Role of Growth Substances in Plants

Plants are coordinated by chemicals which necessarily move from their sites of synthesis and because their effects are usually on some aspect of growth, they are called growth substances. Five major types of growth substances are recognized.

Auxins

These are indole acetic acid (IAA) or their varieties. The discovery of auxins was the result of investigations into phototropism that began with the experiment of Charles Darwin and his son Francis (1880).

Affects Of Auxins

1- In stem, promote cell enlargement in region behind apex. promote cell division in cambium.

2- In root promote growth at very low concentrations. Inhibit growth at very concentrations e.g. geotropism. promote growth of roots from cutting and callus.

3- Promote bud initiation in shoots but sometimes antagonistic to cytokinins and is inhibitory.

4- Promote apical dominance and fruit growth.

5- Sometimes can induce parthenocarpy. Cause delay in leaf aging in a few species.

6- Inhibit abscission

discovery of IAA lead to the synthesis of a wide range of active compounds with similar structure. synthetic auxins have proved commercially useful in a variety of ways. they are cheaper than IAA to produce and are often more physiologically active because plants generally do not have necessary enzymes to break them down.

Gibberellins

The compound extracted from fungus  Gibberella (now called fusarium) is called gibberellins. The third most active gibberellins isolated is called gibberellic acid (GA). Now more than 50 naturally occurring gibberellins are known.

Effects of Gibberellins: (1) The main of gibberellins Is on stem elongation, mainly by affecting call elongation. Cause cell division In apical meristem and cambium (2) Promote bolting in some rosette stage of plant e.g. a lettuce plant, typically formed into a compact head can be made to "bolt" that Is to stretch Its stem upward and separate the leaves. (3) Promote bud (shoot) Initiation in Chrysanthemum callus, Sometimes promote in Intact plant if apical dominance Is broken. (4) Promote leaf growth and fruit growth. Can sometimes induce parthenocarpy. (5) In apical dominance, enhance action of auxins. (6) Break bud dormancy (7) Break seed dormancy e.g. cereals, (8) Sometimes substitute for red light. Therefore promote flowering in long day plants, Inhibit In short day plants (9) Delay leaf senescence (aging) in a few species.

Commercial applications: Some of the commercial applications of gibberellins are (1) They promote fruit setting e.g. in tangerines and pears and are used for growing seedless grapes, (parthenocarpy) and alto Increase the berry size. (2) GA is used in brewing Industry to stimulate a amylase production in barley and this promotes "malting". (3) To delay ripening and improve storage life of bananas and grape fruits.

Cytokinins

Cytokinins are most abundant where rapid cell division is occurring, particularly In fruits and seeds where they are associated with embryo growth.

Effects of cytokinins: (1) Cytokinins promote cell division In the apical meristem, only In the presence of auxins. Gibberellins may also play a role, as in the cambium. (2) Inhibit primary root growth. (3) Promote lateral root growth. (4) Promote bud Initiation and leaf growth. (5) Promote fruit growth but can rarely induce parthenocarpy. (6) Promote lateral bud growth, also break bud dormancy. (7) Cause delay In leaf senescence. (8) Promote stomatal opening.

Commercial Applications: Cytokinins delay aging of fresh crops  such as cabbage and lettuce, as well as keeping flowers fresh. They can also be used to break dormancy of some seeds.

Abscisic acid

The substances which accelerated abscission (an act of cutting off) was called abscisic acid (ABA) in 1967.

Effect of ABA: (1) Inhibits stem and root growth notably during physiological stress e.g. drought, water logging (2) Promotes bud and seed dormancy. (3) Promotes flowering in short day plants, and inhibits in long day plants (antagonistic to gibberellins). (4) Sometimes promotes leaf senescence. (5) Promotes abscission. (6) Promotes closing of stomata under conditions of water stress (wilting)

Commercial applications: ABA can be sprayed on tree crops to regulate fruit drop at the end of the season. This removes the need for picking over a long time-span.

Ethene

Ethene is made by most or all plant organs, and tends to escape more easily from the plant surface.

Effects of Ethene: (1) Inhibits stem growth, notably during physiological stress. (2) Inhibits root growth. (3) Break dormancy of bud. (4) Promotes flowering in pineapple. (5) Promotes fruit ripening. (6) Like ABA it acts as a growth inhibitor in some circumstances and can promote abscission of fruits and leaves.