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Unformatted text preview: Plant Responses to the Environment Chapter 26 What are plant hormones and how do they act? • Plant hormones are chemicals that are produced in one part of a plant’s body and transported to other parts of the plant where they exert their effects • Five major classes of plant hormones have been identified: o Auxins o Gibberellins o Cytokinins o Ethylene o Abscisic Acid Hormone affects on plants (Table 26-1) • Hormones have multiple effects depending on: o Type of target cell o Developmental stage of the plant o Concentration of the hormone o Presence of other hormones How do hormones regulate the pant life cycle? • Maturing seeds produce absicisic acid o Abscisic acid maintains seed dormancy by lowering the metabolic rate of the embryo o Abscisic acid must be removed for the seed to germinate Seeds of the desert plants require a hard rain (washes acid away) Seeds of high latitudes plants require an extended period of cold (destroys acid) • Gibberellin Stimulates Germination o Gibberellin is synthesized when the seed breaks dormancy o Rising levels initiate synthesis of enzymes that digest food reserves (endosperm and cotyledons) • Auxin Controls orientation of the sprouting seedling o Auxin is produced in shoot tips and is transported toward roots Stimulates elongation of stem cells Retards elongation of root cells o Distribution of auxin within shoot and root cells is influenced by gravity and light o Gravitropism and phototropism in shoots (Figure 26-1) Auxin accumulates in cells toward gravity and away from light Since auxin stimulates elongation cells, the stem bends away from gravity and toward light Since auxin retards elongation of root cells, the root bends toward gravity How do plants detect gravity? (Figure 26-2) 1 • Plants apparently detect gravity by means of starch-filled plastids o Located in specialized stem cells and root caps o Orient within cells toward gravity • Changes in plastid orientation are believed to trigger the accumulation of auxin and subsequent bending or root or shoot Hormones interact to determine shape of mature plant (Figure 23-6) • Auxin: Cytokinin ratio regulates branching of shoots o Auxin: produced by shoot tip and transported downward; high levels of auxin near shot tip inhibit lateral bud growth (apical dominance) o Cytokinin is produced by the root tips and transported upward; an optimal concentration of both auxin and cytokinin stimulate lateral growth • Branching of roots (from pericycle) is stimulated by auxin • Auxin and cytokinin create a balance between root and shoot systems o Enlarging root system produces large amounts of cytokinin; stimulates lateral bud growth o Enlarging shoot system produces large amounts of auxin; stimulates root branching and growth Daylight (Duration of Darkness) Controls flowering (Figure 26-4) • Day-neutral plants flower when mature, regardless of daylight • Short-night plants flower when the duration of darkness is shorter than critical value...
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This note was uploaded on 05/06/2008 for the course BIOL 1002 taught by Professor Pomarico during the Spring '08 term at LSU.
- Spring '08