CHART FIVE
2 TENIS 14/11/009 medal la delegación colombiana
Co-ordination and Control in Plants 139
Co-ordination in plants is simpler than in animals. It is done by a chemical known as
plant hormones.They are similar to animal hormones in that they are only needed in small amount to bring about their effects.
Therefore produced in one part of the body and transported to another part where they exert their effects (positively or negatively). Animal hormones are produced in specific glands and bring about very specific responses in specific target organs. In plants, hormones are not produced in tissues specialised only for their
• production, their effects are also more general, varying with concentration and types of target organ. Chemically too, plant hormones are not related to animal hormones, although they are organic compounds.
Plant hormones almost always stimulates or inhibit growth in the target tissues in responses to external stimuli such as light, temperature, day length, gravity and touch. As a result, they are also known as plant growth substances, though they affect other activities such as fruit formation, root development and leaf fall.
Combination of plant hormones bring about responses that are different from what each hormone would produce alone. The responses of plant hormones are usually much slower than those of animals hormones.
This is because responses in plants are mainly brought about by growth.
In this unit, we shall discuss the various type of plant hormones and their functions.
By the end of this unit, you should be able to:
i. explain what plant hormones are.
ii. explain the roles/functions of the hormones.
The growth of plants to a great extent is regulated by hormones. These chemical substances (hormones) influence the plants internally while factors such as light, temperature, water and gravity influence the plant externally. Hormones are needed in small quantities. They are manufactured in some areas (apical meristems of shoots and roots, young growing leaves, developing seeds or fruits) and diffuse to other parts to produce some special effects.
The major or well known plant hormones are auxins, gibberelins, cytokinins, floregens, abscissic acid, ethene (ethylene), traumatin, dormin, formin etc.
3.1 Auxins
The most important naturally occuring auxin is indole acetic acid (IAA). It is produced at
the apices of shoots and is transportaed in one direction away
from the tip. Itmoves across short distances by diffusion and longer distances via the phloem.
3.2 Functions or Effects of Auxin on Plant
The functions or effects of auxin on plant include the following:
root cap C. Root (geotropism)
more auxin collects on lower side - the higher auxin concentration slows down cell elongation on this side of the shoot 140 Flowering Plants
d. It induces flowering: Auxin is known to induce or initiate flowering in plants.
e. It stimulates the vascular cambium
to divide to increase the production of secondary vascular tissues (increase in width of stem).
f. Auxin and fruit development: Once a flower is fertilised, auxins are produced and sent to the ovary causing rapid development of the ovary as it develops into fruits. The presence of auxins also prevent the formation of abscission layer at the base of the flower stalk. This will help the fruits to reach ripening stage. When the fruits are ripe, auxin supply to the flower stalkis reduced and abscission layer is formed leading to the dropping of the fruit.
g. It induces root formation: Auxin is known to induce the formation of roots in plants.
h. Delay abscission or leaf fall: Auxin in petiole of fruits and leaves delays abscission or leaf fall.
i. It breaks dormancy in seed: Auxin equally promotes or help to break dormancy in seed thus ensuring their early germination.
j. It induces pathenocarpy: Auxins induces pathenocarpy or fruiting without fertilisation in plants.
k. Auxin influences the growth of stems toward light (phototropism) and away from the force of gravity (geotropism). Similarly, it also cause the growth of roots towards the force of gravity (see fig. 6.1).
Figure 6.1 How auxin brings about photoropism and geotropism
A. Shoot (phototropism)
terminal bud
o
b 0
terminal bud more auxin collects on shaded side-the higher auxin concentration promotes cell elongation on the shaded region of the shoot
B. Shoot (geotropism) more auxin collects on lower side - the higher auxin
concentration promotes cell elongation on this side of the shoot
3.3 Modern Applications of Auxin in Agriculture
Auxins, both natural and synthetic, are widely used in agriculture and horticulture. Some of the uses are:
1. Weed Control: Synthetic auxin called 2,4 - dichloropheno acetic acid - (2,4-D) at high concentration is used in killing weeds. When applied to lawn, it kills the weeds having broad leaves without killing grasses.
2. Development of Seedless fruits: Synthetic auxin, naphthalene acetic acid (NAA) can be sprayed on certain crops to induce the formation of fruits without fertilization. The causing of flowers to develop into fruits without fertilization is called PARTHENOCARP. Seedless water cucumber, watermelon and tomatoes have been produced in this way
3. Harvesting: If a farmer has a storage problem or the market price of his fruits is low due to surplus supply by other farmers, he can apply auxins to the plants to make the fruits stay longer on the plants.
Reduced amount of auxins stimulates the formation of abscission layer and when auxins are applied to plants, abscission layers will not be formed hence a longer stay of fruits on the plant. Tomato, orange and mango fruits have been prolonged on plants in this way.
4. Preservation: If stored products like yam, cocoyam, potatoes are sprayed with auxin, the auxin
Co-ordination and Control in Plants 141
controls the dormancy of the "eye" which remain dormant. Sprouting is prevented, thus prolonging the storage life of these products.
