Photoperiodism is the phenomenon where the duration of light and darkness influences how plants grow and develop. The photoperiod acts as a reliable seasonal cue for plants to coordinate their life events. This response is not primarily about the total amount or intensity of light, which affects photosynthesis. By accurately tracking the changing day and night lengths, plants can synchronize crucial processes like flowering, dormancy, and tuber formation with the optimal time of year.
Understanding Photoperiodism
Photoperiodism is the physiological reaction of plants to the relative lengths of light and dark periods. This biological mechanism allows plants to detect the passage of seasons with accuracy that temperature and moisture fluctuations cannot provide. Early research mistakenly focused on the length of the day as the controlling factor for plant development.
Modern understanding reveals that plants are actually measuring the duration of the uninterrupted dark period, often called the scotoperiod. The length of this continuous night determines whether a developmental response, such as the induction of flowering, will be triggered. If the dark period is broken by even a brief flash of light, the plant’s internal timekeeping is reset, which can prevent the required response from occurring.
How Plants Sense Light Duration
Plants sense the duration of light and darkness using specialized photoreceptor proteins, the most well-known of which are the phytochromes. Phytochromes exist in two interconvertible forms: an inactive form called Pr (phytochrome red) and a biologically active form called Pfr (phytochrome far-red). When Pr absorbs red light, which is abundant in sunlight, it is quickly converted into the active Pfr form.
The active Pfr form is responsible for triggering physiological responses, such as promoting growth or initiating flowering in certain species. When darkness falls, the Pfr form slowly reverts back to the inactive Pr form in a process known as dark reversion. This dark reversion acts as the plant’s internal timer: the amount of Pfr remaining at dawn indicates the length of the previous night.
Classifying Plants by Growth Response
The most visible effect of photoperiodism is the timing of flowering, which allows botanists to classify plants into three main groups based on their growth response. These classifications are determined by a plant’s specific critical night length, the minimum duration of darkness required to initiate a change.
Short-Day Plants (SDP)
Short-Day Plants (SDP), sometimes called long-night plants, only flower when the night period is longer than their critical threshold. These plants typically flower in the late summer, fall, or winter when the days are short and the nights are long. Classic examples include:
- Chrysanthemums
- Poinsettias
- Rice
- Soybeans
Long-Day Plants (LDP)
Long-Day Plants (LDP) require a night period shorter than a critical threshold to initiate flowering, meaning they bloom when the days are long. These plants are generally summer bloomers in temperate regions, where the nights are brief. Common examples include:
- Spinach
- Lettuce
- Radishes
- Many grains like wheat and oats
Day-Neutral Plants (DNP)
Day-Neutral Plants (DNP) have a reproductive cycle that is unaffected by the length of the day or night. Instead, these plants flower once they have reached a certain age or size, or in response to non-light cues. Examples include common vegetables such as:
- Tomatoes
- Corn
- Cucumbers
- Sunflowers
Practical Applications for Growth Control
Understanding photoperiodism allows growers to manipulate plant life cycles for economic benefit. Greenhouse operators use artificial lighting to extend the effective day length, a technique that can force Long-Day Plants to flower early or continuously. This practice is used extensively in the cut flower industry to ensure blooms are available out of season, such as timing flowers for specific holidays.
Conversely, growers use blackout cloths or movable dark structures to simulate a long, uninterrupted night for Short-Day Plants. This manipulation allows them to induce flowering in plants like chrysanthemums and poinsettias during the summer months, outside their natural autumn bloom time. This precise control over the photoperiod minimizes the time required to bring a crop to market and aligns production with peak consumer demand.