2. Auxins stimulate cell elongation in shoot tips, embryos, young leaves, flowers, fruits, and pollen. Auxins are more concentrated at the main shoot tip, which blocks growth of lateral buds(apical dominance).
3. Gibberellins stimulate cell division and elongation and help break seed dormancy, but act more slowly than auxins.
4. Cytokinins stimulate mitosis in actively developing plant parts.
5. Ethylene speeds ripening, senescence, and leaf abscission.
6. Abscisic acid counters the growth-inducing effects of other hormones by inducing dormancy and inhibiting shoot growth.
29.2 How Plants Move
7. A tropism is a growth response toward or away from an environmental stimulus, usually caused when different parts of an organ or structure grow at different rates.
8. In phototropism, light sends auxin to the shaded portion of the plant, stimulating growth towards the light.
9. Shoot growth is a negative gravitropism, and root growth is a positive gravitropism. The positions of amyloplasts in cells apparently help plants detect gravity.
10. Thigmotropism is a response to touch.
11. Nastic movements are not oriented toward a stimulus. Thigmonasty is response to contact. Nastic response to light and dark is nyctinasty, caused by osmotic changes that differently alter cell volume.
12. Plants sense seasonal and other environmental changes. Photoperiodism is the ability of a plant to measure length of day and night. Flowering can depend upon photoperiodism.
13. Short-day plants flower only when the duration of light is less than a critical length. Long-day plants require a light period of longer than a critical length. Day-neutral plants do not use light or dark cues to flower. The type of plant determines the season when it flowers.
14. Plants may respond to length of darkness rather than length of daylight.
15. A plant pigment, phytochrome, controls response to light. One form, Pr, absorbs red light to become Pfr. Pfr promotes flowering of long-day plants and inhibits flowering of short-day plants. It is reconverted to Pr by absorbing far-red light. The ration between these two forms provides information about daylight because sunlight has more red than far-red light.
16. Phytochrome controls seed germination and early seeding growth, and helps direct shoot phototropism.
17. Senescence is an active and passive cessation of growth. Growth becomes dormant during cold or dry times and resumes when environmental conditions are more favorable.
29.4 Plant Circadian Rhythms
18. Internal biological clocks control daily responses, or circadian rhythms. Environmental changes can alter, or entrain, these clocks.
19. Heliotropism (solar tracking) is a circadian rhythm in which flowers face the sun when cells in the upper stem respond to blue light.
