Plant Growth Regulators
Short Questions and Answers
One mark questions with answers
1. Gibberellins were first discovered from
Answer: (b) fungi
2. Dwarf plants can be made taller by
Answer: (a) gibberellins
3. High concentration of synthetic auxins would
(a) kill the plants
(b) prevent lateral buds from growing
(c) control cell division
(d) cause phototropism
Answer: (a) kill the plants
4. Which one of the following is known to induce flowering in pineapple?
Answer: (a) Ethylene
5. The ripening of fruit can be hastened by treatment with
(a) gibberellic acid
(b) indole acetic acid
(d) ethylene gas
Answer: (d) ethylene gas
6. The growth regulator that retards ageing of plant organ is
(d) abscisic acid
Answer: (c) cytokinin
7. In plants auxin synthesis occurs in
(b) phloem cells
(c) root and shoot tips
(d) xylem cells
Answer: (c) root and shoot tips
8. Apical dominance is caused by
Answer: (a) auxin
9. Hormone used in early ripening of fruit is
(c) ethylene gas
Answer: (c) ethylene gas
10. Induction of cell division and delay in senescence is due to
Answer: (a) cytokinin
Two marks questions with answers
1. What are auxins?
Answer: Auxins are growth promoters. Kogl (1931) isolated auxin. Auxins are weak organic acids. The naturally occurring auxins are indole 3 acetic acid, indole 3 pyruvic acid, indole 3 acetal dehyde etc. The synthetic auxins are indole 3 butyric acid, alpha and beta naphthalene acetic acid, phenol acetic acid, 2,4 dichlorophenoxy acetic acid etc. Auxins are synthesized continuously in the shoot apices and young leaves. Auxin is mainly located in young actively growing tissues like root apices, shoot apices, lateral meristem and enlarging leaves.
2. What are cytokinins?
Answer: Cytokinins are plant growth hormones which are basic in nature, either aminopurine or phenyl urea derivatives that promote cytokinesis or cell division. They have little or no effect on extension growth. The first naturally occurring cytokinin to be chemically identified was from Zea mays grains in 1963. It was named zeatin. Coconut milk, is another common kinetin and is called coconut-milk factor.
3. What is Richmond-Lang effect?
Answer: One of the important properties of cytokinins is their ability to delay the normal process of senescence in leaves. Disappearance of chlorophyll and degradation of protein are two important symptoms of senescence. Cytokinins are useful in delaying the process of aging and cytokinin and senescence of leaves by reducing the breakdown of chlorophyll, protein and nucleic acid. This effect of cytokinins is known as Richmond-Lang effect.
4.What are gibberellins?
Answer: Gibberellins are weakly acidic growth hormones which cause cell elongation of intact plants in general and increased internodal length of genetically dwarfed plants in particular. Gibberellins have gibbane ring stucture. Gibberellins are found in almost all groups of plants such as Algae, fungi, mosses, ferns, gymnosperms and angiosperms. Gibberellins are most abundant in young expanding organs. They are synthesized particularly in growing young shoot tips, young apical leaves, buds, embryos and seeds.
5. What is abscissic acid and why it is known as antigibberellin?
Answer: Abscissic acid is chiefly a growth inhibitor. It was discovered almost simultaneously by Fredericke Addicott and PF Wareing. They named it dormin and abscisin II, respectively. Later both these inhibitory substances were found to be abscisic acid (ABA). It is produced mainly in mature leaves. Besides, it is also synthesized in stems, fruits and seeds and then transported to the rest of the plants through vascular tissue mainly the phloem.
Abscisic acid suppresses the growth promoting effects of gibberellins and hence it is also known as antigibberellin.
Three marks questions with answers
1. Mention some of the physiological effects of auxin:
Answer: (1). Cell elongation and longitudinal growth: Auxin promotes cell elongation in stem and coleoptile. High auxin concentration promotes cell elongation but the same concentration inhibits root growth.
