Class 12 Biology | Plant Growth And Movement

Plant hormones(phyto-hormones)

Plant need water, oxygen sunlight,minerals,etc for their growth and development.These are external factors, Besides there, there are some internal factor, that regulate the growth and development of plants. These internal factors are called Plant hormones.
Plant hormones control the growth and the development activities like the cell division enlargement, flowering, Seed formation, dormency and abscission.
Based on their action, plant hormones are grouped into two categories:-

1. Plant growth promoters

   • Auxin
   • Cytokinin
   • Gibberellins (GA)

2. Plant growth Inhibitors

   • Abscisic Acid
   • Etylene

Auxins

Auxins is defined as a plant growth hormone which is responsible for the elongation of the cells in the shoots. F.W Went first isolated auxin from the Coleoptiles of out seedlings. Auxins (plural of auxin) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development.

Site of auxin formation and its types

The auxin (IAA) is synthesized in the meristematic tissues and transported to the growth regions of the plants. IAA is the main natural hormone, found in the highest concentration at the tips of the stems and the roots, in the young growing leaves, and in the flowers and fruits.

There are two types of auxins:

• Natural auxins
  Indole-3-acetic acid(IAA),Indole butric acid (IBA)
• Synthetic auxins
  2,4-D(2,4-Dichlorophenoxyacetic acid),NAA(Naphtnalene acetic acid)

Functions of IAA

Auxin perform many functions and physiological activities in the plants. Some of their a Important roles are!-

1. Cell division or callus formation

   Auxins initiate and promote the cell division in the tissuses leading to the formation ofundifferentiated mass of the cells called the callus. This activity of auxin I is used in tisssue Culture.

2. Cell elongation

   Auxins brings about the enlargement of the shoot and the root tips especially behind the apical meristem.

3. Apical dominance

   It is observed in many plants that the removal of terminal buds to the growth of one or many lower buds.

4. Suppression of abscission layer

   Another inhibitory effect of auxins is on the abscission of the leaves and the fruits. During abscission, an abscission zone present at the base of the leaves or fruits, cutts of the supply of nutrient and water.

5. Eradication of weeds

   The roots are extremely sensitive to auxins. Application of the high concentration of 2,4-D stimultes the growth promoting activities of the cells of the root.

6. Root initation

   Applications of auxins activates the root initials. Rapid adventitious root formation is absolutely essential in cutting for their Successful development into new plants invegetative propagation

7. Flower initation

   At Normally auxins inhibit the flowering. How- ever, in litchi and pineapple (Ananos sativus) auxins like 2,4-D and NAA have been found to promote the uniform flowering.

Cytokins

Cytokinis are defined as the planthormones that influence the growth and stimulate the cell division. Cytokinins are the major growth promoters of the cell division (cytokinesis). Cytokinin was first identified from young maize (Zea mays) in 1963. So it is also called zeatin. Auxing and cytokining acts anatugoni- stically. Auxins are responsible for stimulating the growth of cipical bud. On the other hand, Cytokinins promote the growth of the lateral buds.

Functions of cytokinin

1. Cell Division

   Cytokinins promote the cell division in the apical meristem as well as in the non-meristematic tissues.

2. Cell enlargement and differentiations

   It plays a vital role in morphogeneis and differentiation of the shoot and the root meristems.

3. Initation of Interfascicular cambium

   Cytokinins induce the formation of interfascicular cambium in the plants.

4. Counteraction of apical dominance

   Auxins and cytokinins act antagonistically in the control of apical dominance. Auxin are responsible for stimulating the apical bud.

5. Breaking of dormancy

   Cytokinis break the dormancy of many Seads and promote their germination.

6. Flowering

   Cytokinins induce flowering in certain species of the plant

7. Delay of senescence

   Cut leaves dipped in cytokinis stay green longer. The effect of cytacining in retareling. the ageing as called the Richmond Lang effect Cytokinins slow down the process of senescence.