Agriculture Reference
In-Depth Information
This mechanism appears to be much more effective in apple than in Asian
pear.Higgins
et al.
(
)foundthattranspirationofAsianpearleavesincreased
linearly with increases in leaf-to-air vapour pressure difference but transpira-
tion of apple (which showed much greater changes in leaf conductance) was
almost constant over a wide range of vapour pressure differences.
!
In general stomata partially close as windspeed increases (Mansfield and
McAinsh,
). This is attributed to the fact that although wind takes water
vapour away from the leaf surface, it brings CO
towards it which could result
in partial stomatal closure. It also seems that plant hormones (ABA, IAA and
cytokinins) can induce changes in the sensitivity to CO
which act to prevent
excessive transpiration under windy conditions.
Irrigation of apple trees under conditions where water supply is otherwise sub-
optimal results in an increase in transpiration per unit leaf area and in stomatal
conductance (Gowing
et al.
,
).
Leaf conductance can be up to about twice as high in well-watered trees as in
those subject to severe, but not irreversibly damaging, water stress.
; Alleyne
et al.
,
; Fernandez
et al.
,
Control of leaf water potential (
ψ
l
)
During the day, apple and pear leaf water potentials are lower when tran-
spiration rates are higher. Landsberg
et al
.(
) showed a linear negative
relationship (
r
=
.
) applying to both control and droughted trees. This is
largely a result of the high resistances in the soil-leaf continuum. The daily
patterns are very similar over widely different environments, with minimum
ψ
l
values of
−
−
to
.
MPa in Griffith, Australia; East Malling, England
(Figure
). This indicates ef-
fective physiological control, primarily by stomatal control of transpiration.
Irrigation under moderate levels of evaporation does not influence
.
), and Michigan, USA (Fernandez
et al.
,
ψ
l
as
long as the soil moisture potential is above
MPa. When soil moisture
stress in unirrigated plots becomes more negative than this,
−
.
ψ
l
becomes
.
-
MPa more negative than that of irrigated trees under English conditions
(Goode and Higgs,
.
). Fernandez
et al.
(
) found the
ψ
l
of water-stressed
trees to be up to about
.
MPa more negative than that of irrigated trees in
Michigan.
Stomatal closure in dry conditions can reduce the effect of drought on
leaf water potential, especially in younger trees. This stomatal closure may,
unusually, be so effective in reducing water loss that droughted trees have
higher leaf water potentials than irrigated trees at the end of a season ( Jones
et al.
,
).
