Grafted plants are commonly used in top-fruit, grapes, roses and
amenity shrubs with novel shapes and colours. Rootstocks resistant
to soil-borne pests and disease are sometimes used when the desired
cultivars would succumb if grown on their own roots, e.g. grapevines,
tomatoes and cucumbers grown in border soils. Grafting is not usually
attempted in monocotyledons, since they do not produce continuous
areas of secondary cambium tissue suitable for successful graft-unions.
In top fruit, grafting is used for several reasons:
- a grafted plant will establish more quickly than a seedling;
- plants derived from seedlings will show different (usually inferior)
qualities of fruiting compared with their commercially useful parent
plants so a means of vegetative propagation is advantageous; the
cultivars are, therefore, clones derived from one original parent;
- to control the size of the tree through the choice of dwarfing rootstock
(see Table 12.1), e.g. the M9 apple rootstock, causes the grafted scion cultivar to be considerably dwarfed. Reduced levels of auxin and cytokinin in the rootstock possibly, bring this about.
|Table 12.1 Fruit rootstock
There are numerous grafting methods that have been developed for
particular plant species. Several principles common to all methods
can be briefly mentioned. Firstly, the scion and stock should be
genetically very similar. Secondly, the scion and stock will need to
have been carefully cut so that their cambial components are able to
come in contact. In this way, there will be a higher likelihood of callus
(resulting from cambial contact), which quickly leads to graft
establishment. Thirdly, the graft union should be sealed with grafting
tape to maintain the graft contact, to prevent drying-out and to keep
out disease organisms such as Botrytis.
Fourthly, the buds on the stem
taken as scion material should, ideally, be dormant (leafy material would
quickly dry out). The rootstock should be starting active growth, and
thus bring water, minerals, and nutrients to the graft area.