AVOCADO - Tropical Fruits - page 171

from grafting nursery-grown trees to grafting container-grown trees. Seeds
are taken from fruit picked from trees free of sun-blotch and are treated in
a water bath at 49-50°C for 30 min, cooled and surface-dried in a partially
shaded area. Seeds are planted (broad side down) in polyethylene bags, with a
well-draining potting mix. Seeds germinate in about a month. The papery seed
coats should be removed. If seeds are not very fresh, cutting of a piece of seed
tip or doing some vertical cuts at the side of the seed will improve germination
speed and percentage; sometimes a slice of bottom can also be cut of .
Seedlings may be cleft or side-wedge grafted 2-4 weeks after germination.
Scion wood from terminal growth of sun-blotch-free cultivars should be used.
Propagation is usually done in shade houses, preferably with temperature
control if the environment has a wide temperature range. Grafted plants
must be hardened for approximately 2 weeks under full sunlight before fi eld
transplanting. Under open-fi eld conditions in the central coast of Peru, with
13-15°C lowest temperatures in winter, Duarte et al. (1975) reduced by 2
months the time to obtain a grafted plant with seeds sown at the start of
autumn and by 1 month with seeds sown at the end of autumn, by spraying
the seedlings three times with 250 or 500 ppm GA at 2-week intervals,
starting when they had reached 15 cm.
Leafy avocado cuttings of some genotypes under mist consistently rooted
nearly 100% under practically any conditions, while others did not root at
all or rooted with dii culty (Kadman and Ben-Ya'acov, 1970b). Generally,
West Indian cultivars with strong resistance to salinity are dii cult to root.
Cuttings from mature trees were dii cult to root and, in those that rooted
well, they took 4-10 months to root. Cuttings from 1-year-old seedlings show
a higher percentage of rooting in 4-12 weeks. A 50% light intensity in the
intermittent-mist system during the summer is better for rooting than full
sunlight. The anatomy of the avocado stem provides a reason for the dii culty
in rooting as the fi bre bundles and the sclereid ring are thicker in the West
Indian types, intermediate for Guatemalan and hybrid types, and least for the
Mexican cultivars.
Air layers and cuttings have been successfully rooted. Variability exists
in ease of rooting air layers between races and even among cultivars of a
race, and the dii culty of large-scale production has discouraged commercial
development. Generally, Mexican cultivars root most easily, followed by
Guatemalan and West Indian, whether by air layering or by cuttings. Time
required for rooting of air layers ranged from 146 to 518 days, depending
upon cultivar and the time of the year. Etiolated stems show few or no sclereid
connections between fi bre bundles, suggesting that the sclerenchyma ring may
be acting as a barrier to root emergence (Gomez et al. , 1973). Hence, shoots of
most avocado cultivars produced in light do not root nearly as well as shoots
produced in darkness.
Seedlings produced upon scions of the desirable rootstock cultivar are
grafted by tip grafting as close to the base as possible. Shoots of the scion