Agriculture Reference
In-Depth Information
more dwarfing the rootstock the lower the proportion of xylem tissues in the
roots (Beakbane and Thompson
; Rogers and Beakbane
; McKenzie
). Moreover, although there are some anomalies, dwarfing rootstocks have
fewer vessels and these are smaller. Vigorous rootstocks may have
%ofthe
xylem tissue composed of vessels whereas the roots of the same age (
-
years)
from dwarf trees have only
% (Rogers and Beakbane,
). The mean cross-
sectional area of vessels of unworked 'M.
' rootstock roots was only
-
%
of that of more vigorous rootstocks in one study (McKenzie,
). Scions
grafted on 'M.
' rootstocks have somewhat smaller vessels than when on more
invigorating rootstocks (McKenzie,
).
Fruiting trees have smaller xylem vessels than deblossomed ones (McKenzie,
). The effects of some rootstocks on precocity and on relative fruitfulness
are therefore likely to affect the resistance to flow of their root systems.
Anatomical differences between graft unions of different stock-scion com-
binations and the high proportion of non-functional xylem in graft unions
involving some dwarfing rootstocks were discussed earlier (pp.
-
).
Root signals controlling leaf growth and
stomatal behaviour
Roots appear to control plant water use and plant water status by sensing
soil water status and sending a chemical signal to the shoots when the soil is
dry. In a split-root experiment on apple, Gowing et al. (
) subjected half
of the roots to soil drying until leaf initiation and expansion in the shoots
was inhibited. Excision of the roots in the dry soil resulted in a recovery of
leaf growth rate, the effect being almost as large as that of rewatering them
(Figure
). When roots are in contact with drying soil they produce ABA
in increased quantities. The ABA enters the xylem and is transported to the
leaves. This can inhibit stomatal opening even before the shortage of soil mois-
ture causes any measurable change in the water status of the leaves (Davies
and Zhang,
.
) and so reduce transpira-
tion and water stress. ABA may also increase the permeability of roots to
water, hence their hydraulic conductivity, and may limit the growth of shoots
and enhance that of roots at low water potentials (Mansfield and McAinsh,
; Mansfield and McAinsh,
).
Drought stress generally reduces the production and transport of cytokinins
from roots. This can be expected to reduce stomatal apertures and transpira-
tion (Davies and Zhang,
).
The fruit tree root system is fairly sparse and is distributed through large soil
volumes with varying water status. The roots are therefore subject to different
soil water potentials, especially under drying conditions. Shoots are therefore
likely to receive steadily increasing signals of moisture deficit and to adapt in
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