CBSE Questions for Class 11 Medical Biology Plant Growth And Development Quiz 5 - MCQExams.com

Correct Option: D
Explanation:

•  If a shoot is placed horizontally in the absence of light: Auxins accumulate on the lower side of the shoot, due to gravity. Cells on the lower side grow more quickly. The shoot bends upwards. This is called negative geotropism. 
• Pineapple is not a kind of climacteric fruit. However, ethylene exposure will activate chitinase (which is a type of enzyme that breaks down chitin, a structural material inside plants), and cause softening of the pineapple.
• The pioneering work has shown that a high auxin/cytokinin ratio induces root regeneration, whereas a low ratio promotes shoot induction (Skoog and Miller, 1957).
• Gibberellic acid makes the grapes long and cylindrical, while other chemicals can give them a rounder physique.

• Hence, the large-sized grapes that can be produced with cytokinin treatment is a false statement.

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. 

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.

• Gibberellic acid is the most common GA used commercially. It can be readily obtained in large quantities from fermentations of the fungus Gibberella fujikori. 
• Almost all seedless grapes on the market are treated with GA$$_3$$. It substitutes for the presence of seeds, which would normally be the source of native GAs for fruit growth. 
• Repeated spraying with GA$$_3$$ increases both rachis length and fruit size. The increased rachis length prevents the cluster from being too compact, and this reduces the chance of fungal growth inside the cluster.

NAA is a synthetic plant hormone in the auxin family and is an ingredient in many commercial plant rooting horticultural products; it is a rooting agent and used for the vegetative propagation of plants from stem and leaf cutting. It is also used for plant tissue culture. The hormone NAA does not occur naturally and like all auxins is toxic to plants at high concentrations. 2,4-D is also called as 2,4-dichlorophenoxy acetic acid which is also a synthetic auxin which can act as herbicide. it was used as defoliating agent as 'agent orange' during Vietnam war.

Gibberellins are plant hormones that regulate growth and influence various developmental processes, including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction and leaf and fruit senescence. It was first recognized in 1926 by a Japanese scientist, Elichi Kurosawa, studying 'bakane', the 'foolish seedling' disease in rice. It was first isolated in 1935 by Teijiro Yabuta and Sumuki, from fungal strains (Gibberella fujikori) provided by Kurosawa. Yabuta named the isolate as gibberellin.

Solution

Correct answer is option C.

Explanation for correct option:

NAA is a synthetic plant hormone in the auxin family and is an ingredient in many commercial plants rooting horticultural products; it is a rooting agent and used for the vegetative propagation of plants from stem and leaf cutting. It is also used for plant tissue culture.