Test: Plant Growth Regulators (Old NCERT) - NEET MCQ

Charles Darwin and his son Francis Darwin (1880 are concerned with the discovery of auxins. They worker on Phalaris canarensis (Canary grass) and found tha the stimulus of unilateral illumination was picked up b; the coleoptile tip of canary grass, whereas a decapitate! coleoptile did not receive the stimulus.
Hori (1918) and Kurosawa (1926) are related to tfv discovery of gibberellins (gibberellic acid, GA).
The first cytokinin was discovered from degraded autoclave Herring sperm DNA by Miller etal, 1955. It is called kineti (6-furfuryl amino-purine). Kinetin does not occur naturall; It is a synthetic hormone. The first natural cytokinin Wc obtained from unripe maize grains or kernels by Letham i al (1964). It is known as zeatin (6-hydroxy 3-methyl trans 2 butenyl amono-purine). It also occurs in coconut milk.Coconut milk is arich source of cytokinin

Isolation of auxins from tips of coleoptiles of oat seedlings is based upon Avena coleoptile curvature test.
This experiment was devised by F.W Went (926). It is based on the quick polar transport of auxin, downwards in Avena coleoptile. The result will be differential growth and formation of curvature. Degree of curvature is almost proportional to the concentration of auxin.

Auxins can be natural or synthetic. Natural auxii are the auxins which occur naturally in plant parts, e. IAA (lndole-3-Acetic Acid). Related chemicals are indc 3-acetaldehyde, indole 3-acetonitrile, indole 3-butyric ac (IBA), etc. Synthetic auxins are the chemical substanc which have been artificially synthesized and cause vario physiological responses common to IAA. Some exampl of synthetic auxins are 2,4-D (2,4-dichlorophenoxy ace acid), 2,4,5-T (2,4,5-trichlorophenoxy acetic acid), α and β naphthalene acitic acid(NAA). IBA is considered as both natural and synthetic.

Auxins stimulate root formation on the stem cuttings, e.g., IBA, IBA-alanine, NAA. NAA is synthetic auxin while IBA is considered as both natural and synthetic.

Apical dominance is the phenomenon by which presence of apical bud does not allow the nearby lateral buds to grow. When the apical bud is removed, the lateral buds sprout. Apical bud inhibits the growth of lateral buds by releasing auxins. When a plant is decapitated i.e., its apical bud is removed then the lateral buds sprout, resulting in dense bushy growth. This phenomenon is widely used in the tea plucking and hedge making.

Callus is an unorganized, undifferentiated mass of diving cells formed during tissue culture which requires both auxins and cytokinins for its growth and differentiation. In callus, shoot regeneration is promoted by a cytokinin, such as BAP (Benzylaminopurine) and root regeneration is promoted by an auxin, such as NAA (Naphthalene acetic acid). Thus, shoot and root regenerations in callus are controlled by auxin-cytokinin balance. Usually, an excess of auxins promote root regeneration and that of cytokinins promote shoot regeneration.

Gibberellic acid increases the yield of sugarcane crop. Cytokinins inhibit apical dominance and act antagonistically to auxins which promote apical dominance. Rhizomes, corms, tubers, seeds (e.g. peanut) and other storage organs can be made to sprout early by exposing them to ethylene. Abscisic acid is a plant growth inhibitor, which inhibits seed germination.

Auxins promote the growth of apical buds and suppress the growth of lateral buds/lateral shoots.

So the correct option is 'Statements (i), (ii) and (iii) are correct.'

The raw material which is used in synthesis of auxin is called auxin precursor. It is tryptophan of IAA.

NA A and 2,4-D are often employed for inducing flowering in litchi and pineapple. Application of auxins (e.g., IA A, IBA) and conjugate auxins (e.g., IBA-alanine) to unpollinated pistils make them develop into seedles fruits (or parthenocarps) which carry a better market price than the normal fruits having seeds. 2,4-D and 2,4,5-T are used as weedicides (herbicides) which destroy broad leaved weeds in cereal crops and lawns. Dalapon (2,2-dichloropropionic acid) Kills grasses in broad leaved crops.

Auxins are mainly responsible for apical dominance. IAA is a natural auxin.

The compounds which inhibit the action of auxins are called as antiauxins, e.g. p-chlorophenoxy isobutyric acid (PCIB)

Auxins are used for inducing root formation in stem cuttings in many commercially important vegetatively propagated plants, and also plants propagated through layering. 
Example- Rose, guava, Ficus etc
Here, synthetic Auxin like NAA (Naphthalene Acetic Acid) is used.

