Agriculture Reference
In-Depth Information
meristematic activity, are in close proximity. The two components must
have the correct polarity, the morphologically proximal end of the
scionwood piece, i.e. its basal end, being grafted on to the distal end of the
rootstock stem, or, if it is grafted on to a root, to the end of this furthest
from the root tip. The graft has to be secured by wrapping, tying, nailing or
wedging. This is to ensure that the scion and stock cannot move relative to
one another and dislodge the interlocking parenchyma cells that develop
from the cut surfaces. The positioning of the rootstock and scionwood
in some of the most widely used techniques of grafting and budding are
shown in Figures
.
It is essential that the cut surfaces of stock and scion which are placed
together do not dry out. Exposure to air at below the saturation point
inhibits callus formation (Shippy,
.
and
.
) because the parenchyma cells of the
callus tissue are thin-walled and cannot resist desiccation. Speedy grafting
or budding reduces the time over which the moist surfaces are exposed
to air. Skilled grafters can average
whip-and-tongue grafts per hour
and budders
buddings per hour when working in a team with a tyer
(Garner,
). Shippy concluded that liquid moisture present as a film
enclosing the cutting, such as is supplied by surrounding it with moderately
moist peat, sphagnum or sand, provides the most favourable conditions
for bringing about uniform callusing. Garner (
), however, warns that
deliberate wetting of the cut surfaces of either stock or scion in the budding
or grafting process may prove detrimental. Where the plants have been
bench-grafted, i.e. dormant scionwood grafted in winter on to rootstocks
which have been lifted and cold-stored, adequate humidity is maintained
by storing the grafted plants in callusing beds or boxes containing moist
peat, sphagnum moss or sand. In countries with mild winters grafting is
done out of doors, as is budding later in the year. Desiccation of these
unions is prevented by sealing over grafts with wax, by smearing heavy-
grade petroleum jelly over buds or by using sticky, watertight plastic tapes
to seal as well as tie.
The cutting of the stock and scion results in the death of at least one
cell layer at each surface. This necrotic material disappears or remains in
pockets between newly formed living parenchyma cells.
Parenchymatous callus cells proliferate, mainly from the primary cortexof
the scion and from secondary phloem of the stock and scion. The spongy
callus cells mingle and growth pressure compresses them until cells from
stock and scion become indistinguishable. The actual cambial layer takes
little part at this stage (Sass,
).
Parenchyma cells at the edges of the newly formed callus mass which are
touching the cambial cells of the stock and scion differentiate into new
cambium cells within
-
weeks after grafting. This cambial formation
