Agriculture Reference
In-Depth Information
rootstocks may show differences in leaf nutrient content these are not system-
atically related to rootstock vigour. It is therefore unlikely that differences in
nutrient uptake are generally responsible for rootstock effects on scion growth.
There may be exceptions under conditions of very low fertility: Ruck and Bolas
(
) found the difference in growth between 'Crab C' (vigorous) and 'M.
'
(dwarfing) to be much greater when in sand culture with
ppm N than with
ppm N.
.
Apple trees on dwarfing rootstocks may suffer more from water stress
. Olien and
Lakso (
) found that the mid-day stem water potential of 'Empire' apple
trees on 'M.
' was significantly corre-
lated with tree size. They attributed the greater water stress in the trees on
dwarfing rootstocks to a greater root resistance at the root surface and in the
xylem. Atkinson
et al.
(
', 'M.
', 'M.
', 'MM.
' and 'M.
), working with 'Queen Cox' on 'M.
', 'M.
' and
'MM.
', found that the hydraulic resistances of the rootstock shanks, root-
stock graft unions and the scions worked on the different rootstocks were posi-
tively related to the rootstock's capacity to dwarf. Trees on dwarfing rootstocks
also crop more heavily in relation to their size and, in general, heavy cropping
is accompanied by higher stomatal conductance and greater water use ( Jones
et al.
,
). These effects on water use and water stress could contribute to the
effects of some dwarfing rootstocks on scion growth. Also the apple rootstock
'MM.
', generally classed as semi-dwarfing, gives much less vigorous scion
trees on dry than on humid sites in comparison with other rootstocks (Parry,
).
.
Cytokinin supply from roots to shoots is a function of rootstock vigour.
The xylem
sap from decapitated apple root systems contains cytokinins which promote
stem elongation and leaf production in isolated apple shoots ( Jones,
) and
are thought largely to control the development of the shoot system in intact
trees (Lockard and Schneider,
). Kamboj (
) and Kamboj
et al.
(
a,
zeatin riboside) in the shoot
xylem sap of both unworked rootstock trees and of 'Fiesta' scions grafted on
rootstocks increased with the vigour of the rootstock (Table
b) showed that the levels of cytokinin (zeatin
+
). It seems likely
that differences in cytokinin production play a major role in causing the effects
of rootstock root systems on scion growth. Cytokinin in the presence of auxin
stimulates early stages of vascular differentiation and increases the sensitivity
of tissues to auxin stimulation (Aloni,
.
).
.
Graft unions between dwarfing rootstocks and scions frequently show unusual fea-
tures of both xylem and phloem
. It is usual for apple graft unions on dwarfing
rootstocks to be swollen (Mosse,
) gives many
examples of poor unions, with restricted vascular connections, which he at-
tributes to bud quality, stage of development or particular tissue development
at the time the rootstock was budded, usually involving dwarfing rootstocks
with much slower growth rates than the scions budded or grafted on them.
; Jones,
). Simons (
