Agriculture Reference
In-Depth Information
above vigorous rootstocks. They found accumulation in the interstock espe-
cially just above the rootstock. Both studies could indicate reduced phloem
transport, that of Dickson and Samuels either greater
P absorption in the
swollen and actively growing interstock tissues or a specific phloem block at
the lower graft union.
.
Transport of carbohydrates from leaves to roots is reduced when the scion is on
a dwarfing rootstock or interstock
. Movement of [
C]-sorbitol (the predominant
transport photoassimilate in apple) from mature leaves of cv. 'Fiesta' to the
root system was greater when the trees were on 'MM.
' or 'MM.
' than
when the root systems were on 'M.
). This was so
even when activity was expressed per unit root weight and so was not just
a consequence of the size of the root system (Table
'or'M.
' (Kamboj,
). This supports the
concept that dwarfing rootstock roots are a less strong sink for assimilates
than those of invigorating rootstocks or that transport to them is restricted
by poorer transport systems or abstraction
en route
. Kamboj's data show the
total
C activity to be higher in the graft union area than above or below
it but this was not the case for specific activity. There was a tendency for
the difference between the above- and below-union levels to be greater with
the dwarfing than with the vigorous rootstocks but not enough to prove a
role for the unions
per se
in the rootstock effect on sorbitol transport. On the
other hand, when
CO
was fed to scion leaves above different interstocks
on 'MM.
.
' dwarfing interstocks had
sixto nine times higher specific activity above the interstock than within or
below it, while trees with the more vigorous 'MM.
' rootstock, trees with 'M.
' and 'M.
' interstocks
had similar specific activity above, within and below these (Young
et al.
,
' and '
).
This indicates a restriction of assimilate movement downwards at the up-
per graft union of a dwarfing interstock. Dana
et al.
(
) found that scion
leaves had higher reducing sugar content when on trees with dwarfing in-
terstocks of 'Clark Dwarf' than when with 'own-stem' interstocks of 'Golden
Delicious'. The concept that the root systems of dwarfing rootstocks have
a limited capacity to make use of assimilates, or that the graft union limits
transport to them, is supported by White (
) who described a physiolog-
ical disorder, Coxdisease, accompanied by high leaf dry matter and sugar
content and low concentrations of major nutrients, which occurs when trees
on dwarfing rootstocks bear too little crop for this to be an effective sink for
carbohydrates.
.
Experimental treatments which interrupt phloem transport can partially mimic dwarf-
ing rootstock effects.
Girdling the bark of apple trees or inverting a ring of bark has
a dwarfing effect until new phloem develops (Sax,
). Even removing a ring
of bark then replacing it in the normal position allows the growth of dormant
buds below the disturbed bark, presumably as a result of interference with
auxin transport. These effects are compatible with the finding that as much
