Phototropism & photoperiodism
- Plants have a variety of developmental, physiological, and growth responses to light—sometimes only to particular wavelengths of light.
- In phototropism a plant bends or grows directionally in response to light. Shoots usually move towards the light; roots usually move away from it.
- In photoperiodism flowering and other developmental processes are regulated in response to the photoperiod, or day length.
- Short-day plants flower when day length is below a certain threshold, while long-day plants flower when day length is above a certain threshold.
- In many plants, photoperiodism is controlled by the overlap between the day length cue and the plant's internal circadian rhythms.
- Phototropism is a directional response that allows plants to grow towards, or in some cases away from, a source of light.
- Photoperiodism is the regulation of physiology or development in response to day length. Photoperiodism allows some plant species to flower—switch to reproductive mode—only at certain times of the year.
Phototropism involves a mobile signal
- He first cut off the tip of a coleoptile, covered the cut section with a block of gelatin, and replaced the tip. The coleoptile was able to bend normally when it was exposed to light.
- When he tried the experiment again using an impermeable flake of mica instead of gelatin, the coleoptile lost the ability to bend in response to light.
Phototropins and auxin
- Plants that flower only when day length drops below a certain threshold are called short-day plants. Rice is an example of a short-day plant.
- Plants that flower only when day length rises above a certain threshold are called long-day plants. Spinach and sugar beets are long-day plants.
What is the plant actually measuring?
- They flower when the day is short and the night is long.
- They do not flower when the day is long and the night is short.
- They do not flower when the long night is interrupted by a brief period of light.
- They do not flower when the long day is interrupted by a brief period of dark.