AVOCADO - Tropical Fruits - page 172

are allowed to grow and then cut back to near the base. When new buds
show signs of growth, the entire plant is placed in a dark room, with the
temperatures maintained at 21-23°C. When the new shoots reach about
8-10 cm in the dark, the plants are again placed in light, with a tar-paper
collar placed around each etiolated stem and fi lled with vermiculite to
continue exclusion of light from the base of the shoots. Only the tips of the
shoots are exposed to light, in order to produce green leaves. This procedure
is done under shade to prevent sunburn. Shoots are then allowed to grow
until several leaves have matured. The collar is then removed and shoots are
detached for rooting in propagation frames. Rooting hormones have shown no
benefi cial ef ects, although some reports from Australia indicate that a 360 o
scrape of the etiolated shoot prior to the application of the potassium salt
of indole butyric acid consistently gave a crown of roots. Rooted shoots are
transplanted into 10 cm peat pots and grown in an enclosed area for further
root and top growth and gradually hardened. These rooted plants are then
transplanted into larger polyethylene bags for more growth and hardening,
then grafted when they attain appropriate size. The side-wedge technique is
usually used, so some terminal foliage on the rootstock is retained until scion
growth begins.
In California, South Africa and to a lesser degree in Spain and Chile, clonal
rootstocks are used and propagated based on the double-grafting-etiolation
method. A large avocado seed such as 'Lula' is grown in a container to serve
as a nurse stock. When the nurse seedling that was sown into a planting sleeve
is large enough for grafting, a splice or cleft graft with a tolerant rootstock is
made. A metal girdle is loosely put above the bud or graft union so that it will
eventually girdle the stem; the grafted rootstock is forced to root above the
girdle (Brokaw, 1975). The nurse seed with the grafted tolerant rootstock is
then grown in the dark till the graft is 20-40 cm tall. The container is then
removed from the dark, and sterile rooting medium is supplied to cover the
base of the bud union. Adventitious roots develop from the tolerant rootstock.
Once the rootstock growth hardens of , it is grafted with the desirable scion
variety and grown for 2-3 months. The double-grafted plant is then grown
for several months before planting. Rooting hormone or minor cuts near the
bud union between the tolerant rootstock and the nurse seed can enhance
the rooting success of the rootstock. Once in the fi eld it is assumed that the
rootstock will extend its roots and the nurse seed will be excluded from the
system due to the girdle. In South Africa, an improvement to this method
was developed (de Villiers and Ernst, 2007). This consists of positioning
a 55 ml micro-container on the etiolated shoots coming from the grafted
rootstock. Roots will form in these containers, and after grafting the desired
cultivar on to these shoots, they will be separated from the nurse seedling just
above the nurse graft and taken to the nursery for hardening; after this they
will be transplanted into nursery containers or bags. The advantages of this
improvement are that more than one plant can be obtained from a nurse seed;