Page 261 - Biology_F5

Biology for Advanced Level Secondary Schools

a certain function). For example, auxins (i) They stimulate development of fruits
are known as growth promoters, since without fertilisation (parthenocarpy).
they influence plant growth and assist in (j) They inhibit abscission in leaves and
producing a phototropic response which fruits.
results into growth. Sometimes, auxins
and gibberellins act together to bring about Application of synthetic auxins in crop
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cell elongation. This is called synergism. production
Alternatively, two plant hormones may Auxins can be synthesised for agricultural
work opposing one another, like auxins, use. It is a proven fact that synthetic auxins
which induces apical dominance, while are more useful and cheaper than naturally
cytokinins prevent it. This is known as produced auxins. Synthetic auxins play
antagonism. the following roles in agriculture:

Auxins (a) Promote fruit setting
Auxins are chemical compounds produced The synthetic auxins such as Indole Butyric
in the root and shoot apices of the plants. Acid (IBA) and Naphthalene Acetic Acid
One of their common forms is Indole Acetic (NAA) are used to promote fruit setting.
Acid (IAA). Auxins have the following role Fruit setting involves a series of changes
in plants: taking place after fertilisation in the ovary,
(a) They influence cell enlargement, bud resulting into development of a fruit.
formation, and root initiation. Auxins can be used for setting some fruits
such as tomatoes and pepper, resulting into
(b) They facilitate production of phototropic production of fruits without fertilisation.
response. NAA can also be used to control pre-

(c) Auxins together with cytokinins control harvest fruit drop and fruit thinning in
the growth of stems, roots, and fruits. various crops such as apples.
(d) They affect cell elongation by altering
cell wall plasticity. (b) Promote rooting in cuttings

(e) They stimulate cambium of meristematic Auxins (IBA and NAA) are very effective
cells to develop stems, leaves and flower in stimulating root development from stem
buds, while in stems, they differentiate cuttings. This is commonly applied in crops
secondary xylem. that are propagated through stem cutting,
(f) They inhibit the growth of lateral buds, such as cassava. This form of asexual
reproduction ensures that no change can
hence they promote apical dominance. occur in the genetic make up of the plants.
(g) They promote lateral and adventitious
root development and growth. (c) Weed killers

(h) They regulate specific protein synthesis Phenoxyacetic acid is used as a selective
in seeds as they develop within the flower weed killer as it kills broad leaf plant
after pollination. species, especially dicotyledons; hence it is

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