Control and Coordination

Growth substances are also called the phytohormones. The phytohormones have been put in five different categories based on their actions.

They are:

• auxins
• gibberellins
• cytokinins
• ethylene
• abscissic acid

The regulators associated with cell enlargement and differentiation are: auxins and gibberellins

• The regulators associated with cell division: cytokinins
• The regulators associated with ageing: ethylene
• The regulators associated with dormancy of buds: abscissic acid

Auxins

Auxins are phytohormones that are mainly concerned with cell enlargement. They affect the plasticity of the cell walls and induce them to grow. The name 'auxin' was derived from the Greek word 'auxein' meaning to increase.

They were discovered through a series of experiments. Darwin and his son's experiments with oat coleoptile gave the first indication of the existence of auxins.

More experiments were conducted by Boysen-Jensen (1913) and Went (1928) which confirmed their presence and showed that they are involved in the growth of the shoots.

Chemically, auxin is Indole Acetic Acid (IAA).

It is produced by shoot apex, young leaves and roots (to some extent). They only move in upward direction through phloem or xylem.

Functions

• It causes elongation of stem (high concentration) and root (low concentration).
• It promotes root initiation in cuttings and callus. (Callus is an undifferentiated mass of cells from which the entire plant body can be grown by tissue culture techniques.)
• It also promotes root development.
• It causes differentiation of xylem cells in calluses.

Apical Dominance

All shoot tips end in an apical bud, the division of which results in the growth of the stem. In the presence of apical buds, the lateral buds (present in the axils of leaves) do not grow into branches. This is called apical dominance. If the apical bud is removed, the lateral buds show growth. On application of IAA to the cut ends, the lateral buds are again inhibited. So, auxins cause apical dominance.

Parthenocarpy

It is the development of fruits without fertilization and formation of seeds or embryo. Therefore, the fruits are seedless. Auxins help in parthenocarpic development of fruits like guava, papaya, banana, orange and tomato which are seedless.

Delay in Abscission

If the leaves and fruits stop producing auxin, they fall. Presence of auxins gives them maximum time for fruits to ripen by maintaining them on the trees.

Commercial Applications of Auxins

Many synthetic auxins have been developed because they are found useful in many ways. These synthetic auxins are cheaper to develop than the natural auxins. They are also more effective as the plants are not able to degrade them easily.

Some of the uses of synthetic auxins are given below:

• Fruiting: Naphthalene acetic acid (NAA) and indolebutyric acid (IBA) help in natural or parthenocarpic fruit setting that increases crop yield in tomato, pepper, figs, etc.
• Rooting: It is an important technique to reproduce genetically similar plants, especially ornamental plants. The cuttings are dipped in rooting powders containing NAA or IBA. This promotes root initiation and stimulates their development.
• Weeding: 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,3,6-trichlorobenzoic acid (benzoic acid) are used to kill dicot weeds among the monocot (cereal) crops as the latter are unaffected.
• Storage: 2-methyl-4-chlorophenoxyacetic acid (MCPA) inhibits sprouting of buds in potatoes and hence is used in their storage.

Gibberellins

Gibberellins are plant hormones that are mainly responsible for cell elongation. They cause the cells to grow in length. They were for the first time isolated from the extract of the fungus, Gibberella and hence the name.

Chemically, they are a class of compounds called the terpenes (related to lipids) and are weak acids.

They are synthesised in embryos, young leaves, root tips, buds and seeds. They move up or down in the plant body through xylem or phloem.

Functions of Gibberellins

• Gibberellins cause stem elongation by affecting cell elongation. This can result in stem elongation of dwarf varieties (peas and maize) and rosette plants (cabbage). In the latter, it is called bolting.
• Causes leaf expansion
• Promotes fruit growth
• Breaks bud dormancy
• Breaks seed dormancy
• Promotes flowering in long-day plants and inhibits the flowering in short-day plants. (Long-day plants are those which require sunlight for a longer period during the day.)

Commercial Applications of Gibberellins

• It is used along with auxins to induce parthenocarpy. For example, it is used to develop seedless grapes.
• It is used to increase fruit size and bunch length in grapes.
• GA-3 (gibberellic acid), a gibberellin that has been most studied, is used in the brewing industry. It causes the barley seeds to produce the starch-digesting enzymes like maltase, amylase. This process is called the malting.

Cytokinins

They are phytohormones that induce cell divisions even in mature tissues. They were named 'cytokinins' as the cell division is also called cytokinesis (Cyto-cell, Kinesis-breaking). They belong to a group of compounds called the kinetins. They were isolated from coconut milk by Skoog when he was looking for chemicals that would induce cell division in tissue cultures.

There are many types of cytokinins present. For example, zeatin, a cytokinin present in maize grains.

Chemically, they are similar to adenine, a nucleotide in DNA and RNA. The structure is as given:

Cytokinins are synthesized in the fruits and seeds where rapid cell division takes place.

Functions of Cytokinins

• They promote cell division, in the presence of auxins
• They delay the process of ageing (senescence) in leaves
• They promote growth of lateral bud
• They break bud and seed dormancy

Commercial Applications of Cytokinins

• They are used in tissue culture to induce cell division in mature tissues.
• They also induce development of shoot and roots along with auxin, depending on the ratio.
• They are used to delay senescence in fresh leaf crops like cabbage and lettuce.
• They are used to keep flowers fresh.

Abscissic Acid (ABA)

It is a growth inhibitor that results in dormancy and abscission. It was discovered in the 1960s and was initially called dormin. Then another compound abscisin-II was discovered. They are all now commonly called as abscissic acid. It is synthesized in stem, leaves, fruits and seeds.

Functions of Abscissic Acid

• It is a growth inhibitor, causing bud and seed dormancy.
• It results in abscission of leaves and fruits.
• It is produced during stress.

For example: During drought, it causes growth suppression that conserves energy. It also controls water loss during dry conditions by causing closure of stomata.

Commercial Applications of Abscissic Acid

It is used as a spray on trees to regulate dropping of fruits.

Know Something

Ethylene Ethylene is a gaseous growth regulator that speeds up the ripening process. It is a gas produced by most of the plant organs. Chemically, ethylene (ethene) is an unsaturated hydrocarbon and has the following structure: Functions of Ethylene It promotes ripening of fruit. It sometimes promotes flowering. It inhibits stem growth. It promotes abscission of fruits and leaves. Commercial Applications of Ethylene It is used to stimulate ripening of fruits. For example, tomatoes and citrus fruits. It is applied to rubber trees to stimulate flow of latex.