Question

Root’s development is promoted by
(a) Ethylene
(b) Abscisic acid
(c) Auxin
(d) Gibberellic acid

Answer 1

Hint: Root’s development is promoted by a plant hormone which was the first one to be discovered. It exhibits a phenomenon that is exploited by all gardeners for dense bushes in the growth of their plantations.

Complete answer:
Frits Went discovered the first plant hormone ‘Auxin’ in the tip of oat (Avena sativa) coleoptiles as a growth-promoting chemical. As it promoted growth, it was named auxin by Fris Went which is derived from a Greek word ‘auxein’ that means to increase.
The growing apices of the stems and roots synthesize auxins or indole-3-acetic acid or IAA. It then migrates to the required regions in the plant.

Additional Information: -Auxins initiates the growth of roots or rooting. It also stimulates root initiation on stem cuttings i.e adventitious roots and lateral root development. Auxin stimulates the pericycle cells to divide which is the origin of lateral roots in a plant.
- Its functions include inducing parthenocarpy, preventing fruit and leaf drop at early stages but abscission of older mature leaves and fruits. It also helps to promote flowering and initiates cell division.
- In most plants, auxin exhibits a phenomenon named as ‘apical dominance’ wherein growing apical bud inhibits the growth of lateral buds. Removal of the shoot apex results in the growth of lateral buds. This property is exploited by gardeners in ‘hedge making’ and ‘tea plucking’ to produce dense bushy growth.
- IAA is a natural auxin. Some are now being manufactured. 2,4-D (2,4- dichloro phenoxy acetic acid) is a synthetic auxin.
So, the correct answer is ‘Auxin.’

Note: There are three varieties of root systems in nature:
-In the tap root system, the primary root or taproot from the radicle persists, which is the thickest and largest of all. Later, lateral roots i.e. primary and secondary roots develop from the main branch. This type of root system is observed in dicots e.g. Pea, Sunflower.
-In the fibrous root system, tufts of roots arise from a radicle which is nearly equal in size. E.g. wheat.
-Adventitious root system involves the growth of roots from parts other than radicle. They may be aerial or underground. E.g. Lower nodes of the stem bear roots to provide support to the plant in maize.