Presence of cytokinin in an area caues preferetial movement of nutrients towards it. When applied to lateral buds, they help in their growth despite the presence of apical bub. They thus act antagonistically to auxin which promotes apical dominance. Therefore, cytokinin can overcome apical dominance, caused by auxins.

Cytokinins play a vital role in morphogenesis in plants. It opposes initiation of roots in stem cuttings treated with auxin and results in the formation of callus at the cut end. It is now well known that kinetin-auxin interaction controls the momhogenetic differentiation of shoot and root meristems.

The most important function of cytokinins is the promotion of cell division. The varying amounts of cytokinins, alongwith sufficient auxins is required for the growth of callus in tissue culture experiments. Cytokinins delay the senescence of leaves and other organs by mobilisation of nutrients. it is known as Richmond-Lang effect.

Cytokinins are plant growth hormones which are basic in nature, cytokinsis (=cell division) either alone or in conjunction with auxin. Kinetin, a synthetic cytokinin, is 6-furfuryl amino purine. Zeatin, a natural cytokinin, is 6-hydroxy 3-methyl trans 2-butenyl amino purine.

ABA (Absisic acid) is a stress hormone which is synthesised by the plant during drought or other stress conditions. It causes rapid movement of K+ ions out of the transpiration rate.

The term 'auxin' is applied to the indole-3-acetic acid (IA A) and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stem, from where they migrate to the regions of their action.

Gibberellic acid induces de novo (a new) synthesis of various hydrolytic enzymes, such as, α-amylase, ribonuclease, α-1,3-glucanase and protease in aleurone cells of barley grains. This has been experimentally shown by isolating the aleurone layer and treating it with gibberellin solution. These enzymes are mobilised to endosperm during germination and hydrolyse the storage reserve.

Phototropic curvature is the result of uneven distribution of auxin. Darwin and Darwin observed that the coleoptiles of canary grass responded to unilateral illumination by growing towads the light source (phototropism). After a series of experiments, it was concluded that the tip of coleoptile contains auxin that caused the bending of the entire coleoptile in relation to the direction of light.

One of the most striking effects of the gibberellins is the reversal of dwarfism in many genetically dwarf plants. Rosette plant of sugar beet, when treated with GA₃, undergoes marked longitudinal growth of axis. Similarly, dwarf pea and dwarf corn attain almost normal size when treated with gibberellins. The dwarf plants have very low GA level which appears to account for the dwarfing habit. Thus, the external supply of GA to such plants causes rapid elongation.

In Japan (in early 1800's), certain rice plants suffers from bakanae (foolish seedling) disease. Such rice plants were thin, pale green, spindle shaped, longer by 50% than the healthy plants, and were sterile. The disease was found by Hori and Kurosawa to be caused by a fungus. Gibberella fujikori. The fungus is the perfect stage of Fusarium moniliforme.

ABA is a plant growth inhibitor. It promotes senescence and ageing in plant parts.

ABA is known as dormin as it induces dormancy in buds, underground stems and seeds in a variety of plants. e.g., ABA induces winter bud dormancy in duckweed plant.

Gibberellins specifically induce internodal elongation in genetically dwarf verieties of plants like pea and maize.

Shedding of mature leaves from the stem or ripe furits from the stem is called abscission. Generally a layer of tissue is formed at the base of the organ. This layer of tissue is called abscission zone. Abscission zone does not occur when the concentration of auxin is high, particularly when the gradient of auxin is steep, i.e, more auxin on distal side and less auxin on proximal side. the abscission zone formation occus rapidly when the auxin gradient become slight or netural. Moreover, the plant hormone ethylene is found to promote the abscission. Thus, a high concentration of auxin prevents the formation of abscission layer and the phenomenon is controlled by the concentrations of auxin and ethylene. Apical dominance is promoted by auxin.

Some of the gibberellins are commercially used in the delayed ripening of fruits. GA; delays senescence so thal fruit can be left on the tree for longer period. It extendi period of marketing. Ripening of Citrus fruits can be delayed with the help of gibberellins. This is useful in storing the fruits.

About 18 types of cytokinins have been discovered till now. Some of them are constituents of tRNAs. Roots are considered as the major source of cytokinin synthesis, from roots, the cytokinins pass upwardly through xylem. Cytokinin synthesis also occurs in the areas where cell divisions takes place e.g., growing embryos and developing seeds, endosperm of seeds, young fruits, develpoing shoot buds, etc.

Rooting on cut stem is a function of auxin not cytokinin.