5. Root Initiation:
Stem cutting which are dipped in auxin solution of appropriate concentration are capable of initiating roots.6. Ripening of Fruits:
Fruits like tomatoes are picked unripe and stored in an atmosphere of carbon dioxide (which prevents ripening). Auxin can be applied to ripen them artificially and uniformly when needed.Briefly explain the functions/roles of auxin.
3.4 Gibberellins
Gibberellins are produced in young foliage leaves and roots, apices of roots and stems as well as in embryo.
3.4.1 Functions or Effects of Gibberellins
1. Gibberellins stimulate rapid stem elongation in dwarf plants and other plants that undergo little stem elongation.
2. It induces the production of fruits without fertilisation.
3. Dormancy in seeds and buds are broken when gibberellins are released to them.
4. It stimulate the growth of roots.
5. Stimulate some biennial plants to flower during their fruit year of growth.
6. Stimulate the embryos in germinating seeds to produce enzymes that hydrolyse starch reserves in the seeds.
Explain the main functions of Gibberellins
3.5 Cytokinins (Kinins)
Cytokinins are produced in roots, embryos and fruits that are actively undergoing growth. Cytokinins are also growth promoting hormones like auxins, and gibberellins.
3.5.1 Functions or Effects of Cytokinins
1. It controls cell division.
2. It can stimulate the development of roots.
3. Help in breaking dormancy in some seeds.
4. Promote flowering in some plants.
5. Promote fruits development in some plants.
6. Help to retard the onset of ageing in leaves by maintaining protein and nucleic acid synthesis.
7. It stimulates mitosis in meristems and in embryos during germination.
8. It increases resistance of some plant to harmful effects such as viral infections, radiation and low temperature.
9. It promotes auxilliary bud growth in plants.
10. Cytokinins and auxins act together in promoting the replication of DNA.
In your own words, explain six function of cytokinins.
3.6 Abscisic Acid
The abscisic acid is a hormone produced in mature green leaves, fruits and root caps. It is a growth inhibitor whose effects generally oppose those of auxins, gibberellins and cytokinins.
3.6.1 Functions or Effects of Abscisic Acid 1. It suppresses the growth of buds.
2. It induces dormancy (especially when environmental conditions are unfavourable).
3. It brings about ageing in leaves.
4. It may play certain roles in abscission.
5. It controls the opening and closing of stomata.
6. It inhibits mitosis in vascular cambium.
3.7 Ethene (Ethylene) Gas
Ethene gas is a simple hydrocarbon produced in leaves, stems and young fruits.
3.7.1 Functions or Effects of Ethene 1. It retards lateral bud development.
2. It hastens the ripening of fruits.
3. It inhibits stem elongation.
4. It accelerates abscission of leaves, flowers and fruits.
5. It accelerates ageing of plant organs.
3.8 Floregens
Floregen is found in flowers and fruits of plants - most botanists are convinced that flower initiation is a result of a hormonal influence and the tentative name floregen has been assigned to this kingdom material.
3.8.1 Functions or Effects of Floregens It controls initiation of flowers in plants
Explain the importance of abscisic acid to your study mate.
Plant hormones play significant roles in the development of agriculture. For increase in food production, the use of plant hormones especially auxins should be encouraged.
Co-ordination and Control in Plants 143
In this unit, we have learnt that;
1. Co-ordination in plants is done by a chemical known as plant hormones
2. This hormones are produced in one part of the plant and transported to another part where they exert their effects (positively or negatively).
3. The major or well known plant hormones are auxin, gibberellins, cytokinins, abscisic acid, ethene, traumatin, dormin, formin.
4. Auxins, both natural and synthetic ones, are widely used in agriculture and horticulture for weed control, harvesting crops, root initiation, formation of fruits, preservation of stored products.
5. Gibberellins control growth in plants, induces production of fruits without fertilisation, help to break dormancy in seeds, stimulate stem elongation, promote development of flowers and stimulates dwarf bean plant to grow into large plants.
6. Cytokinins are produced in roots, embryos and fruits that are actively undergoing growth.
7. Cytokinins help to slow down ageing of plant part, it stimulates mitosis in meristems and in embryo during germination, increases resistance of some plants to harmful effects such as viral infection, radiation and low temperature.
8. Abscisic Acid are hormone produced in mature green leaves, fruits and root caps. It suppresses the growth of bud, induces dormancy, bring about ageing in leaves and controls the opening and closing of stomata.
9. Ethene gas is a simle hydrocarbon produced in leaves, stems and young fruits. It retards lateral bud development, hastens the ripening of fruits, accelerates ageing of plant organs.
10. Floregen is found in flower and fruits of plants. It controls initiation of flowers in plants.
List the importance of auxin in the agricultural development of Nigeria.
Give four functions each of the following hormones
* Gibberellin
* Cytokinins
Essential Biology