Agriculture Reference
In-Depth Information
weeks of inflorescence emergence the female fruits re-orientate themselves from
pointing downwards to pointing upwards. This negatively geotropic response may
possibly be auxin-mediated, and explains the curvature of the banana fruits (com-
mercially called 'fingers'). Male flowers remain tightly packed in the bell together
with their bracts for the entire bunch life, although in commercial practice the bell
is usually broken off to prevent further meristem growth and elongation of the pe-
duncle axis. Hermaphrodite flowers situated between female flowers and male bell,
usually abscise at their base, leaving a callus scar on the peduncle axis but, in some
cultivars, they remain attached to the peduncle until harvest.
The number of hands per bunch and fingers per hand is determined at flower
initiation by the number of female flowers laid down on the transformed meristem.
This in turn is controlled by: (a) genome group, (b) crop cycle, (c) temperature, (d)
vigour of the plant and (e) the level of management. For AAA Cavendish bananas
(which comprise 95 % of all commercial bananas in the world) in the subtropics,
flower initiation during cool temperatures can result in as few as 5-6 hands whereas
flower initiation on ratoon plants in summer can produce 16 hands or more. Simi-
larly, the number of individual fruits per hand, also determined at flower differentia-
tion, can vary from as few as 10 under stress conditions, to over 30 under optimal
growing conditions. The eventual size attained by each finger is a function of condi-
tions prevailing after flower initiation, and this is determined by temperature, leaf
number and leaf area during bunch development, soil fertility and water supply, as
well as the stage of maturity at harvest. As a result of these variations, the mass of a
mature Cavendish bunch can vary from 15 to 70 kg in weight.
Fruit Development
The individual banana fruit, despite originating from an inferior ovary, can be bo-
tanically characterized as a berry with a pericarp. The exocarp is composed of the
epidermis and an aerenchyma layer, the mesocarp forms the pulp and the endocarp
is limited to the inner epithelium adjacent to the ovarian cavity. In contrast with
wild seeded bananas, edible bananas are parthenocarpic developing a mass of ed-
ible pulp without pollination. Most of the pulp develops from the outer edge of the
three locules (inner face of the peel where the vascular bundles are situated) pres-
ent in the ovarian cavity. Pulp parenchyma also develops from the placental septa.
Starch grains are deposited initially in these pulp cells that form in the vicinity of
the vascular bundles, but thereafter starch deposition moves centripetally and con-
tinues until fruit maturity. Despite the early shrivelling of the ovules, they may be
recognized in the mature fruit as minute brown flecks embedded in the edible pulp
adjoining the central fruit axis.
The seedless nature of most banana fruits is attributed to specific female steril-
ity genes and a lack of pollen due to triploidy, but differences between genetically
close cultivars can be observed. Thus, while AAA Cavendish subgroup cultivars
are highly female sterile, and cannot normally be pollinated successfully, the AAA
'Gros Michel' cultivar gives 1 or 2 seeds per bunch if pollinated with pollen from
