Agriculture Reference
In-Depth Information
volume is generally low. Hughes and Gandar (
) found root length den-
sities (
L
v
values) in the top metre of soil to average only
cm cm
−
even in the excellent growing conditions of New Zealand. Expressed as m
m
−
ground surface (
L
A
values), the rooting density of apples is generally less
than
.
-
.
×
whereas that of
Gramineae
can be up to
×
(Landsberg and
Jones,
). This low root density is likely to lead to local depletion of soil
moisture and relatively high adverse effects of resistance to water flow in the
soil. The mycorrhizal nature of the roots and the fact that they can proliferate
in moisture-rich soil zones several metres below the surface may compensate.
Apple roots also become concentrated in other areas of high moisture status,
e.g. near trickle irrigation drippers. Levin
et al.
(
) found about three times
as many roots per m
at
or
cm from a trickle line than at a distance of
cm.
There is evidence that some roots preferentially supply water and nutrients
to particular parts of the shoot system. However, experiments with split root
systems of apple showed that when only a quarter of the root system was
supplied with water the transpiration rate was
% of that when all four root
quarters has access to water (West
et al.
,
). Use of tritium-enriched water
showed lateral transport in the tree to be initiated only after appreciable soil
moisture deficits around the unwatered roots had built up.
In the older literature it was often assumed that the absorption of water
occurs entirely through the younger regions of roots, e.g. root tips and the
root hair zone. However, evidence has accumulated that all of the tree roots,
woody as well as white, take up water (Atkinson and Wilson,
). There do,
however, appear to be differences between different types of root. Baxter and
West (
) found that the water flow through intact root systems along a fixed
pressure gradient was two or three times as high per unit root surface area
when the root system included new white roots as when it did not (cf. p.
).
The intact root system has a high resistance to water flow. This resistance is
clearly dependent on membrane or symplast resistance because killing a root
system under water results in a large increase in water uptake at low transpi-
ration rates (Stoker and Weatherley,
). Under
and
kPa imposed
pressure gradients, Baxter and West (
) found that longitudinal resistance
to water flow was very high in apple fibre roots with poorly developed sec-
ondary xylem, very low in main roots and low in stems. The capacity of trunks
to transport water was more than
times that of the intact root systems
under a low (
kPa) applied pressure gradient.
As transpiration rates increase, so do the conductances (reciprocal of re-
sistances) of many living root systems (Stoker and Weatherley,
; Steudle,
). There is some evidence that this is so for apple (Powell,
) but the ma-
jority of reports (Baxter and West,
) show an apparently
constant resistance, with leaf water potential declining linearly as transpiration
; Jones
et al.
