Photoperiodism, Gravitropism, and Thigmotropism

1. Photoperiodism, Gravitropism, and Thigmotropism AP Biology Unit 5

2. Photoperiodism • How a plant responds (with respect to flowering) to the relative amount of light (“photoperiod”) • In reality, plants are responding to the relative amount of night. Slide 2 of 15

3. Photoperiodism: Types of Plants • 3 different types of plants: • “Short Day”  flower when days are short, nights are long (Ex. poinsettias, chrysanthemums) • “Long Day”  flower when days are long and nights are short (Ex. Spinach, Radish) • “Day Neutral”  flowering does not depend on length of day or night (Ex. tomato) Images taken without permission from http://www.fernlea.com/xmas/pix/poinsettia.jpg, and http://www.illinoiswildflowers.info/weeds/plants/garden_radish.htm Slide 3 of 15

4. 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

5. Phytochromes • Plants absorb light via blue-light photoreceptors and phytochromes (Pr and Pfr). • Pr and Pfr play a significant role in the flowering and germinating responses Slide 5 of 15

6. Phytochromes • Germination and flowering occurs in response to red and far-red light • effects of both lights are reversible • Pr and Pfr are isomers (alternate forms) • red light (660 nm) activates Pr to become Pfr • far-red light (730 nm) activates Pfr to become Pr Slide 6 of 15

7. Flowering • Pfr • inhibits flowering in short day plants • promotes flowering in long day plants • Sunlight consists of quite a bit of red light, not much far red light • During the day, which form of phytochrome is in? • Pfr Slide 7 of 15

8. Flowering • At sunset, most of the phytochrome is in the Pfr form • During the night, Pfr gets converted back into Pr or breaks down • Whether a plant flowers or not depends on the amount of Pfr left (which relates to the amount of night) Slide 8 of 15

9. Flowering Hormone? • There also appears to be a flowering hormone called florigen – not fully understood yet Slide 9 of 15

10. Flowering • Photoperiodism Animation Slide 10 of 15

11. Germination • Red light stimulates germination • Far red light inhibits germination • What matters is the last light the seeds are exposed to Slide 11 of 15

12. Gravitropism Stem placed on its side • Response of a plant to gravity • Shoots will grow against gravity (upwards) • Roots will grow with gravity (downwards) Root placed on its side Slide 12 of 15

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

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

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