Auxin

• Chemically, auxin is indoleacetic acid (IAA), which is synthesized from indole or tryptophan

Role of Auxins

Ø Apical Dominance: Removal of apical bud stimulates lateral buds. Auxins inhibit lateral bud formation since they are synthesized in apex. This phenomenon is called apical dominance. Eg: Potato tubers for apical buds forming.

Ø Cell Division and Elongation: Shoot and Root growth.

Ø Xylem Differentiation: Auxins helps in establishing contact between vascular tissues of the callus and that of the bud and makes it possible for the bud to grow properly in callus. By adding Auxins and sugar continued growth of callus may be obtained and new shoots and even new plant can be produced.

Ø Nucleic Acid Activities of IAA increases total RNA – synthesizes specific enzymes lead to cell enlargement.

Ø Manifold Activities Play specific role in seed germination, growth, rooting, flowering (Reproductive phase), abscission, parthenorcarpy and tissue culture.

Practical Applications of Auxins

Ø Germination: IAA, IBA, NBA, 2,4-D are mostly used in soaking seed for germination- at low concentrations promotes germination but these effects are subjected to variation depending on form and species of plants.

Ø Root: NAA, 10% induces 100% rooting in mango: Dashri, langra IBA+SUGAR application leads to greater number of roots-structure of roots also changed (Vascular bundles).

Ø Flowering: Play floragenic role in day neutral plants IAA promotes formation of female flowers. Increased spikelet number, leaf number and weight and number of grains in wheat. NAA & IAA increases boll-set (G.hirsutum) induced more pine-apple. Fruit weight increases.

Ø Parthenocarpy: IBA, NAA produces seed less/fruits – smaller sized fruits, but more in number, hence yield not affected.

Ø Fruit setting: By using 2,4,5 T fruit setting and yield of ber/fruit increased. IAA, IBA, and NAA induce high percentage fruit set.

Ø Prevention of pre-mature drop of fruits: 2,4,D,IAA,IBA, 2,4,5-T, are used to prevent preharvest drop of sweet oranges( 100 to 500 ppm)

Ø Tissue and Organ culture: IAA & Kinetin

Ø Auxins as inhibitors: High concentration of auxins inhibit the growth and exert toxic effect on plants. In normal case, self-produced auxins inhibit the growth and development of lateral buds, and as a result apical buds, remains dormant.

Auxins use in Agriculture and Horticulture

Ø Propagation of plants by hormone treatment of cuttings

Ø Prevention of pre harvest drops of plants.

Ø Increasing parthenocarpy.

Ø Increasing fruit set.

Ø Prevention of sprouting by inhibiting buds.

Ø Inhibition of prolonged dormancy.

Ø Control of flowering.

Ø Defoliation of plants

Ø Prevention of leaf fall or abscission.

Ø Thinning of compact fruits.

Ø Selective weed killer.