Studies of vegetative propagation of the lychee (Litchi chinensis ...

Vegetative propagation of the Iychee (Litchi chinensis Sonn.)
contents. The contribution of the ray cells to the
callus could also not be ascertained.
Once the callus was formed, cell differentiation
within it proceeded rapidly. A bridging callus from
a Brewster stock 64 days after grafting is shown in
Fig. 7. By the 64th day, vessels and xylem ray cells
were well developed in the callus (Fig. 8). Other
tissues such as sclereids (Fig. 9) developed later.
The differentiation of the bridging callus into
permanent tissues in lychee corroborated the
findings of Mendel (1936) who observed that in
citrus, the primary callus is not a transitory tissue
which is destroyed but is transformed through
differentiation into permanent tissue. Cambium
regeneration in all the grafts occurred between the
74th and the 115th day after grafting. The determination
of the exact time when the cambium
differentiation started could not be ascertained due
to the insufficiency of successful unions for
sectioning.
Within this period (74th to the 115th day) the
two callus projections, one from each side of the
bark (Fig. 10) grew actively and eventually met.
Pith cells (Fig. 10) did not contribute to the graft
union. By the 115th day the healing process was
completed and the cambium regenerated (Figs
11 and 12). The extent of callus development in
Kwai Mi and Brewster side-wedge grafts harvested
115 days after grafting is also shown in Figs 13,
14 and 15.
In all cases callus was contributed by stock and
scion but the former contributed a greater portion
towards the eventual healing of the wound in the
case of side-wedge grafts. In the case of approachgrafts
callus contribution seemed to depend upon
the cultivars used. Where a fast growing cultivar
such as Brewster was approach-grafted to a slow
grower such as Hak Ip, the greater contribution
was made by the fast grower.
Discussion
The results of the various experiments showed a
cultivar trend in the three subjects. A few cuttings
from both girdled and non-girdled branches of
Kwai Mi and Brewster from Kona and Poamoho
rooted, whereas none of the Hak Ip cuttings
rooted in any of the experiments. As pointed out
earlier Hak Ip is the slowest grower among the
three cultivars. The slow growth rate may be due
to slow differentiation of tissues in the stem and
Fig. 7. Transverse section through a 64-day-old callus
tissue (ct) from Kwai Mi scion approach-grafted to
Brewster rootstock (R) (x 38).
Fig. 8. Detailed structure ofthe callus tissue (ct) from the
Kwai Mi scion showing vessels (U) (X 136).
Fig. 9. Further detailed structure ofthe callus tissue showing
well-developed sclereids (m) (x 635).
branches coupled perhaps with other factors not as
yet determined. Histology of the graft union
between Hak Ip and the other two cultivars also
revealed the slow rate of proliferation of callus
tissue, a condition which may explain the rooting
failures.