Plant Growth and Hormones

One effect of cytokinins on plant cells is to delay the aging process. Plant cells, like all living cells, go through a natural aging process known as senescence. Senescence is accelerated in cells of plant parts that are cut, such as flower stems. Botanists think that plants must have a continual supply of cytokinins from the roots. Cut stems, of course, lose their source of cytokinins and therefore age rapidly. When cytokinins are sprayed on leaves of a cut stem of many species, they remain green, whereas unsprayed leaves turn yellow and die.

During the 1940s and 1950s, researchers were trying to find substances that might induce plant cells to divide in tissue culture, a technique in which cells are isolated from plants and grown in a nutrient medium. They discovered that cells would not divide without a substance found in coconut milk. Because coconut milk has a complex chemical composition, investigators did not chemically identify the division-inducing substance for some time. Finally, researchers isolated an active substance from a different source, aged DNA from herring sperm. They called it cytokinin because it induces cytokinesis, or cytoplasmic division. In 1963, researchers identified the first plant cytokinin, zeatin, in corn. Since then, similar molecules have been identified in other plants. Biologists have also synthesized several cytokinins. Cytokinins are structurally similar to adenine, a purine base that is part of DNA and RNA molecules.

The ageing process in plants is known as senescence. Cytokinins delay the process of scenescence. The ability of cytokinins to delay the process of senescence is known as Richmond-Lang effect.

One important effect of cytokinins on plant cells is to delay the aging process. Plant cells, like all living cells, go through a natural aging process known as senescence. Senescence is accelerated in cells of plant parts that are cut, such as flower stems. Botanists think that plants must have a continual supply of cytokinins from the roots. Cut stems, of course, lose their source of cytokinins and therefore age rapidly. When cytokinins are sprayed on leaves of a cut stem of many species, they remain green, whereas unsprayed leaves turn yellow and die.

Cytokinins promote cell division and differentiation of young, relatively unspecialized cells into mature, more specialized cells in intact plants. They are a required ingredient in any plant tissue culture medium and must be present for cells to divide. In tissue culture, cytokinins interact with auxin during the formation of plant organs such as roots and stems. For example, in tobacco tissue culture a high ratio of auxin to cytokinin induces root formation, whereas a low ratio of auxin to cytokinin induces shoot formation. Cytokinins and auxin also interact in the control of apical dominance. Here, their relationship is antagonistic: auxin inhibits the growth of axillary buds, and cytokinin promotes their growth.

Cytokinin causes delay of senescence called Richmond Lang effect. It checks disappearance of chlorophyll and degradation of protein synthesis in leaves. 
Senescense of leaves leads to yellowing and finally leads to the fall from the plant.  If a young excised leaf is kept in water, it slowly changes its color to yellow and dies.  If such leaves are provided with cytokinin, the yellowing is significantly delayed and such an effect is called Richmond and Langs effect; named after the discoverers. 

During the 1940s and 1950s, researchers were trying to find substances that might induce plant cells to divide in tissue culture, a technique in which cells are isolated from plants and grown in a nutrient medium. They discovered that cells would not divide without a substance found in coconut milk. Because coconut milk has a complex chemical composition, investigators did not chemically identify the division-inducing substance for some time. Finally, researchers isolated an active substance from a different source, aged DNA from herring sperm. They called it cytokinin because it induces cytokinesis, or cytoplasmic division. In 1963, researchers identified the first plant cytokinin, zeatin, in corn. Since then, similar molecules have been identified in other plants. Biologists have also synthesized several cytokinins. Cytokinins are structurally similar to adenine, a purine base that is part of DNA and RNA molecules.
Morphogenesis means generation of proper morphological form of a plant body. Cytokinins promote cell division and differentiation of young, relatively unspecialized cells into mature, more specialized cells in intact plants (morphogenesis). They are a required ingredient in any plant tissue culture medium and must be present for cells to divide. In tissue culture, cytokinins interact with auxin during the formation of plant organs such as roots and stems. For example, in tobacco tissue culture a high ratio of auxin to cytokinin induces root formation, whereas a low ratio of auxin to cytokinin induces shoot formation. Cytokinins and auxin also interact in the control of apical dominance. Here their relationship is antagonistic: auxin inhibits the growth of axillary buds, and cytokinin promotes their growth.

Kinetins are a type of phytohormone (cytokinin). One of the important physiological effect of cytokinins is that they delay senescence. This delay in senescence of mature plant parts is known as Richmond Lang effect.

The primary physiological effect of cytokinins is the promotion of cell division (cytokinesis) in plants.

Cytokinin is a type of phytohormone. It encourages the process of cell division and cell differentiation. They promote cytokinesis, and so the term 'Cytokinin'. They affect apical dominance and axillary bud growth.

Auxin promotes stem elongation. It is present in high concentration in apical meristem. Gibberellin controls shoot elongation, seed germination and delay the senescence in leaves and fruit. Cytokinin promotes cell growth and division. It is present in higher concentration in developing areas such as young fruits and seeds. Ethylene helps in fruit ripening, senescence and abscission.

Plants have certain chemical substances in them that transmit messages for maintaining different functions of the body. 

Abscisic acid is a plant growth hormone involved in seed dormancy and responses of plants to stress. This makes option A incorrect. Auxins promote the formation of lateral and adventitious roots, regulate the flower bud development and induce vascular differentiation. This makes option B incorrect. Gibberellins are responsible for stem elongation, breaking seed dormancy, mobilization of endosperm reserves as well as regulate sex determination, flower initiation and fruit set. This makes option C incorrect. Cytokinins are the plant growth hormones that regulate the cell division in root and shoot of the plant both in vivo and in vitro. They do so by regulating the controls that govern the transition of the cell from one stage of the cell cycle to the next one. This makes option D correct. 

Cytokinin is the plant growth regulator which promotes cell division. The hormone retards the process of ageing which is known as senescence. There is stimulation for the production of chlorophyll and the fruits and leaves are prevented from falling. There is breaking of the bud dormancy which allows reproductive growth in the plants.

Cytokinins delay destruction of chlorophyll and proteins and thereby delaying the leaf senescence/ageing. Thus, option C is correct .