Photoperiodism

Not every plant flowers at the same time of year, and one of the main environmental cues a plant uses to decide when to flower is the relative length of day and night it experiences. This response of flowering to the duration of light and darkness is called photoperiodism.

Long-day, short-day and day-neutral plants

Based on how they respond to day length, flowering plants fall into three broad categories:

• Long-Day Plants (LDP) flower only when the length of daylight they receive exceeds a certain critical duration — typically plants that flower in the late spring and summer, as days are progressively lengthening. Examples include henbane (Hyoscyamus niger) and spinach.
• Short-Day Plants (SDP) flower only when the daylight period is less than their critical duration — that is, when the night is sufficiently long. These typically flower in autumn/winter, as days shorten. Examples include Xanthium (cocklebur) and rice; tobacco (Nicotiana tabacum) is another classic short-day plant.
• Day-Neutral Plants (DNP) show no such dependence on day length at all and can flower whenever they reach the appropriate stage of vegetative maturity, regardless of the photoperiod. Examples include sunflower, tomato and maize.

Some species show a more complex response that still gets grouped with one of the three categories for classification purposes. Bryophyllum, for example, is generally classified as a short-day plant, but its actual flowering behaviour is more nuanced than a typical SDP: it is sometimes described as needing a sequence of long days followed by short days (a "long-short-day" response) before it flowers. For classification and exam purposes it is still grouped under SDP, since short days are the final, immediately inductive requirement.

The critical role of the dark period

Careful experiments have shown that it is not simply the length of the light period that a plant is sensing, but really the length of the uninterrupted dark period. A short-day plant that is kept in what would otherwise be a qualifying long night, but has that night briefly interrupted by a flash of light, fails to flower — while interrupting the day period with a brief spell of darkness has no such disruptive effect. This is why the response is sometimes more accurately described in terms of a "critical night length" rather than a critical day length, even though the categories themselves are conventionally named for day length.

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