Agriculture Reference
In-Depth Information
Hansen (
) observed that the uptake of water by fruiting trees of 'Golden
Delicious' apple was about
% more than that of non-fruiting trees in August
and
timesasgreatinOctober.InlateAugusttheaveragesizeofthestomatal
apertures, measured microscopically, was
.
times as great in the fruiting as
in the non-fruiting trees. Buwalda and Lenz (
.
) found the water uptake
per unit root weight was more than twice as high in fruiting as in non-fruiting
trees of apple cvs. 'Golden Delicious', 'Cox's Orange' and 'Gloster'. These
crop load effects may involve effects on internal CO
concentrations and the
lower ABA concentrations found in leaves of fruiting than non-fruiting trees.
Giuliani
et al
.(
) found canopy conductances of fruiting trees of 'Smoothee
Golden Delicious'/'Pajam
' to be much higher at all VPD levels than those
of de-fruited trees and fruiting trees to transpire at up to twice the rate of
de-fruited trees.
Warrit
et al.
(
) found that the stomatal conductance of 'Golden Delicious'
apple was not usually affected by leaf temperature over the range
◦
C,
-
provided the humidity gradient was kept constant. West and Gaff (
) found
◦
C and then a progressive pronounced decline
a maximum conductance at
up to
), however, found canopy conductance to
increase with temperature over the range
◦
C. Giuliani
et al.
(
◦
C with a positive linear rela-
-
tionshipatatmosphericVPDofboth
kPa.Thenegativerelationship
between atmospheric VPD and canopy conductance was pronounced at
.
and
.
,
◦
C for fruiting trees but was only shown at
◦
C by non-fruiting
,
and
trees.
−
Stomatal conductance declines as the leaf-to-air vapour pressure difference
increases (Warrit
et al.
,
; Fanjul and Jones,
). The effects are large,
conductances at a
.
kPa vapour pressure difference being only about a
fifth of those at a
). This results
in transpiration initially rising with increases in the vapour pressure gradient
and then declining with increasingly severe vapour pressure deficits as the
effects on stomatal closure outweigh those on evaporation
per se
(West and
Gaff,
.
kPa difference (Fanjul and Jones,
). The stomatal response to changes in atmospheric humidity is
very rapid, most of it occurring within
minute and some within
seconds
(Fanjul and Jones,
). The leaf therefore seems to have some mechanism
for sensing the ambient humidity and triggering a rapid stomatal response. It
is possible that stomata respond to direct cuticular loss of water from guard
and subsidiary cells, not passing through the stomata, which will be directly
related to ambient humidity (Farquhar,
).
