Plant Growth Regulators In Horticulture crops

Table of Contents

plant growth regulators in horticulture-

It is a complex organic compound other than nutrients which when applied in minute quantities are able to promote or inhibit growth. Growth harmones are the substances synthesized in particular cells and are transferred to other cells where in extremely small quantities influence development process. Plant hormones affect gene expression and transcription levels, cellular division, and growth. They are naturally produced within plants, though very similar chemicals are produced by fungi and bacteria that can also affect plant growth. Synthetic plant hormones or PGRs are commonly used in a number of different techniques involving plant propagation from cuttings, grafting, micro propagation and tissue culture

Types of plant growth regulators in horticulture crops

Auxin
Gibberellins.
Cytokinins
Abscisic Acid
Ethylene

1-Auxin

Auxins are compounds that influence cell enlargement, bud formation and root initiation. They also promote the production of other hormones and in conjunction with cytokinins, they control the growth of stems, roots, and fruits, and convert stems into flowers. Auxins were the first class of growth regulators discovered.
They affect cell elongation. Auxins act to inhibit the growth of buds lower down the stem and also to promote lateral and adventitious root development and growth. Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. Synthetic auxin herbicides including 2,4-dichlorophenoxyacetic and 2,4,5-T have been developed and used for weed control. Auxins, especially Naphthalene acetic acid and Indole-3-butyric acid are also commonly applied to stimulate root growth when taking cuttings of plants. The most common auxin found in plants is indole-3-acetic acid. In majority of fruit plants fruit drop is controlled by spraying of NAA in different fruit crops in different concentration. It is applied after blossom fertilization. Application of NAA 10- 50 ppm in mango, citrus reduce fruit drop by preventing formation of abscission layer.

2-Gibberellins

Gibberellins include a large range of chemicals that are produced naturally within plants and by fungi. They were first discovered when Japanese researchers, including Eiichi Kurosawa, noticed a chemical produced by a fungus called Gibberella fujikuroi that produced abnormal growth in rice plants. It was later discovered that GAs are also produced by the plants themselves and they control multiple aspects of development across the life cycle. The synthesis of GA is strongly upregulated in seeds at germination and its presence is required for germination to occur. In seedlings and adults, GA strongly promote cell elongation. GAs also promote the transition between vegetative and reproductive growth and are also required for pollen function during fertilization. Application of 50- 100 ppm GA in grapes increases the berry size.

3-Cytokinins

Cytokinins are a group of chemicals that influence cell division and shoot formation. They were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence of tissues are responsible for mediating auxin transport throughout the plant and affect internodal length and leaf growth. Cytokinins and auxins often work together and the ratios of these two groups of plant hormones affect most major growth periods during a plant’s lifetime. Cytokinins counter the apical dominance induced by auxins. They in conjunction with ethylene promote abscission of leaves, flower parts, and fruits. It is used to induce development of shoot and roots along with auxin, depending on the ratio.

4-Abscisic Acid

Abscisic acid is one of the most important plant growth regulators. It was discovered and researched under two different names before its chemical properties were fully known, it was called dormin and abscicin II. The name “abscisic acid” was given because it was found in high concentrations in newly abscissed or freshly fallen leaves. This class of PGR is composed of one chemical compound normally produced in the leaves of plants, originating from chloroplasts, especially when plants are under stress. In general, it acts as an inhibitory chemical compound that affects bud growth, and seed and bud dormancy. It mediates changes within the apical meristem, causing bud dormancy and the alteration of the last set of leaves into protective bud covers. In other plants, as ABA levels decrease, growth then commences as gibberellin levels increase.

5-Ethylene

Ethylene is a gas that forms through the breakdown of methionine, which is in all cells. Ethylene has very limited solubility in water and does not accumulate within the cell but diffuses out of the cell and escapes out of the plant. Ethylene is produced at a faster rate in rapidly growing and dividing cells, especially in darkness. The resulting thicker stem can exert more pressure against the object impeding its path to the surface. If the shoot does not reach the surface and the ethylene stimulus becomes prolonged, it affects the stem’s natural geotropic response, which is to grow upright, allowing it to grow around an object. Synthesis of ethylene is inhibited by carbon dioxide and requires oxygen. Ethylene affects fruit ripening. Normally, when the seeds are mature, ethylene production increases and builds-up within the fruit, resulting in a climacteric.

Method of application of plant growth regulator

i) Quick dip method
ii) Prolong dip method
iii) Powder method
iv) Paste method

a. Quick dip method

Weigh 1 gm of IBA on a piece of butter paper or any other non-sticky glazed paper to prepare 5000 ppm of IBA solution.
Transfer it carefully in a volumetric flask of 200 ml.
Pour a small quantity of 95% alcohol in the flask.
Place the stopper over the bottle and shake slowly till the IBA is dissolved. If it does not dissolve readily, add more alcohol and shake.
Make up the volume by adding more alcohol. Add alcohol very slowly and carefully so that you can stop when the volume reaches up to the graduated mark.
Close the bottle and shake vigorously so that the solution is homogenous.
Pour approximately 150 ml in a beaker of 250 ml capacity.
Dip 10 cuttings for 5 seconds in the solution. If all the ten cuttings cannot be accommodated together, dip 5 cuttings and after taking out, dip 5 more to make 10 cuttings. There is no harm in using the same solution again and again, but care should be taken that the whole operation must be finished in a short time, otherwise the evaporation of alcohol changes the concentration.
Hold the cuttings for a while to dry the alcohol.
Plant in a pot or nursery bed carefully.

b. Powder method

Weigh 1 gm of IBA on a glazed or butter paper.
Transfer the material into a mortar.
Grind throughly so that the IBA crystals become a fine powder.
Weigh 49 gms of tale or any other inert powder and transfer the same to the mortar.
Mix the inert material and IBA throughly in the mortar.
Transfer the mixture into a petri dish.
Take 10 cuttings and make a fresh cut at the basal end so that the powder sticks to the cut surface.
Dip the basal cut end of the cuttings in the growth regulator at or near the basal end mixture, so that the powder sticks Tap lightly to remove excess powder.
Plant in a pot or nursery bed.
In case of layers also it could be applied similarly for rooting.

c. Paste method

Weigh 1 gm of IBA.
Weigh 49 gm of lanolin paste on a butter paper and transfer it to a 250 ml beaker.
Heat the beaker so that paste starts melting.
Add 1 gm of IBA into the melted paste and mix it by strirring it with glass rod.
Remove it from heater and cool it.
Take freshly prepared 10 cuttings and apply small quantity of paste to the cut end with the help of glass rod or forefinger.
Plant the cuttings in the prepared bed for rooting. In case of layers also it could be applied similarly for rooting