NEET Biology Plant Growth and Development MCQs Set 13

Answer: (c) Tryptophan

Explanation:
1. IAA (indole-3-acetic acid) is made from the amino acid tryptophan, in the presence of zinc.
2. The other amino acids listed are not its precursors.

Teacher's Note:
1. Zinc deficiency lowers auxin and causes little-leaf disease.
2. Ethylene comes from methionine.

Answer: (b) Avena curvature test

Explanation:
1. Went's Avena (oat coleoptile) curvature test measures auxin: agar blocks with auxin placed on one side of a decapitated coleoptile make it bend.
2. The degree of bending is proportional to the auxin amount.

Teacher's Note:
1. Alpha-amylase test is for gibberellins.
2. Chlorophyll preservation test is for cytokinins.

Answer: (b) Auxin and gibberellin

Explanation:
1. Auxin causes cell elongation and gibberellins cause internode elongation.
2. Florigen is a hypothetical flowering stimulus and cytokinin mainly promotes cell division.

Teacher's Note:
1. Gibberellins are the characteristic internode elongation hormones.
2. Both are growth promoters.

Answer: (b) Suppression of growth of axillary buds by the presence of the apical bud

Explanation:
1. The apical bud makes auxin that flows down the stem and keeps the axillary buds from growing.
2. Option c describes what happens on decapitation, not the definition.

Teacher's Note:
1. Removing the shoot tip lets lateral buds grow, as in tea and hedge plants.
2. Cytokinins overcome apical dominance.

Answer: (a) 2,4-D

Explanation:
1. 2,4-D is a synthetic auxin.
2. GA2, gibberellins in general and IAA are natural plant hormones.

Teacher's Note:
1. Other synthetic auxins are NAA and IBA.
2. 2,4-D kills dicot weeds.

Answer: (b) Auxin decreases

Explanation:
1. Auxin keeps leaves attached; when its level falls the abscission layer forms.
2. Abscisic acid rises at this stage, so the others are wrong.

Teacher's Note:
1. Auxin prevents abscission of young organs.
2. ABA and ethylene promote abscission.

Answer: (c) Apical shoot meristem

Explanation:
1. Auxin is made mainly at the growing tips, chiefly the shoot apex.
2. It then moves basipetally down the stem.

Teacher's Note:
1. Root tips also make some auxin, but the shoot apex is the main site.
2. Cytokinins are made mainly in root tips.

Answer: (d) All of the above

Explanation:
1. Plant growth is localised at meristems, it is an irreversible increase and it involves a permanent increase in size.
2. All three statements are correct.

Teacher's Note:
1. Plants show open, indeterminate growth.
2. Growth is irreversible; swelling by water uptake is not growth.

Answer: (a) Cell division

Explanation:
1. The sequence is formative (meristematic) phase, elongation phase and maturation phase.
2. So cell division comes first.

Teacher's Note:
1. The meristematic cells have dense protoplasm and prominent nuclei.
2. Vacuolation drives the elongation phase.

Answer: (d) Cells on the shaded side elongate more

Explanation:
1. Light moves auxin to the shaded side of the stem tip.
2. The extra auxin makes the shaded cells elongate more, so the stem curves toward the light.

Teacher's Note:
1. This is positive phototropism.
2. The Darwins first studied it in coleoptiles.

Answer: (c) Some as promoters and some as inhibitors

Explanation:
1. Auxins, gibberellins and cytokinins promote growth.
2. Abscisic acid and ethylene generally inhibit it.

Teacher's Note:
1. Plant growth regulators are grouped as promoters and inhibitors.
2. Their effects often depend on concentration.

Answer: (b) Basipetal

Explanation:
1. Auxin moves from the shoot tip downwards, away from the apex, which is basipetal transport.
2. It is polar and needs energy.

Teacher's Note:
1. Cytokinins move acropetally.
2. Auxin transport in roots is mostly acropetal in the central cylinder, but the stem pattern is basipetal.

Answer: (a) Auxin

Explanation:
1. 2,4-dichlorophenoxyacetic acid acts like IAA but is not made by plants.
2. It is a selective weed killer for dicots.

Teacher's Note:
1. 2,4-D plus 2,4,5-T is called Agent Orange.
2. NAA is another synthetic auxin.

Answer: (a) Ethylene

Explanation:
1. Ethylene promotes female flowers in cucumber and so raises fruit yield.
2. Gibberellins tend to favour male flowers.

Teacher's Note:
1. Ethylene is the feminising hormone.
2. Cytokinins also favour female flowers in some plants.

Answer: (a) Cytokinin

Explanation:
1. Cytokinin delays leaf senescence by improving nutrient mobilisation and keeping chlorophyll.
2. This delay is called the Richmond-Lang effect.

Teacher's Note:
1. Cytokinins also promote lateral buds.
2. ABA and ethylene hasten senescence.

Answer: (c) Florigen

Explanation:
1. Florigen is thought to be made in leaves and carried to the shoot apex to start flowering.
2. It has not been isolated from plants.

Teacher's Note:
1. Vernalin is the similar idea for cold treatment.
2. Phytochrome is the pigment that detects the photoperiod.

Answer: (b) Gibberellin

Explanation:
1. Gibberellins make rosette plants bolt and flower even in short days.
2. They therefore act in place of long-day exposure.

Teacher's Note:
1. Auxin promotes flowering in pineapple.
2. Gibberellins also cause bolting in cabbage and beet.

Answer: (c) Gibberellin

Explanation:
1. Gibberellins cause the internodes to lengthen, as in bolting.
2. This is their characteristic effect.

Teacher's Note:
1. Genetic dwarfs become tall with GA.
2. Sugarcane gives more yield with GA.

Answer: (b) Gibberellin

Explanation:
1. Bolting is rapid internode elongation just before flowering in rosette plants such as cabbage and beet.
2. Gibberellins bring this about.

Teacher's Note:
1. It is due to gibberellin.
2. The rosette stem lengthens as the flower stalk emerges.

Answer: (b) Growth promoters

Explanation:
1. Gibberellins promote stem elongation, seed germination and fruit growth.
2. They are used widely in agriculture, for example in grapes, sugarcane and malting.

Teacher's Note:
1. More than 100 gibberellins are known.
2. GA3 is the most studied.