PLANT RESPONSES

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 Slide 12 of 15

Auxin and Gravitropism Auxin is responsible for gravitropism inhibits cell elongation in roots stimulates cell elongation in shoots In stems Slide 13 of 15

Gravitropism (geotropism) & Starch Statoliths (dense starch-containing plastids) may accumulate in the direction of gravity to assist in gravitropism Slide 14 of 15

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. Slide 15 of 15 Image taken without permission from http://images.botany.org/set-08/08-004v1.jpg

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

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 http://hoopermuseum.earthsci.carleton.ca/vegetation/photoperiodism.gif

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

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 Slide 4 of 15

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.

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.