Agriculture Reference
In-Depth Information
Within this framework the following appear to make major contributions
to the dwarfing effect of dwarfing rootstocks.
.
The limited size of dwarfing rootstock root systems.
In general, the more dwarfing
an apple rootstock the less vigorous it is as an 'unworked' tree and the smaller
its root system. This characteristic is retained when the rootstock is grafted
with a scion. Bane
et al.
(
) excavated the
root systems of mature trees of 'Bramley' and 'Worcester' on various rootstocks
and found that with the more vigorous scion the root systems were larger but
the rank order of root system size of the different rootstocks was the same. This
was also shown, much later, in trials in which rootstock clones ('M.
) and Beakbane and de Wet (
' crosses)
of differing vigour were grafted on each other in all factorial combinations
(Table
'andsomeotherdwarfing
rootstocks may be associated with their low capacity to produce roots. This is
shown even
in vitro
when auxin and carbohydrate are supplied (Webster and
Jones,
.
).Thelimitedsizeoftherootsystemof'M.
), after layering in the nursery or after auxin treatment of hardwood
cuttings (Doud and Carlson,
' show limited initial
root growth following planting compared with those on some more vigorous
rootstocks (Young and Werner,
). Grafted trees on 'M.
). The small root systems of orchard trees
on 'M.
' and other dwarfing rootstocks (Rogers and Vyvyan,
; Coker,
) reflect this inherently limited growth potential
and control scion growth. If a vigorous scion grafted on a dwarfing rootstock
is allowed to develop scion roots into the soil the tree then grows much more
rapidly, showing the predominance of root influence (Rogers and Beakbane,
; Fernandez
et al.
,
).
.
The special anatomical features of dwarfing rootstock roots.
The roots of dwarfing
rootstocks have smaller xylem vessels and less than half as many xylem fibres
as those of more vigorous rootstocks, and also a much higher percentage of
bark and of wood ray tissue per unit of root cross-sectional area (Beakbane and
Thompson
' with more
vigorous scions increased its mean vessel diameter this remained consistently
smaller than that of other, more vigorous, rootstocks grafted with the same
scions.
,
; McKenzie,
). Although grafting 'M.
.
Root system size is reduced by the presence of a dwarfing interstock
. The root systems
of vigorous rootstocks are greatly reduced in size if they are separated from the
scion by a piece of stem, an interstock, of a dwarfing rootstock such as 'M.
',
'M.
' or 'Clark Dwarf' (Swarbrick
et al.
,
; Dana
et al.
,
; Parry and
Rogers,
). The interstock, in these studies, reduced the growth of the root
system more than that of the scion.
.
The size of the root system directly affects scion growth.
Rootstocks can be in-
vigorating as well as dwarfing, as shown by stock-on-stock trials (Table
)
and by comparing trees of cultivars on their own roots (with no graft unions)
with grafted trees. Trees of 'Starking Delicious' on 'M.
.
' rootstock are about
