a higher concentration of solutes than
the soil solution, with a consequently lower osmotic potential. This results in
a gradient of osmotic potential between the soil solution and the xylem and a
flow of water from the former to the latter. This water movement gives rise to
the phenomenon of root pressure and the 'bleeding' of decapitated stumps.
The water fluxinto and out of cells is also determined by water potential
gradients (equations
ψ
soil
is the soil water potential,
where
E
l
is the transpiration rate,
ψ
s
) is related to cell sap
concentration measured in number of moles of solute. As water potential in the
transpiration stream is lowered water should flow from adjacent tissues into it.
This actually happens, with measurable trunk diameter shrinkage during the
.
,
.
). The osmotic potential (
