Agriculture Reference
In-Depth Information
cm s
−
but have fairly frequently been reported
are usually in the range
.
-
.
cm s
−
and occasionally up to
cm s
−
(Palmer,
in the range
).
One-year-old apple trees subjected to water stress had a higher water use ef-
ficiency (CO
uptake/water loss) than controls at any given vapour pressure
deficit (Flore
et al.
,
.
-
.
.
).
Under orchard conditions water stress effects on photosynthesis per unit leaf
may only occur under severe stress conditions or be very limited. Fanjul (
),
for example, found little relationship between photosynthesis of 'Bramley'
apple leaves and water potential over the range
MPa. Whole-
tree photosynthesis, which incorporates effects of leaf area, shows much more
consistent negative effects of imposed droughting than does individual leaf
photosynthesis (Fernandez
et al.
,
−
.
to
−
.
).
The osmotic adjustment of mature leaves contributes to keeping their stom-
ata open, so enabling photosynthesis to be maintained, while the concomitant
check to shoot growth and new leaf production helps conserve water use.
The maintenance of root growth enables the tree to tap ever greater soil
volumes.
Fruiting simultaneously increases water use through its effects on stomatal
conductance and through reducing root growth. This obviously accentuates
the effect of drought, i.e. potential water stress, on vegetative growth. This may
partly explain the adverse interaction between limitations to water supply and
crop load with respect to fruit size (Goode
et al.
,
b). Effects of differences
in fruiting may also contribute to differences between cultivars in some aspects
of tree water relations (Higgs and Jones,
).
Irrigation
Irrigation is carried out to achieve economic benefit through the reduction of
plant water stress. The amount and frequency of water application is based
on assessment of crop needs, adjusted to take account of inefficiencies in the
irrigation system (e.g. evaporation from open water channels and surface run-
off) and effective natural precipitation, i.e. rainfall adjusted for losses from
surface run-off and drainage.
Maintenance (depletion) irrigation
This is the traditional system based on full replenishment of soil moisture in
the active rooting zone, to field capacity, at timings such that the soil moisture
in the rooting zone is never depleted by more than a given percentage of
available soil water. The objective is to supply sufficient water, while avoiding
run-off or deep percolation or the creation of anaerobic conditions.
