40.1 Plants respond to light.
Photomorphogenesis
• Photomorphogenesis describes nondirectional, light-triggered development. (p. 808)
• Photomorphogenesis is often triggered by red photoreceptors. (p. 808)
• Phytochrome is a light receptor involved in many plant growth responses, such as seed germination, shoot elongation, and plant spacing. It exists in two forms, red and far-red. (p. 808)
Phototropisms
• Phototropism describes directional growth toward sources of blue-wavelength light. (p. 809)
• In general, stems are positively phototropic while roots are nonresponsive or weakly negatively phototropic. (p. 809)
40.2 Plants respond to gravity.
Gravitropism
• Gravitropism refers to the response of a plant to the earth's gravitational field. (p. 810)
• Gravitropism occurs at germination when shoots grow upward and roots grow downward. (p. 810)
• Four steps lead to a gravitropic response: Gravity is perceived by the cell; signals form in the cell; a signal is transduced intra- and intercellularly; and differential cell elongation occurs. (p. 810)
40.3 Plants respond to touch.
Thigmotropism and Thigmonasty
• Thigmotropism is directional growth due to contact with another object. (p. 811)
• Thigmonastic responses are growth due to contact with another object, but the direction of the growth response is independent of the direction of the contact. (p. 811)
• Coiling tendrils can be examples of either thigmotropism or thigmonasty. (p. 811)
• Other tropisms have been recognized, including electrotropism (electricity), chemotropism (chemicals), traumotropism (wounding), thermotropism (temperature), aerotropism (oxygen), skototropism (darkness), geomagnetotropism (magnetic fields), and hydrotropism (water). (p. 811)
Turgor Movement
• Some plant movements are due to turgor pressure changes in specific cells that may be induced by several factors, including touch and light. (p. 812)
• Circadian clocks are endogenous clocks that keep plant movements and systems synchronized on a 24-hour cycle. (p. 813)
40.4 Water and temperature elicit plant responses.
Dormancy
• The ability to enter a dormant phase provides a survival advantage, especially under harsh conditions. (p. 814)
• Environmental signals trigger both the start and the end of dormancy. (p. 814)
Surviving Temperature Extremes
• Some plants have the ability to undergo deep supercooling where ice crystals are stored in extracellular spaces. (p. 815)
• Heat shock proteins can be produced to stabilize proteins under high temperatures. (p. 815)
40.5 The hormones that guide growth are keyed to the environment.
Plant Hormones
• Hormones are chemical substances produced in small quantities in one part of an organism and then transported to another part, where they bring about a response. (p. 816)
• In animals, hormones are usually produced in specialized tissues, but this is not the case in plants. (p. 816)
• Auxin, the first plant hormone to be discovered, increases the plasticity of plant cell walls and is involved in stem elongation. Auxin is synthesized in the apical meristems of shoots and helps cause stems to bend toward light. (p. 818)
• Cytokinins stimulate plant cell division and differentiation in combination with auxin, promote growth of lateral buds into branches, and inhibit the formation of lateral roots. (p. 822)
• Gibberellins, which are named after the fungus Gibberella fujikuroi, function endogenously as hormones. They are synthesized in apical portions of stems and roots and have effects on stem elongation. (p. 824)
• Brassinosteroids, which are very similar to animal steroid hormones, exhibit a broad spectrum of effects that can be triggered by environmental signals. (p. 825)
• Oligosaccharins function as signaling molecules and structural wall components when degraded by pathogens. They signal defense responses and inhibit auxin-stimulated stem elongation. (p. 825)
• Ethylene plays a major role in stimulating fruit development. (p. 826)
• Abscisic acid, which is mainly synthesized in mature green leaves and fruits, suppresses growth of dormant lateral buds and plays a role in promoting senescence, seed dormancy, and stomatal control. Levels of abscisic acid increase in plants under stress, and accordingly may play an important role in desiccation tolerance. (p. 827)