We now know, from the 1926 experiments of Frits Went, that auxin, a plant hormone produced in the stem tip (auxins promote cell elongation), moves to the darker side of the plant, causing thecells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.
Alterations in Growth Patterns Generate Tropisms
Charles Darwin and his son Francis studied the familiar reaction of plants growing toward light: phototropism. The Darwins discovered that the tips of theplant curved first, and that the curve extended gradually down the stem. By covering the tips with foil, they prevented the plant from curving. They concluded that some factor was transmitted from the tip of the plant to the lower regions, causing the plant to bend.
Phototropism in the coleoptile of a monocot. Image from Purves et al., Life: The Science of Biology, 4th Edition, by SinauerAssociates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
We now know, from the 1926 experiments of Frits Went, that auxin, a plant hormone produced in the stem tip (auxins promote cell elongation), moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plantstem tip toward the light, a plant movement known as phototropism.
Geotropism is plant response to gravity. Roots of plants show positive geotropism, shoots show negative geotropism. Geotropism was once thought a result of gravity influencing auxin concentration. Several new hypotheses are currently under investigation.
HORMONE- a signaling molecule released by one cell and transported to TARGETCELLS (usually nonadjacent). The target cell contains receptor sites.
Hormones that Promote Growth
moves upward and downward in vascular system
too little- dwarf
too much- long, spindly
important in bolting (sudden stem lengthening)
induce seeds of some grasses to germinate (breaks dormancy)
can break dormancy in buds
can stimulateflowering in mature plants
Auxin moves only downward but gravity does not cause its movement.
It is generally produced by apical shoot meristems and developing leaves.
Auxin promotes growth by stimulating cell elongation.
It triggers enzyme activities that loosen the cell wall fibers (normally tightly woven).
Auxin also stimulates cell division. It activates the vascular cambiumand promotes the formation of lateral roots by the pericycle.
Auxin inhibits the growth of lateral buds.
small quantities promote root growth. Slight increases inhibit it.
Auxin production by seeds stimulates fruit growth. If eggs are not fertilized, ovules do not become seeds and auxin cannot be produced. The lack of auxin results in abscission of the flower.
Auxin alsoprevents fruits and leaves from off prematurely. It can be sprayed on to prevent from falling off.
high concentration ® uncontrolled growth, death
example: some herbicides- 2,4-D, 2,4,5-T (components of agent orange)
Apical dominance is the inhibition of lateral buds by auxin from the apical bud.
Auxin is broken down as it moves down stem, so its concentration decreases.lowest buds- least inhibited.
Removal of apical bud releases buds farther down
stimulate cell division
major source is the roots
oppose auxin by:
promote growth of lateral buds
as plant grows, lower buds are more under the influence of cytokinins
prevents leaf senescence
Senescence refers to the aging and eventual death of a plant or plant parts.