Plant Growth and Development Test - 39

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.

Gibberellins are involved in seed germination in certain plants. They induce hydrolytic enzymes. Their effect on seed germination, however, is species specific. In a classic experiment involving barley seed germination, researchers showed that the release of gibberellin from the embryo triggers the synthesis of alpha-amylase, an enzyme that digests starch in the endosperm. As a result, glucose becomes available for absorption by the embryo. Although enzymes mobilize starch reserves in many types of seeds, gibberellin control of seed enzymes appears restricted to cereals and other grasses. In addition to mobilizing food reserves in newly germinated grass seeds, application of gibberellins substitutes for low-temperature or light requirements for germination in seeds of plants such as lettuce, oats, and tobacco.

Gibberellins promote stem elongation in many plants. When gibberellin is applied to a plant, particularly certain dwarf varieties, this elongation may be spectacular. Some corn and pea plants that are dwarfs as a result of one or more mutations grow to a normal height when treated with gibberellin. Short-stemmed, high-yielding varieties of wheat have short stems because they have a reduced response to gibberellin. These varieties put less of their resources into stem height and more resources into grain production. Gibberellins are also involved in bolting, the rapid elongation of a floral stalk that occurs naturally in many plants, like cabbage, when they initiate flowering. These plants are biennials and show vegetative growth in first year and reproductive growth (flowering) in second year.

Gibberellins affect several reproductive processes in plants.They stimulate flowering, particularly in long-day plants. Gibberellins are known to stimulate maleness in flowering plants as compared to auxin, cytokinin and ethylene all three of which are known to promote femaleness. In addition, gibberellins substitute for the low temperature that biennials require before they begin flowering. If gibberellins are applied to biennials during their first year of growth, flowering occurs without exposure to a period of low temperature. Gibberellins, like auxin, affect fruit development. Agriculturalists apply gibberellins to several varieties of grapes to produce larger fruits.

In recent years, there have been indications that in many plants GA$$_1$$ is the most important native biologically active gibberellic acid, primarily responsible for stem elongation. So, is widely distributed in higher plants. It is generally easily transported in plants, they have been detected in phloem sap, xylem sap etc.  

Ethylene is a simple gaseous plant growth regulator (PGR). It is synthesised in large amounts by tissues undergoing senescence and ripening fruits. Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings. Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called as respiratory climactic.

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.

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.

Gibberellins are involved in seed germination in certain plants. In a classic experiment involving barley seed germination, researchers showed that the release of gibberellin from the embryo triggers the synthesis of a-amylase, an enzyme that digests starch in the endosperm. As a result, glucose becomes available for absorption by the embryo. Although enzymes mobilize starch reserves in many types of seeds, gibberellin control of seed enzymes appears restricted to cereals and other grasses. In addition to mobilizing food reserves in newly germinated grass seeds, application of gibberellins substitutes for low-temperature or light requirements for germination in seeds of plants such as lettuce, oats and tobacco.

Correct option - A