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membrane and tonoplast. Stomatal opening reflects a net accumulation of
K + into guard cells, stomatal closing a net loss of K + . ABA both inhibits the
inward movement of K + and activates its efflux(Mansfield and McAinsh,
). It appears that ABA need not enter the cytosol of the guard cells to
induce these fluxes. The guard cells seem to have two sites of ABA percep-
tion involved in the regulation of stomatal aperture, one located at the plasma
membrane and one intracellularly (Leung and Giraudat,
).
a) found that spray application of ABA to apple trees
resulted in a very large but short-lived reduction in stomatal conductance
and a corresponding reduction in transpiration. The leaf water potential was
increased by
Goode et al. (
bar in both unirrigated and trickle-irrigated trees but the
effects were of too short duration to be of practical value.
The ABA concentration in seedling apple leaves can be increased several-
fold by imposed water stress (Landsberg and Jones,
-
). Fernandez et al.
(
) found increased ABA in the leaves of droughted trees of 'Imperial Gala'
apple, especially when on 'M.
EMLA' rootstock.
It is noteworthy that ABA is produced in leaves as well as roots and the ABA
content of excised leaves of a number of plant species increases at critical levels
of water deficit (Davies et al. ,
).
With other plants it has been found that the inhibitory action of ABA can
be overcome by IAA and by the cytokinins kinetin and zeatin. Paradoxically
synthetic auxins, e.g. NAA, applied as foliar sprays reduce stomatal CO con-
ductance and transpiration of apple (Stopar et al. ,
) as well as other plants.
ψ l )
In general stomata are insensitive to reduction in water potential until a thresh-
old is passed, then they close rapidly and almost completely. In apple the
threshold is usually between
!
(
.
and
.
MPa (West and Gaff,
; Warrit
et al. ,
, Atkinson et al. ,
). Landsberg et al. (
), however, concluded
that stomatal conductance, k s , decreased at least
% when
ψ l fell below
.
to
.
MPa, and West and Gaff (
) found no effect of changes in leaf water
potential above
MPainCO -free air. The relationship between g l and
ψ l varies between different genotypes (rootstock cultivars) of apple, and also
with leaf age and leaf preconditioning to water deficits (Atkinson et al. ,
.
).
Lakso et al. (
) found that the water potential at which stomata closed (to a
cm s ) was above
conductance of
.
MPa in June but around
MPa
in September in a dry year (Figure
.
). This change was associated with
increasing osmotic potential.
"
The intracellular concentration of CO in the leaf is a major factor controlling
stomatal aperture. In controlled environment studies, Warrit et al. (
) found
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