Tropic movement

1. Plant hormones are chemicals that are required in small amounts to promote and influence growth, development and differentiation of plant cells and tissues. They are active and effective even at very low concentrations.
2. They are produced in certain parts of the plant and transported to other parts of the plant where they elicit specific biochemical, physiological or morphological responses.
3. They are transported within the plants by four different types of movements namely; localised movement, cytoplasmic streaming, slow diffusion of ions, and through vascular tissues (xylem and phloem).
4. Each plant hormone evokes many different responses.
5. The effects of different plant hormones overlap and may be stimulatory or inhibitory.
6. Each plant hormone performs specific functions in the plant body.
7. The biosynthesis of plant hormones within plant tissues is always diffuse, not localized.
8. The production of plant hormones occurs very often at sites of active growth within meristems, before cells have been fully differentiated. After the production, they are sometimes moved to other parts of the plant, where they cause an immediate effect; or they can be stored in cells to be released later.

There are five main types or groups of plant hormones. These are auxins, gibberellins, cytokinins, ethene (ethylene), and abscisic acid (ABA). These hormones can perform their functions either independently or synergistically (working together to effect a certain function). For example, auxins are known as growth promoters, since they influence plant growth and assist in producing a phototropic response which results into growth. Sometimes, auxins and gibberellins act together to bring about cell elongation. This is called synergism. Alternatively, two plant hormones may work opposing one another, like auxins which induces apical dominance while cytokinins prevent it. This is known as antagonism.

Auxins are chemical compounds produced in the root and shoot apices of the plants.

One of their common forms is Indoleacetic Acid (IAA). Auxins have the following role in plants:

1. They influence cell enlargement, bud formation, and root initiation.
2. They facilitate production of phototropic response.
3. Auxins together with cytokinins control the growth of stems, roots, and fruits.
4. They affect cell elongation by altering cell wall plasticity.
5. They stimulate cambium of meristematic cells to develop stems, leaves and flower buds, while in stems, they differentiate secondary xylem.

Gibberellins such as Gibberellic Acid (GA) include a wide range of chemicals that are produced naturally within plants. Like auxins which can be produced in industry, gibberellins are also commercially produced from fungal culture. They perform the following role in plants:

1. They stimulate stem elongation and pollen tube growth.
2. Influence cell division.
3. They promote flowering, seed germination and differentiation after germination.
4. They are important in seed germination, as they effect enzyme production that mobilises food production needed for growing new cells. This is done by modulating chromosomal transcription in the growing seedling.
5. Promote bolting in rosette plants, hence increasing inter nodal lengths

Cytokinins (CKs) are a group of chemicals that influence cell division and shoot formation in plants. In the past, when the cytokinins were first isolated from yeast cells they were called 'kinins'. They are found mostly in regions of rapid cell division, particularly in seeds and fruits, where embryos develop. They perform the following role in plants:

1. They promote cell division in the presence of auxins.
2. They induce delayed senescence of leaves and fruits.
3. They are responsible for mediating transportation of auxins throughout the plant and affecting internodal length and growth.
4. They promote lateral bud growth.
5. They counter the apical dominance induced by auxins.

Unlike other plant hormones, ethene is in gaseous state. It is formed from the amino acid methionine and released from ripening fruits, nodes of stems, ageing leaves, and flowers. It is produced as a metabolic byproduct of most plant organs. It plays the following role in plants:

1. It stimulates ripening of fruits.
2. It is involved in axillary bud inhibition. This occurs when auxins are transported from the apical meristem of the stem downward, stimulating the production of ethylene, which suppresses axillary bud development.
3. It suppresses stem and root elongation, especially during physiological stress as in drought.
4. It breaks bud dormancy.

ABA is also known as stress hormone; because its production is stimulated by environmental stress or other adverse conditions, such as drought and water logging. In contrast to other growth substances such as auxins, gibberellins, and cytokinins, which are plant growth promoters, ABA is a growth inhibitor; hence it acts antagonistically to the growth promoters. It is produced by any tissue containing chloroplasts. It is concentrated in leaves, fruits, and seeds.

The term abscisic acid originates from the belief that it was a direct cause of leaf fall (abscission), although this is now known to be not true. Abscisic acid has the following role in plants:

1. It promotes dormancy and inhibits growth.
2. It causes abscission (fall of leaves and fruits). When fruit ripens, the level of auxins which inhibits abscission falls, while that of abscisic acid which promotes abscission increases.
3. It promotes closing of stomata under water stress conditions.