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15.1 Plant Responses to Stimuli p. 554-565
Plant Responses to Stimuli
Internal Regulators
plant growth regulators= plant growth hormones
definition: chemical substances that regulate the growth of cells and tissues inside the plant
action: can either stimulate or inhibit growth
source: produced by the growing tissue
AUXINS
Action
-stimulate cell division and cell enlargement in apical meristems
eg. root formation on stems, cell division of vascular cambium, fruit growth
Natural Example: IAA (indole-3-acetic acid)
Man-Made Example: 2,4-D the weed control agent
-stimulates growth beyond control
Effects:
Phototropism: when light shines on one side of the growing stem, auxins move to the dark
side and cause the cells there to elongate and the stem bends toward the light
Geotropism: auxins concentrate on the underside of the developing root and inhibit
elongation. The top side cells elongate and the root grows down.
Apical Dominance: auxins inhibit lateral bud growth in high concentration. They are
produced at the top and diffuse down the stem.
GIBBERELLINS
Action: promote cell division and elongation; stimulate seed germination
Produced by: growing shoot tips
CYTOKININS
Action: stimulate cell division, prevent leaf aging
Found in plant tissues with rapidly dividing cells.
Use: biotechnology; "cloning" identical plants from a "callus"
ETHYLENE
Action: interacts with other growth regulators; encourages fruit ripening; stimulated by auxin
can induce flowering
How it works: increases rate of respiration and conversion of starch to glucose
ABISCISIC ACID
Action: inhibition; maintains dormant states; cause stomate to close;
Tropisms: External Regulation of Growth
Growth Responses to a Directional Stimulus
Phototropism:
-the growth of a plant toward light
-occurs because the cells on the dark side elongate more
Geotropism:
-positive geotropism- the growth of a root toward the direction of gravity
-negative geotropism- the growth of a shoot away from the direction of gravity
*both are regulated by chemicals
Non-directional Movements
Nastic Movements
-occur in response to a stimulus but the movement is independent of the direction of the stimulus
eg. venus fly trap
-tactile stimulus cause certain cells to lose turgor pressure and they become flaccid
Photoperiodism
-response to the length of day
-plants contain a blue pigment called phytochrome
-light converts phytochrome to another form-this new form stimulates flowering in long-day plants
and inhibits flowering in short day plants
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Section Review p. 565 #1-8