(2). Cell division in the cambium: During secondary growth the initiation and promotion of cell division in cambium is brought about by auxin.
(3). Cell division and tissue culture: Initiation and promotion of cell division brought about by auxin is very useful in tissue culture for the formation of callus.
(4). Root formation: Cell division leading to the formation of adventitious and lateral roots is stimulated by auxin.
(5). Prevention of abscission layer: Auxin in the cell prevents the formation of abscission layer which may otherwise result in the fall of premature leaf, flower and fruit from the stem.
(6). Parthenocarpy: External application of auxin in flower induces parthenocarpy and causes development of seedless fruits.
2. Explain the role of gibberellins in bolting and flowering in long-day plants.
Answer: Gibberellins play an important role in controlling a balance between internode growth and leaf development. In rosette-forming plant such as cabbage, internode growth is poor but leaves are large. Therefore, leaves appear to arise in tufts. The internodes suddenly elongate and the stem becomes normal just before flowering. This is called bolting. Generally, such a rosette plant is a long-day plant or requires cold nights. Under the long-day conditions and low-temperature nights, these plants maintain rosette form. However, gibberellin treatment causes them to bolt or elongate and flower under non-inducing conditions. Thus, long-day plants can be made to flower in off seasons by gibberellin treatment.
3. Mention some of the applications of gibberellins.
Answer: (1) Cell division in the cambium: Gibberellin can stimulate cell division of vascular cambium in many deciduous trees.
(2) Parthenocarpy: Gibberellins have been found to be more effective than auxins in inducing parthenocarpy in tomato, apple, pear etc. They are commonly used commercially in increasing the yield and fruit size of seedless grapes.
(3). Breaking of dormancy: Gibberellins can effectively break the dormancy of potato tubers and winter buds of many trees. Gibberellins also act antagonistically to abscisic acid.
(4). Germination of seeds: Light sensitive seeds like lettuce and barley can also be germinated in dark when treated with gibberellins.
(5). Synthesis of hydrolytic enzymes: It induces synthesis of various hydrolytic enzymes such as alpha amylase, ribonuclease and protease.
4. What are the major roles of cytokinin?
Answer: (1).Cell division: Cytokinin promotes cell division, however, only in the presence of auxins. The balance of these two hormones determines the growth of root and shoot systems. With high concentration of both cytokinin and auxins, the tissue gives rise to a lump of undifferentiated tissue known as callus. If the concentration of auxins is higher as compared to the cytokinin concentration, roods develop in the cutlure tissue.
(2). Counteraction of apical dominance: Auxins and cytokinins act opposite to one another in relation to apical dominance. Auxins allow dominance of apical bud while cytokinins stimulate the growth of the lateral buds. Application of cytokinins to lateral buds release them from apical inhibition.
(3). Delay in senescence: One of the important properties of cytokinins is their ability to delay the normal process of senescence in leaves. Disappearance of chlorophyll and degradation of protein are two important symptoms of senescence. Cytokinins are useful in delaying the process of aging and cytokinin and senescence of leaves by reducing the breakdown of chlorophyll, protein and nucleic acid.
5. What is the role of ethylene as a plant growth regulator?
Answer: Ethylene is the only gaseous natural plant growth regulator. It is effective in extremely low concentrations. A variety of plant tissues, such as roots, leaves, flowers, fruit and seeds also produce ethylene. It is known for its effects on fruit ripening and it is used on commercial scale to stimulate ripening of bananas, apple, mango, citrus fruits, tomatoes and many other fruits. These fruits are picked up before ripening and subsequently ripening can be regulated with ethylene when needed. The ripening of fruits by ethylene is accompanied by increase in their rate of respiration. This is known as respiratory climacteric. Ethylene also promotes seed germination, flower induction in pineapple and melon, development of root hairs, the number of female flowers in cucumber and abscission of leaves, flowers and fruits. It also increases the diameter of tree trunk.