Agriculture Reference
In-Depth Information
day followed by expansion at night. Such tissue water losses, with potentially
unfavourable reductions in cell turgor, are minimized in, for example, mature
appleleavesbyactiveosmoticadjustmentinwhichthereisactiveaccumulation
of solutes in cells (Lakso,
). This can result from the production of sorbitol,
glucose and fructose from starch (Wang and Stutte,
).
If there is no osmotic adjustment then turgor potential (
ψ
t
) (the cellular
version of
ψ
w
; if there is osmotic
adjustment it changes to a lesser extent. Although changes in
ψ
p
in equation
.
) changes directly with
ψ
w
in themselves
are unlikely to affect cell enzymes or metabolism, changes in
ψ
t
in the guard
cells have a large effect on stomatal aperture, which controls photosynthesis
as well as transpiration, and on cell expansion and growth. The relative water
content (RWC, earlier known as Relative Turgidity), can be measured directly
as the water content of the tissue as a proportion of its water content at full
hydration.
The growth of tissues, e.g. leaves, depends on cell division followed by ir-
reversible cell expansion. The expansion phase of this growth is governed by
equation
.
.
Expansion Growth
=
M
(
P
−
Y
)
(
.
)
where
M
iscellwallextensibilityandistheslopeofthecurverelatingexpansion
to turgor,
P
is the turgor pressure and
is the minimum turgor for growth,
Y
or yield threshold (Taylor and Davies,
). The tissue extensibility is often
described as a process of cell wall loosening and is sometimes referred to as
plasticity.
The leaf water potential (
ψ
l
) which is the usual starting point for plant water
relations studies depends on three factors, the soil water potential (
ψ
soil
), the
resistance of the plant-soil system to the flow of water (
R
sp
) and the rate of
evaporation from the leaves (
E
l
), i.e. equation
.
can be re-written:
ψ
l
=
ψ
soil
−
E
l
R
sp
(
.
)
The key plant factors controlling this are the resistances to flow in the pathway
for liquid water movement and control of the stomatal aperture.
Roots and tree water relations
Water uptake and flow through roots
Apple root systems can explore all the space between the trees to a depth
of at least
) but commonly, especially in
the case of young trees, they explore only a small part of the available
soil volume (Atkinson,
.
m (Hughes and Gandar,
). The rooting density within the exploited soil
