Auxins: Discovery and Functions - Practice Questions & MCQ

• Auxins are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation.

• Auxins were the first hormones to be discovered.
• The discovery started with the observation of Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass (Phalaris canariensis) responded to unilateral illumination by growing towards the light source (phototropism). 
• After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile.
• Boysen-Jensen showed that transmittable influence of the tip of coleoptile could be transmitted to the subapical region through a block of gelatin but not through the mica plate.
• Paal replaced the previously exposed excised tip eccentrically over the stump of the coleoptile. He observed greater growth on that side even in the dark.
• Paal discovered that the hormone travels basipetally (from the tip to base).
• Went showed that agar block containing the chemical caused bending of a decapitated coleoptile of Avena according to its concentration. 
• This growth-promoting chemical was named as Auxin.
• The first naturally occurring auxin was isolated by Kogl and Haagel-Smit from human urine. It was identified as auxin-a.
• Indole 3-acetic acid (IAA) is the universal natural auxin.
• It was discovered by Kogl et al.

1. Cell enlargement: 

• Auxin promotes elongation and growth of stems and roots, and enlargement of fruits by stimulation elongation of cells in all directions. 
• The auxin causes cell enlargement due to solubilization of carbohydrates, loosening of wall microfibrils, synthesis of more wall materials, increased membrane permeability and respiration. 

2. Apical Dominance: 

• In many plants, the apical bud grows while the lateral buds are inhibited.
• If these apical buds are removed, the lateral buds begin to grow. 
• This inhibitory effect of the apical bud on the lateral buds is due to auxin and the effect is called apical dominance.
• This property is used in agriculture, like the sprouting of the eyes of potato is checked by applying the synthetic auxin.

3. Control of abscission layer:

• Auxin delays the abscission of young leaves and fruits. 
• Abscission layer is produced when the levels of auxins fall.
• The premature drop of fruits like apples and citrus can be prevented by applying a diluted solution of IAA or NAA. 

4. Tissue culture:

• They help to initiate rooting in stem cuttings, an application widely used for plant propagation.

5. Tropic movements:

• Differential distribution of IAA produces tropical plant responses such as phototropism and geotropism.

6. Weed control:

• Application of 2,4-D and 2,4,5-T removes broad-leaved weeds in cereal crops.
• Monocots are not affected by them.

7. Parthenocarpy:

• Application of auxins to unpollinated pistils make them develop into seedless fruits or parthenocarps which carry a better market price than the normal fruits having seeds, e.g., tomato

8. Control of lodging:

• Auxin reduces the lodging of plant stems caused due to rains and winds.
• 9. Flowering:
• In pineapple, NAA promotes flowering.
• In lettuce, auxins can delay flowering.
• In cotton plants, auxins can increase the seed production.

10. Sex expression:

• Auxins increase the number of female flowers in cucurbits.
• In maize, NAA induces formation of hermaphrodite or female flower in male flower.
• So, auxins have a feminizing effect on the plants.