Photoperiodism is the physiological response of plants to the relative lengths of light and dark periods within a 24-hour cycle. This mechanism acts as a biological calendar, allowing plants to sense the change of seasons based on day length, which is a highly predictable environmental cue. By tracking the photoperiod, plants synchronize developmental stages, such as flowering, with the most favorable time of year for reproduction and survival. This process is a sophisticated adaptation that ensures a plant’s activities align with seasonal shifts.
The Role of Phytochrome in Sensing Light
Plants detect the duration of light and darkness using phytochrome, a specialized photoreceptor pigment. Phytochrome exists in two interconvertible forms: an inactive form called Pr (phytochrome red) and an active form called Pfr (phytochrome far-red). The Pr form absorbs red light and is immediately converted into the active Pfr form upon exposure to sunlight. Since sunlight contains a high proportion of red light, the active Pfr form accumulates during the day.
The active Pfr form is unstable and slowly reverts back to the inactive Pr form during periods of continuous darkness, a process called dark reversion. This slow conversion rate is the mechanism by which the plant measures the length of the night. If the night is short, a significant amount of the active Pfr remains at dawn; if the night is long, almost all of the Pfr has reverted to Pr. The duration of this uninterrupted dark period, often called the “critical night length,” primarily governs the plant’s photoperiodic response. A brief flash of red light during the dark period can instantly convert accumulated Pr back to Pfr, effectively “resetting” the night clock and preventing the long-night signal.
Categorizing Plant Responses to Light
The requirement for a specific critical night length allows classification into three categories based on their flowering response. Short-Day Plants (SDP) initiate flowering only when the night period is longer than their critical night length. These plants, which include chrysanthemums and poinsettias, typically flower in late summer, autumn, or winter when the nights are naturally long. Exposure to a night shorter than the critical length inhibits their flowering.
Long-Day Plants (LDP) require a night period shorter than their critical night length to flower. This means they flower when the days are long, such as during late spring and early summer. Common examples of LDPs are spinach, lettuce, and oats, which need the uninterrupted dark period to be curtailed. In these plants, the active Pfr form promotes flowering, which explains why a short night or a flash of light interruption promotes blooming.
Day-Neutral Plants (DNP) have a flowering process that is not regulated by the length of the light or dark periods. They flower once they have reached a certain stage of maturity or in response to other cues, such as specific temperatures. Tomatoes, corn, and sunflowers are classic examples of day-neutral species. Their reproductive timing is governed by internal developmental programs rather than the seasonal light calendar.
Key Biological Actions Controlled by Photoperiodism
While flowering is the most widely recognized outcome, photoperiodism governs several other major physiological actions. The photoperiodic signal ensures that the plant’s reproductive transition is timed to coincide with optimal conditions for pollination and seed dispersal. This seasonal timing increases the chance of successful reproduction.
Beyond reproduction, the shortening days of autumn trigger preparations for winter dormancy in many perennial species. Trees use the lengthening night to initiate the formation of protective winter buds, a process known as growth cessation. This response occurs before cold temperatures arrive, allowing the plant to anticipate the change in season.
Photoperiodism also controls leaf senescence and abscission, which is the process of leaves changing color and dropping off in the fall. The decreasing day length stimulates the breakdown of chlorophyll, revealing the underlying pigments, and promotes the formation of the abscission layer. This coordinated shedding of leaves is a strategy for water conservation and protection against freezing damage during the winter months.