12th botany neet school

Phytohormones Auxins and Physiological effects of auxin

Phytohormones Auxins and Physiological effects of auxin

Phytohormones Auxins and Physiological effects of auxin are explained with detail notes.

Phytohormones Auxins

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Auxin was the first plant hormone to be discovered.

They were isolated initially from human urine.

The term auxin is given to generally IAA and other natural and synthetic compounds having similar structure and growth regulating properties.

Generally, auxins are produced in the growing apices of stem and root where from they migrate to the other parts of the plant. Auxins such as IAA and phenyl acetic acid (PAA) are natural auxins.

Synthetic auxins are chemicals synthesised in the laboratory.

They are considered as plant growth regulators. eg. Naphthalene acetic acid, 2,4 – Dichlorophenoxy acetic acid.

Physiological effects of auxin

  1. Auxins are well known to promote elongation of stem and coleoptile.It promotes the growth by cell enlargement in stems, particularly by elongation of cells behind the apical meristem.
  2. Growth in lateral bud is inhibited when the apical bud of a tall plant
    remains intact. However, the lateral bud grows rapidly on removal
    of apical bud.
  3. Suppression of growth in lateral bud by apical bud due to auxin
    produced by apical bud is termed as apical dominance. The reason for this is due to auxin produced in growing tip and it stimulates growth but as it moves downward, suppresses growth in the stems below.
  4. Auxin is responsible for initiation and promotion of cell division in
    cambium, which is responsible for the secondary growth. This
    property of induction of cell division has been exploited for tissue
    culture techniques and for the formation of callus.
  5. Auxin promotes growth of root only at extremely low concentrations. At higher concentrations, it always inhibits growth of root.
  6. When leaves and fruits mature, they shed from the stem. This is
    called abscission. Auxin prevents abscission.
  7. Seedless fruits are produced in tomato and apple, by external
    application of auxin on flowers. Such seedless fruits are called
    parthenocarpic fruits.
  8. 2,4 – Dichlorophenoxy aceticacid, a synthetic auxin is used to eradicate weeds in the field.

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Other links 

Plant tissue culture – origin and techniques

Plant physiology – photosynthesis and its significance

Site of photosynthesis and Mechanism of photosynthesis

Electron transport system and photophosphorylation types

Dark reaction

C3 and C4 pathways

Photorespiration or C2 cycle

Factors affecting photosynthesis

Test tube and funnel experiment, Ganong’s light screen experiment

Mode of nutrition – Autotrophic, Heterotrophic


Mechanism of Respiration – Glycolysis

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

Mechanism of Respiration – Electron Transport Chain, Energy Yield

Ganong’s respiroscope, Pentose phosphate pathway

Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment

Plant growth and Measurement of plant growth

Phytohormones Gibberellins

Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects

Photoperiodism and vernalization, Phytochromes and flowering


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