Plant Growth and Development Test - 28

IBA - Indole-3-butyric acid : It is an auxin hormone. It is white to light yellow crystalline solid. It is a plant hormone in the auxin family and is an ingredient in many commercial horticultural plant rooting products. It is not soluble in water.

Phenylacetic acid is an active auxin. An active auxin results in the growth in length of stems and hypocotyl. It is also involved in the growth of plant organs such as the Avena coleoptile. Indole acetic acid and phenylacetic acid are the commonly found active auxins in plants. Thus, the correct answer is option A.

Charles Darwin (1880) the great evolutionist had hypothesized the presence of growth substances in plants. In his book, 'The power of movement in plants', Darwin has described the experiments concerning the growth movements in canary grass. He had noticed the unilateral bending of the shoot tip (coleoptile) towards the source of light. Charles Darwin and his son Francis discovered in 1880, that the phototropic stimulus is detected at the tip of the plant. The Darwins used grass seedlings for their experiments. When grass seeds germinate, the primary leaf pierces the seed coverings and the soil while protected by the coleoptile, a hollow cylindrical sheath that surrounds it. Once, the seedling has grown above the surface, the coleoptile stops growing and the primary leaf pierces it. The Darwins found that the tip of the coleoptile was necessary for phototropism but that the bending takes place in the region below the tip.

The rosette habit of cabbage can be changed by application of gibberellic acid. This hormone has the induce shoot elongation. It even causes bolting in plants.

Ethylene does not contain cytokinin. It is also a phytohormone, which induces fruit ripening. Whereas cytokinin contains t-RNA, m-RNA and het-RNA. It promotes growth in cells cytokinesis and karyokinesis.

Gibberellin is the plant hormone which is produced by the plastids to regulate the growth of plants. The identifying gibberellins are based on a common principle- a high response of the stem or its individual portions to this phytohormone. Michalski carried out extensive comparative work to investigate various gibberellin bioassay systems. He found most sensitive system was lettuce which responded to 10$$^{ -10}$$ g/ml of gibberellic acid. The response of the phytohormone is but one property of the bioassay. Michalski showed that the specialised property of gibberellins, namely its ability to affect an entire plant, disappears if only part of a plant is used for a bioassay. This regularity was observed on intact plants of rice, maize, pea, cucumber and lettuce. 

Barley endosperm test is a bioassay for gibberellin. Nicholis and Paleg used barley endosperm for gibberellin bioassay. In this, Barley grains are cut in half along their equatorial axis. The embryo halves are discarded and the remaining endosperm halves are grouped into two lots. One serves as a control and the other lot is treated with the test solution (gibberellin containing). Endosperm halves in presence of GA show high amylase activity and production of reducing sugars.

The gibberellins are plant growth hormones which enhance the longitudinal growth of stem when applied to intact plants. Gibberellic acid helps in the formation of male flowers on genetically female plants such as Cannabis. It also causes replacement of female flowers with male flowers on monoecious plants such as Cucurbits. 

Solution:

Correct answer is option C.
Explanation for correct option:

• Gibberellin is a type of plant hormone. In seed the stored food is starch Gibberellins stimulate the synthesis of two enzymes i.e protease and α-amylase which are involved in the formation of sugar from starch.
• The proteases convert an inactive β-amylase to the active form. The active β-amylase and α-amylase together convert starch to glucose which is mobilized to meet the metabolic demands of the embryo during the germination of seed.
• Hence, during seed germination, its stored food is mobilized by gibberellin.