Photoperiodism, Gravitropism, and Thigmotropism AP Biology Unit 5

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

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 and Slide 3 of 15

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

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

Phytochromes Germination and flowering occurs in response to red and far-red light –effects of both lights are reversible –P r and P fr are isomers (alternate forms) –red light (660 nm) activates P r to become P fr –far-red light (730 nm) activates P fr to become P r Slide 6 of 15

Flowering P fr –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? –P fr Slide 7 of 15

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

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

Flowering Photoperiodism AnimationPhotoperiodism Animation Slide 10 of 15

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

Gravitropism Response of a plant to gravity Shoots will grow against gravity (upwards) Roots will grow with gravity (downwards) Root placed on its side Stem 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 & 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. Image taken without permission from Slide 15 of 15