Short-day plants: These plants flower when the day length is shorter than a critical threshold. They typically flower in the fall or winter when days are shorter. Examples include:
* Chrysanthemums
* Poinsettias
* Asters
* Rice
Long-day plants: These plants flower when the day length is longer than a critical threshold. They typically flower in the spring or summer when days are longer. Examples include:
* Spinach
* Lettuce
* Wheat
* Barley
Here's how it works:
* Phytochrome: Plants use a pigment called phytochrome to detect the length of day and night.
* Light sensitivity: Phytochrome is sensitive to red and far-red light. During the day, it absorbs red light and converts to an active form. During the night, it converts back to its inactive form.
* Critical threshold: The length of the night determines how much phytochrome is in its active form. If the night is longer than the critical threshold for a short-day plant, enough phytochrome will be in its active form to trigger flowering. The opposite happens for long-day plants.
Important note: Not all plants are strictly short-day or long-day. Some plants are day-neutral, meaning they flower regardless of day length. Others are intermediate-day plants, requiring a specific range of day length to flower.
The terms "short day" and "long day" can also refer to the actual length of daylight hours in a particular place, but the context usually makes it clear which meaning is intended.