Agriculture Reference
In-Depth Information
concentration of the cell sap raises its osmotic water potential enough for
cell sap water potential to be in equilibrium with that of the transpiration
stream. In this new equilibrium, attained by 'passive osmotic adjustment', the
cells inevitably have lower turgor than before (equation
).
In apple a second type of osmotic adjustment may take place in response to
water stress: the cell osmotic potential may rise over and above that due to the
concentration of sap by water loss. This 'active osmotic adjustment' can be
calculated following measurement of the osmotic potential of expressed sap
(Wang and Stutte,
.
). Carbohydrates appear to be the primary osmotic
solutes that change with increasing water stress.
Sorbitol concentration increases linearly with increasing drought stress as
measured by increasingly negative leaf water potentials. Sucrose and, espe-
cially, starch content declines. Sorbitol is a very effective osmoticum. It has
a small molecule (it is a
-carbon alcohol) so that it has a greater effect on
osmotic potential per unit mass than compounds with larger molecules, and it
has little effect on cell metabolism. The amount of active osmotic adjustment
in mature leaves can be as much as
.
MPa (Lakso,
), although in many
circumstances it is much lower.
This increase in osmotic potential results in an increase in leaf turgor po-
tential at any given level of
), for example, found
that the water potential at which there was zero turgor was
ψ
w
. Fanjul and Rosher (
MPa more
negative in field-grown trees grown under stress-inducing conditions than in
those irrigated. At a typical 'stress'
.
MPa turgor potential of leaves
on trees grown under drought conditions was about
ψ
w
of
−
.
.
MPa while that on
trees which had not osmotically adjusted was about
.
MPa.
Control of leaf growth
Leaf expansion is a linear function of
ψ
p
(Figure
.
), being negligible at
ψ
p
=
.
ψ
p
=
.
MPa and increasing steadily up to
MPa (Davies and Lakso,
). Young leaves and shoot tips do not show active osmotic adjustment, as
the season progresses, in the same way that mature leaves do (Figure
). As
a consequence shoot and leaf growth is checked under water stress conditions
which have little or no effect on stomatal apertures of adapted mature leaves.
.
Fruit water relations
Fruits have a very high content of osmotica and show increasingly negative
osmoticpotentialsastheseasonprogresses.Thisenablesmaintenanceofturgor
potential in the face of declining water potential (Mills
et al.
,
). Fruit water
potential itself is generally much higher than leaf water potential, maximum
