Agriculture Reference
In-Depth Information
sources of cytokinins and gibberellins. Root restriction reduces apple fruit
growth and size (Atkinson
et al.
,
,
; Webster
et al.
,
) as well as
vegetative growth.
Nutrient deficiencies
Rogers and Head (
) attributed very shallow root systems on light sandy
soil at Wisley, where more than three-quarters of the root weight was in the top
cm of soil, to lack of nutrients at greater depths. Head (
b) showed that
in the absence of applied fertilizers, 'Worcester'/'MM.
' apple trees did not
show any root growth in spring and Weller (
) found that adding mineral
fertilizer close to the tree trunk increased root density there at the expense of
root growth elsewhere.
Soil moisture deficits
In general soil moisture appears to be available over the full range from field
capacity to permanent wilting point. Root production and extension tend to
be greater when water is limiting than under conditions of ample water supply
(Jones
et al.
,
). Indeed with other plants the increasing root growth rate
in water-stressed plants is maintained up to the point where the leaf water
potential decreases to a value at which stomata close and net photosynthesis
is zero (Cruz Romero and Ramos,
). This is clearly an adaptive feature
leading to tolerance of dry conditions. However, there is also evidence that
apple and pear root growth is reduced under dry conditions. Rogers (
)
found reduced root growth at soil water potentials of
−
to
−
kPaorlower
and Afensev (
% in summer and autumn
very harmful to root growth. Roots proliferate and concentrate where water
is available but do not specifically grow towards it. Tamasi (
) found soil moisture below
,
) found
that apple trees on 'M.
cm above
the soil water level produced a more superficial and much more dense root
system compared with those on a similar sandy soil with identical cultural
practices but planted
' rootstocks planted in a sandy soil
-
cm above the water table. The roots of the latter
trees did not, in general, reach the soil water table or the layer dampened
by it and relied on precipitation and irrigation. When a limited part of a soil
with apple or pear roots is irrigated the roots proliferate mainly in the wetted
soil. Huguet (
) found that drench irrigation limited rooting to a superficial
zone. Levin
et al.
(
) found that the size of apple tree root systems and
root distribution pattern responded very quickly to changes in irrigation pat-
tern. Trees growing in a dry Mediterranean climate which had been irrigated
by surface irrigation for many years and had widespread root systems adjusted
their roots to the small volume wetted by drip irrigation within one season.
The soil volume wetted by the tricklers was about
,
% of that wetted by
surface irrigation and the density of roots became related to distance from
the tricklers rather than the tree trunk, the highest concentration being within
-
