Agriculture Reference
In-Depth Information
as
% of the bark of the graft union with very dwarfing rootstocks can be
non-conducting phloem (Simons,
).
Conclusion and comments
Itisclearthatthevigourofacompoundtreereflectsthevigourofitscomponent
rootstock and scion. The use of a dwarfing rootstock as a management tool
to control scion vigour is primarily dependent on the reduced vigour of the
root system. This may be inherent, as when the rootstock provides the root
system, or may be induced by limitation of root growth as a result of interstock,
rootstock stem or graft union effects on the supply of auxins, carbohydrates
and other metabolites. The greater check to growth as the length of rootstock
stem or of dwarfing interstock is increased is compatible with abstraction from
the downward fluxes to the root. The main effect of root system size on shoot
growth is probably via supply of cytokinins but may also involve nutrient and
water supply. The graft union
per se
may also restrict upward movement of
water and nutrients.
Pointsofpracticalimportancearisingfromotheraspectsofrootstockcontrol
of tree growth include the following.
When a vigorous scion is grafted on a dwarfing rootstock it must not be
planted with the graft union below soil level. If this is done a vigorous scion
root system may develop and the tree as a whole becomes vigorous.
The limited competitiveness of dwarfing rootstock root systems as sinks
and their independent effect in stimulating precocious fruiting brings the
risk of excessive dwarfing. This makes it important to plant trees which are
already large and well branched and to manage their early fruiting so as
to achieve the best balance between early economic returns and the rapid
growth needed to attain the desired orchard canopy dimensions as soon
as possible.
The effect of a dwarfing interstock on the size of the root system renders
invalid the general concept of the use of a vigorous rootstock to give good
anchorage coupled with a dwarfing interstock to control vigour. It does not
rule out the possibility of such a combination being effective, but only if the
rootstockrootsystemhasspecificcharacteristicswhichimproveanchorage,
including mechanical toughness, not just vigour.
The potential to control tree growth by the use of dwarfing rootstocks,
management of cropping level and appropriate tree training and pruning
has greatly reduced the risks of excess vigour and consequently of excessive
shade in orchards planted at high tree densities to achieve early yields and
economic returns.
