1. PLANT RESPONSES

2. Gravitropism (geotropism)
Response of a plant to gravity
Shoots will grow against gravity (upwards)
Roots will grow with gravity (downwards)
Stem placed on its side
Root placed on its side
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3. Auxin and Gravitropism
Auxin is responsible for gravitropism
inhibits cell elongation in roots
stimulates cell elongation in shoots
In stems
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4. Gravitropism (geotropism) & Starch
Statoliths (dense starch-containing plastids) may accumulate in the direction of gravity to assist in gravitropism
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5. Thigmotropism Response of plant to touch.
Ex. Tendrils of vines wrapping around things, venus fly traps, “shy plant”
Response is similar to nervous response in humans.
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6. Photoperiodism
Some plants flower only when the day length is shorter than some critical period
Short-day plants
Rice, chrysanthemum, poinsettia, violet
Some plants flower only flower after being exposed to day lengths that are longer than a certain critical minimum
Long-day plants
Wheat, cabbage, hibiscus, spinach
Some plants flower regardless of the length of day
Day-neutral plants
Corn, potato, rhododendron, tomato

7. Eg, Poinsetta, which flowers only when the day length becomes less than 12.5 hrs
Eg, Hibiscus, which flowers only when the day length becomes greater than 12 hrs
Eg, Cocklebur, which flowers only when the night length exceeds 9 hrs

8. In plants which require an extended night length (ie, the cocklebur), if the dark period is interrupted even briefly by a flash of light, the plant no longer flowers
Uninterrupted dark is important for flowering plants
Short-day plants are really long-night plants
Long-day plants are really short-night plants
Phytochromes, light-sensitive pigments in leaves, react to a photoperiod that is appropriate for the particular plant  hormone is released when travels to the buds which then flower

9. Question…
Poinsettias are short day plants– how could nurseries make sure they bloom just before Christmas?
Control the amount of light and dark they experience
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10. PLANT HORMONES Auxins Gibberellins
Promote stem elongation, inhibit growth of lateral buds (maintains apical dominance). They are produced in the stem, buds, and root tips. Example: Indole Acetic Acid (IA).. Auxin 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 plant stem tip toward the light, a plant movement known as phototropism. Auxin also plays a role in maintaining apical dominance.
Gibberellins
Promote stem elongation. They are not produced in stem tip. Gibberellic acid was the first of this class of hormone to be discovered.

11. Cytokinins Abscisic Acid Ethylene
Promote cell division. They are produced in growing areas, such as meristems at tip of the shoot. Zeatin is a hormone in this class, and occurs in corn (Zea).
Abscisic Acid
Promotes seed dormancy by inhibiting cell growth. It is also involved in opening and closing of stomata as leaves wilt.
Ethylene
A gas produced by ripe fruits. Why does one bad apple spoil the whole bunch? Ethylene is used to ripen crops at the same time. Sprayed on a field it will cause all fruits to ripen at the same time so they can be harvested.