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Plant Growth & Development

Plant Growth & Development

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Plant Growth & Development

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  1. Plant Growth & Development Chapter 27

  2. Plant Growth and Development How do cells differentiate and form organs of the primary body of a plant? • Internal and external factors Growth • cell enlargement Development • genetically programmed succession from simple to more advanced form Differentiation • progressive, developmental change to more specialized form Totipotency • can undifferentiate and redifferentiate into other cell types

  3. Plant Hormones • Chemical signal that regulates and coordinates metabolism, growth, and morphogenesis • Plant hormones = plant growth regulating substances • Organic compound molecule made in one part of a plant • where it elicits a response • and/or transported to another part where it elicits a response • made in several locations in the plant body • Five classic groups Auxin Gibberellins Cytokinins Ethylene Abscisic acid

  4. Important Points 1. Single hormone elicits multiple responses 2. Effect of hormone depends on many factors • presence of other hormones • amount • sensitivity of tissue 3. Difficult to predict specific (single) response 4. Small quantities are required for response • 6µg IAA/kg plant • weight of needle/44,000lbs

  5. Initial hormone experiments (Charles & Francis Darwin, 1881) 1. Auxin • “when seedlings are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend.”

  6. 1. Auxin • Synthesized primarily in shoot apical meristems, young leaves, developing fruits/seeds • Cellular elongation • Stem elongation • Root growth • Phototropism & Gravitropism • Apical dominance • Differentiation of vascular tissue • Fruit development • Formation of adventitious roots tryptophan

  7. 1. Auxin • IAA (indole-3-acetic acid) is most abundant auxin in nature • Synthetic auxinsare economicallyimportant • NAA (1-Naphthaleneacetic acid) • induces adventitious roots in cuttings • reduces fruit drop in commercial crops • 2,4-D (2,4-dichlorophenoxyacetic acid) • herbicide • kills broadleaf dicots but not monocots • Agent orange • 1:1 auxin and 2,4,5-T (2,4,5-trichlrophenoxyaceticd acid) • sprayed to kill vegetation during Vietnamese war • may have long-term health effects on soldiers and Vietnamese citizens

  8. Auxins

  9. Differentiation of Vascular Tissue 1. Auxin • Physiological response to many hormones is influenced by non-hormonal factors • auxin with 2% sucrose • differentiation of xylem • auxin with 3% sucrose • xylem and phloem • auxin with 4% sucrose • differentiation of phloem

  10. Apical Dominance 1. Auxin • Axillary buds near shoot apex grow more slowly than those further from shoot apex • remember axillary buds at base of petioles • High IAA high ethylene  inhibition of bud growth  cone-shaped plant • IAA is released from apical bud, thus highest levels close to that bud • Single aspect of growth and development influenced by several hormones

  11. 2. Gibberellins a. Extensive growth: internode elongation • Growth retardants are manufactured to inhibit action of gibberellins • leads to stunted plants • chrysanthemums to produce flowers with thicker, sturdier stalks • turf-grass for slowing stem elongation Promotes seed/bud germination b. Promotes seed and bud germination c. Promotes flowering • biennials are rosettes first year • bolt to produce flowers 2nd year • GA can cause bolting in 1st year

  12. 3.Cytokinins a. Stimulates cellular division • hastens G2 to Mitosis transition • depends on presence of auxin also b. Organogenesis (formation of organs) • high auxin:cytokinin ratio  roots • low auxin:cytokinin ratio  shoots c. Senescence (leaf or other organ death) • delays breakdown of chlorophyll and thus initiation of senescence

  13. H2C=CH2 ethylene 4. Ethylene a. Fruit ripening • what happens if you put an orange and an unripe banana in a paper bag? • “one bad apple spoils the barrel” b. Opposes or reduces some effects of auxin c. Promotes or inhibits growth and development of roots, leaves, flowers, depending on species

  14. 5. Abscisic Acid a. Inhibits growth b. Closure of stomata c. Counteracts breaking dormancy (buds, seeds)