PLANT GROWTH & HORMONES

What is plant hornmones? Plant hormones are chemicals produced in very small amounts within the plant body, and play a very important part in regulating plant growth and development. Most growth hormones are produced at the tip of a shoot and transported downwards to the root. The root tip produces very small quantities of the hormones.

Type of plant hormones

What are the types of plant hormones? There are many different types of plant hormones and one well-known group is the auxins. Indoie acetic acid (IAA) is one best known auxin.

Auxins

Auxins are produced at the shoot and root tips.Maximum influence on growth in plants occurs when auxins are produced simultaneously with other plant hormones e.g. gibberellins. Maximum growth response in stems requires more IAA than tn roots.

Auxins are known to have various effects on the growth and development in plants. They stimulate cell division and cell elongation in stems and roots leading to primary growth. Auxins cause tropic responses, which are growth responses in plants due to external stimuli acting from a given direction.

On the other hand IAA stimulates the growth of adventitious roots which develop from the stem rather than the main root. Cuttings can be encouraged to develop roots with the help of IAA. If the cut end of a stem is dipped into IAA, root sprouting is faster. IAA is also used to induce parthenocarpy. This is the growth of an ovary into a fruit without fertilization. This is commonly used by horticulturalists to bring about a good crop of fruits particularly pineapples.

 

Auxins are known to inhibit development of side branches from lateral buds. They therefore enhance apical dominance. During secondary growth auxins play an important role by initiating cell division in the cambium and differentiation of these cambium cells into vascular tissues.

Auxins in association with other plant hormones such as the cytokinins induce the formation of callus tissue which causes the healing of wounds.

When the concentration of auxins falls in the plant, it promotes formation of an abscission layer leading to leaf fall. A synthetic auxin, 2, 4-dichlorophenoxyacetic acid (2,4-D) induces distorted growth and excessive respiration leading to death of the plant. Hence it can be used as a selective weed killer.

Gibberellins

Is another important group of plant growth hormone.  GibbereHins are a mixture of compounds and have a very high effect on growth. The most important in growth is gibberellic acid. Gibbereilins are distinguished from auxins by their stimulation of rapid cell division and cell elongation in dwarf varieties of certain plants.

Dwarf conditions are thought to be caused by a shortage of gibberellins due to a genetic deficiency.

Gibberellins are important in fruit formation. They induce the growth of ovaries into fruits after fertilization. They also induce parthenocarpy. Gibberellins also promote formation of side branches from lateral buds and breaks dormancy in buds. This is common in species of temperate plants whose buds become dormant in winter.

This hormone also inhibits sprouting of adventitious roots from stem cuttings, it retards formation of abscission layer hence reduces leaf fall. Gibberellins also break seed dormancy by activating the enzymes involved in the breakdown of food substances during germination.

Cytokanins

Cytokanins also known as kinetins, are growth substances which promote growth in plants when they interact with auxins. In the presence of auxins, they stimulate cell division thereby bringing about growth of roots, leaves and buds. They also stimulate formation of the callus tissues in plants.

The callus tissue is used in the repair of wounds in damaged parts of plants. Cytokinins promote flowering and breaking of seed dormancy in some plant species. They also promote formation of adventitious roots from stems and stimulate lateral bud development in shoots. When in high concentration cytokinins induce cell enlargement of leaves but in low concentration they encourage leaf senescence and hence leaf fall.

Ethylene

Ethylene is a growth substance produced in plants in gaseous form. Its major effect in plants is that it causes ripening and falling of fruits.

This is widely applied in horticultural farms in ripening and harvesting of fruits. It stimulates formation of abscission layer leading to leaf fall, induces thickening of stems by promoting cell division and differentiation at the cambium meristem. But it inhibits stem elongation. Ethylene promotes breaking of seed dormancy in some seeds and flower formation mostly in pineapples.

Abscisic acid

Abscisic acid is a plant hormone whose effects are inhibitory in nature. It inhibits seed germination leading to seed dormancy, inhibits sprouting of buds from stems and retards stem elongation. In high concentration, abscisic acid causes closing of the stomata.

This effect is important in that it enables plants to reduce water loss. It also promotes leaf and fruit fall. Another hormone, florigen is produced in plants where it promotes flowering.

Apical Dominance

Although auxins, particularly IAA are important stem and root elongation, they are known to exert profound effects on other aspects of plant growth and development.

If an apical bud which normally contains high concentrations of auxins is removed, it is observed that more lateral buds lower down the stem sprout, producing many branches. This shows that high concentrations of auxins have an inhibitor}’ effect on sprouting of lateral buds and therefore hinders growth of many branches.

This forms the basis of pruning in agriculture where more branches are required for increased harvest particularly on crops like coffee and tea. The failure of lateral buds to develop in the presence of an apical bud is due to the diffusion of auxins from the shoot apex downwards in concentrations higher than that promoting lateral bud development.

Practical Activity

1. To investigate apical dominance in plants
2. Requirements
3. Tomato seedlings growing in a tin.

Procedure

1. Cut off the terminal buds from 3 seedlings in the tin, leaving the other seedlings with the terminal buds intact,
2. Leave the seedlings to continue growing for five more days.
3. List the differences noticed between the two groups of seedlings? Explain how the differences come about.
4. From your observations, explain the basis for pruning tea and coffee.

Evaluation

1. What is plant hormone and growth?
2. What are plant hormones?
3. What is the growth of plant?
4. What are the hormones that promote growth in plants?
5. What is plant hormone roles?
6. What are the 5 plant hormones?
7. How are plant hormones used?
8. Characteristics of Plant Growth Regulators?
9. What are the 3 stages of plant growth?
10. How many hormones are in plants?