EnvReg213-page6 - gravitropism Homeowners have often...

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Plant Environmental Regulators - 6 Heliotropism Sun-tracking plants have pulvini (pulvinus) in the peduncle or petiole that undergo changes in turgor to keep flowers and/or leaves facing the sun throughout the day. Plants that have positive heliotropism (or diaheliotropism) orient the broad part of leaves or petals toward the sun. An alternative is an orientation to minimize surface exposure to sun by having leaves parallel to the sun throughout the day. This is called paraheliotropism. Paraheliotropism is assumed to be a way to minimize dehydration during drought times. Hydrotropism Roots grow toward regions of higher water potential. The response to water potential is less than the response towards gravity, but has been studied in mutant plants that do not respond to gravity. Little is known of the mechanism other than Calcium is involved. The mechanism may be similar to that postulated for
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Unformatted text preview: gravitropism. Homeowners have often experienced consequences of root growth towards moisture, when roots “invade” water or sewer pipes. Phototropism, Photomorphogenesis and Photoperiodism The primary response to light by plants is the bending of shoots through differential growth towards a light source, as shown by Charles Darwin and his son in 1880. The hormone, auxin, migrates towards cells away from the light source. Auxin moves from cells in the light side of the plant to cells on the dark side. The differential distribution of auxin promotes elongation of the cells away from the light, causing a curvature or bending of the plant towards the light source. The pattern of development of tissues in response to light is called photomorphogenesis. But how do plants detect light?...
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