PLANT HORMONES

The growth of plant to a great extent is regulated by hormones. This chemical substance influences the plant internally while a factor such as light temperature water and gravity influences the plant externally. They are produced or manufactures in the apical meristem of shoots         and roots, young growing leaves, developing seeds and fruits and diffuse to other part to produce some special effects.

There are about five hormones produced by plant but three of them are very important. They are the auxins, gibberellins and cytokinin.

AUXINS

Among the neutrally occurring auxins that are known is the indole acetic acid (IAA) is the most important. The auxins is produced n the actively growing and developing part such as apexes of shoot and roots , developing leaves ,flowers ,fruits cambial cells from where it is transferred t for short distance by diffusion while for long distance through the phloem tissues .Auxins has been artificially synthesized .an example of synthesized auxins is Dichlorophenocy ethanoic acids (2,4,-D).

Auxins affects the rate of cells division, elongations and differentiation of newly produced cells.

TYPES OF PLANT HORMONES

I    Auxins

ii   GIBBERELLINS– are produced in young foliage leave and roots, a pieces of roots and system as well as in embryo .

iii    CYTOKININS-are produce in the roots, embryo and fruits that are actively undergoing growth. Cytokinins are also growth promoting hormones like auxins and gibberellins.

1. ABSCISSIC ACID: The hormone is produces in matured green learned and fruits. It differs from auxin, gibberellins and cytokinin in being a growth INHIBITOR. It induces dormancy in bud and short such as under unfavorable condition of weather as well as playing a role in leaf and fruit fall.

ethylene (ETHENE): The hormone produces hydrocarbon in young shoots and fruit the hormone stimulates the ripening of fruit and suppress the development of internal buds when activated by auxin

Functions of Auxins

PHOTOTROPISM of shoot: when light shines  on one side of the shoot of a plant, the auxin on that side is reduced and consequently the growth of that side slows down. The  side receives more auxins resulting in faster growth. This unequal growth produces bending of the shoot towards the side receiving light.

1. Inhibition of lateral buds growth auxins produced in the terminal bud stimulate produced in the terminal bud stimulate the growth or elongation of the main stem while at the same time move downward to the stem to inhibit the growth of lateral bud.

iii. FRUITS DEVELOPMENT: After fertilization auxin are produced and sent to the ovary causing rapid development of the ovary as it develops into fruit.

iv.PREVENT FORMATION OF ABSCISSION LAYER– the presence of auxin at the base of flower stalk will help to fruit to reach ripening stage.

reduction of auxin at the base of petiole will allow formation of abscission layer will allow shedding of leaves

1. `PATHENOCARPY– Auxins and other plant hormones induce pathenocarpy i.e fruiting without fertilization.

vii.-Auxins break dormancy in seeds and and ensure their early germination

Function of gibberellins

1. It promotes flowering in some plants
2. It stimulates mitosis in meristematic and in embryo during germination.

iii.        It helps to retard the onset of aging in leaves by maintaining protein and nucleic acid synthetic

1. It increases resistance of some plants to harmful effects such as viral infection, radiation and low temperature.

MODERN APPLICATION OF AUXIN IN AGRICULTURE

1. The synthetic auxin commonly known as 2,4-D is used as a selective weed killer (herbicide) to control weeds.

ii.Cytokines which prevent the aging process is used to store and preserve vegetables

iii.Naphthalene acetic acid (NAA) is a synthetic auxin is used to stimulate fruit production in flowering fruit trees. In some species it causes the flower to produce fruit without fertilization, a phenomenon called pathenocarpy.

1. Yam and potatoes have preserved for a long time by the application of auxin to tubers.
2. Synthetic auxin is used to seal up cut area of plant after grafting by inducing new tissues formation
3. Auxin are used as rooting powders which are applied to the ends if cutting to induce root formation.