AVOCADO - Tropical Fruits - page 163

(20°C), there is sui cient overlap of fl ower types to permit self-pollination of
up to 93%. The possibility of self-pollination is enhanced by relatively long
pollen viability. There is also evidence that the stigma remains white and is
still receptive on the second day (Davenport et al. , 1994). The avocado can
be insect-pollinated, although self- or wind pollination is possibly the norm
(Davenport et al. , 1994). Insects, including honey bees, can carry the sticky
pollen on their bodies and are capable of depositing viable pollen from one
fl ower cycle to another within the same cultivar.
In spite of the fact that some studies have shown relatively high rates of
self-pollination, most investigators have advocated interplanting of pollinator
cultivars of the complementary class in orchards, in order to increase fruit set
(Bower, 1981c). In the case of areas with mild climates, such as Michoacan,
México, there is no need for interplanted pollinator cultivars and most
orchards are made up by solid blocks of 'Hass' (Salazar-García, 2000).
Some cultivars are more ef ective in increasing fruit set than others.
This may indicate the presence of varying degrees of incompatibility among
cultivars, with some complementary cultivars superior to others. Mexican
cultivars are more ei cient pollen providers in California and Israel, while
Guatemalan cultivars are more ef ective in Florida, because of greater cold
tolerance of pollen improving the chances in the fertilization process (Gazit,
1976). Under warmer conditions, West Indian hybrids are good as pollen
donors, probably due to their greater tolerance of higher temperatures.
However, fruit set may be a poor measure of fi nal matured fruit, as all cultivars
lose fl owers and developing fruitlets, regardless of pollination.
Following pollination, considerable fruit drop occurs due to poor
pollination and excessive vegetative growth (Wolstenholme and Whiley,
1992). There is probably sui cient carbohydrate available from mature leaves
to support the growth of both developing fruitlets and young leaves (Finazzo
et al. , 1994). This supply during early fruit development does not limit fruitlet
growth or stimulate fruitlet abscission. However, the lack of an adequate
spring shoot growth, though competitive, will mean that fruit set and early
fruit growth will be dependent upon older leaves and stored carbohydrate,
which may be depleted by fl owering. The later vegetative growth fl ush near
harvest is crucial for fi nal fruit growth, build-up of carbohydrates and the
following period of root growth (Fig. 7.3). This is discussed further with
respect to maturity of fruit at harvest and biennial bearing.
Fruit
The avocado fruit is a one-seeded berry (Fig. 7.2). The single large seed is
composed of two cotyledons enclosing an embryo and is surrounded by a thick
fl eshy mesocarp. The skin varies in thickness up to 6 mm, depending upon the
race (Table 7.1) and has 20,000-30,000 stomata per fruit, less than on a leaf.