Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10

Biology: Plant Hormones, Nutrition, and Transport: Part Four Gibberellins Gibberellic acid was the first of a large class of hormones, called gibberellins, to be discovered. In 1926, Japanese scientist Ewiti Kurosawa was studying a fungal disease of rice plants. The disease was called bakanae, or "foolish seedling disease." The fungus Gibberella fujikuroi infects rice plants and causes disease by producing an excess of a chemical named gibberellins. Japanese scientists isolated gibberellin from plants by 1939, but the outbreak of World War II interrupted international scientific communication. There are now more than 80 known naturally produced gibberellins. Gibberellin hormones promote stem elongation. Unlike auxin, gibberellins are not produced in the stem tip and they move through the plant in all directions. Applying gibberellins to dwarf plants causes them to grow to normal heights. Applying gibberellins to germinating barley seedlings promotes the conversion of stored starch (in the cotyledon) into sugar. Commercially, applying gibberellins to seedless grapes makes the fruits larger. Application of GA to several plants. Cytokinins Cytokinins are the plant hormones that promote cell division. They are produced in growing areas, such as meristems at the tip of the shoot. During the 1950s, University of Wisconsin plant physiologist Folke Skoog and his student, Carlos Miller, confirmed that a cell division stimulating hormone did exist. This chemical was isolated from yeast. It was named kinetin because it stimulated cell division, or cytokinesis. The first naturally occurring plant cytokin was discovered in 1964. It was named Zeatin since it is a hormone in this class of plants and occurs in corn (Zea).Since then, many other natural and artificial cytokinins have been found or synthesized. Cytokinins are made in the roots of plants. They also occur in seeds, fruits, and young leaves. They cause cells to transition from the G2 phase of the cell cycle (if auxin is also present). Cytokinins also promote the development of plant organs and changes in the cotyledons of seeds. Green leaves in floral arrangements can remain green longer if a cytokinin is applied. Abscisic Acid Swedish botanist Torsten Hemberg reported the presence of an inhibitory plant chemical in the dormant buds of ash and potato plants. Philip Wareing and F.T. Addicott independently discovered abscisic acid, a chemical that promotes seed dormancy by inhibiting cell growth. It is also involved in the opening and closing of stomata as leaves wilt. The name, abscisic acid, is unfortunate because it implies that this hormone is involved with leaf abscission. In fact, ethylene, not abscisic acid, causes leaves to drop. Ethylene Back when gas lamps lighted homes, people began to note that plants that grew near the lamps dropped most of their leaves. In 1901, Russian plant physiologist Dimitry Neljubow identified ethylene as the chemical responsible for this. In 1934, it was discovered that ethylene, a gas combustion by-product, was the plant hormone that causes fruits to ripen. In that year, botanist R. Gane demonstrated that this chemical was indeed made by plants. Ethylene is a gas produced by ripe fruits. It is also made by many other parts of the plant. Why does one bad apple spoil the whole bunch? It’s because of ethylene. The absence of ethylene can be used to store “green” or unripe apples over the winter until they are needed at the market in March. Exposure of the apples to a very low amount of ethylene causes the “green” apples to ripen. Changes in color, smell, and hardness of the fruit are all consequences of the action of ethylene. Ethylene is used to ripen crops so that all plants in a crop will ripen at the same time. Sprayed on a field, it will cause all fruits to ripen at the same time so that the crop can be harvested all at once. Pineapples are produced year round in Hawaii by the application of a chemical that breaks down to become ethylene, causing the pineapple plants to flower “out of season.” Ethylene is also sprayed onto a number of crops, such as cotton, cherries, grapes, and blueberries, to induce leaf drop as a prelude to harvest. Now go on to the next part.

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