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Download Unit 4 - Lesson 7 - Plant Growth Regulators
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Grade 11 College Biology – Unit 4 Factors affecting Plant Growth – Plant Growth Regulators Auxins • Auxins are the best known growth regulators. They are found in all plant tissue. Indole-3-acetic acid (IAA) is the most common auxin. • Different concentrations have different effects, and different cells respond differently to same concentration of auxins • Many functions – • Primarily involved in cell elongation • e.g., phototrophic response – When light shine on one side of the growth, auxins move to the shaded side to stimulate elongation of the cells on the shade side causing the growth to bend towards the light • e.g., geotrophism in root and stem – In stem cells, auxins concentrate on the lower side causing the cells on the bottom to elongate and the stem to bend upwards. In the root, higher concentrations of auxins on the downward side inhibit cell elongation, and elongation of cells at the top cause the root to bend downward. • Stimulates apical bud growth, as well as growth and ripening of fruit • Inhibits lateral bud growth • Regulates cell division in the vascular cambium, as well as dropping of fruit and leaves • Regulates lead abscission, root formation and fruit growth • Synthetic auxins cause unwanted plants to grow very fast so they exhaust their carbohydrate supply and die (e.g., 2, 4-D or 2, 4-dichloro-phenoxyacetic acid – commonly used to eliminate broad leafed plants such as dandelions). Gibberellins • Best known for promoting cell division and elongation in plant shoots • 40 different types have been found • Cause shoot to elongate just before the plant flowers (called BOLTING) so a long stem is produced to raise the flower up for pollinators and wind seed dispersal • Stimulates seed germination (e.g., added to barley malt in brewing beer to promote synthesis of enzymes in the protein layer of the barley endosperm. The enzymes break down the starch, causing release of energy-rich sugars that nourish the growing barley embryos and the growing yeast). • Used commercially to stimulate seed germination Cytokinins • Hormones that stimulate cell division and leaf mesophyll growth • Concentrated in the endosperm tissue and young fruit • Used synthetically in field of biotechnology Ethylene (CH2=CH2) • Role in fruit ripening. Many fruits begin to produce ethylene gas just before the rise in cellular respiration that corresponds to – • The mass conversion of starch to sugar (increases sweetness of the fruit) • Colour change on surface of fruit causing chlorophyll to break down and new red and orange pigments to form • The softening of fruit tissue by breaking down the substance holding adjacent cells together. • Interacts with other growth hormones • To keep fruit from ripening, keep it in air and temperature conditions that keep ethylene concentrations low • To ripen fruit, do the opposite Abscisic Acid • Inhibitor of growth • Promotes the closure of stomata, inducing seed and bud dormancy, and provides resistance to water stress NOTE: Environmental conditions will determine the amounts of regulators that a plant will synthesize, as well as the plant tissues’ sensitivity to the regulator. This ensures the plants will respond to the environment in a way that ensures the plant’s survivial External Regulation of Plant Growth Geotropism • Regardless of the orientation of the seed, the seedling root grows downward and the seedling shoot grows upward. The growth of the root toward gravity is POSITIVE GEOTROPISM, while the growth of the seedling away from gravity is NEGATIVE GEOTROPISM. • To grow downward from a horizontal position, the cells of the root on the upper side elongate faster that the cells on the lower side. Microscopically, starch-containing plastids called AMYLOPLASTS and the endoplasmic reticulum are always found on the side of the cells of the root cap toward the direction of gravity regardless of the orientation of the root suggesting these organelles have some involvement with the perception of gravity. Phototropism • The bending of plants towards light is called PHOTOTROPISM. The COLEOPTILE is very light sensitive. When it pushed its way through the seed, it immediately bends towards the source of light. The coleoptile is able bend because the cells of the side away from the light elongate more than the cells facing the light.