Horticulture Principles and Practices

Terminal bud
removed
Unbranched
plant
FIGURE 5–15
Fuller plant
with branching
Axillary
buds grow into
branches
The effect of abolishing apical dominance in plants.
Normal erect
growth
Plant bending
towards light
FIGURE 5–16 Phototropism displayed by a plant
located near a window.
5.6.2 GIBBERELLINS
Gibberellins are produced in the shoot apex and occur also in embryos and cotyledons of
immature seeds and roots. They occur in seed, flowers, germinating seed, and developing
flowers. The highest concentration occurs in immature seeds. This class of hormones
promotes cell division, stem elongation, seed germination (by breaking dormancy), flowering,
and fruit development. In carrot (Daucus carota) and cabbage (Brassica oleracea
var. Capitata), among others, exposure to long days or cold is required to induce flowering
(bolting). Application of gibberellic acid eliminates the need for these environmental
treatments. Gibberellins are noted for their ability to overcome dwarfism in plants,
allowing compact plants to develop to normal heights. Gibberellic acid is used to induce
seedlessness in grapes; the size of seedless grapes is also increased through the application
of this hormone. An example is gibberellic acid (GA 1 ), one of the numerous (more
than seventy) closely related terpenoid compounds that occur naturally.
5.6.3 CYTOKININS
Cytokinins are hormones that stimulate cell division and lateral bud development. They
have been isolated mainly from actively dividing tissue. Cytokinins occur in embryonic
or meristematic organs. Examples of natural cytokinins are isopentenyl adenine (IPA)
and zeatin (Z). Zeatin, isolated from the kernels of corn, is the most active naturally occurring
cytokinin. Kinetin was first isolated from yeast. Benzyl adenine (BA) is also a
commonly used cytokinin. Cytokinins interact with auxins to affect various plant functions.
A high cytokinin-auxin ratio (i.e., low amounts of auxin, especially IAA) promotes
lateral bud development because of reduced apical dominance. Relatively high amounts
of auxin induce root formation in callus. The principal role of cytokinins in plant physiology
is the promotion of cell division. They are important in tissue culture work and are
more effective when IAA is also added. The effect of cytokinins when used this way is
to cause cells to remain meristematic (undifferentiated) in culture, producing large
amounts of callus tissue.
5.6.4 ETHYLENE
Ethylene is a gas found in the tissues of ripening fruits and stem nodes. It promotes fruit
ripening and leaf abscission. In the horticultural industry, ethylene is used to aid in uniform
ripening of apple, pineapple, and banana, and in changing the rind color of fruits (as
in orange and grapefruit from green to yellow and tomato from green to uniform red).
5.6 Plant Hormones 179