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Modern biologists believe that it is the coincidence of the active forms of phytochrome or cryptochrome, created by light during the daytime, with the rhythms of the circadian clock that allows plants to measure the length of the night. Each plant has a different length critical photoperiod, or critical night length.

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LSDPs flower after a series of long days followed by short days whereas SLDPs flower after a series of short days followed by long days. These plants are either long-short-day plants (LSDP) or short-long-day plants (SLDP). Along with long-day plants and short-day plants, there are plants that fall into a 'dual-day length category'. Photoperiodic flowering plants are classified as long-day plants or short-day plants even though night is the critical factor because of the initial misunderstanding about daylight being the controlling factor. Allard published their discoveries on photoperiodism and felt it was the length of daylight that was critical, but it was later discovered that the length of the night was the controlling factor.

This shows how important both of the photoreceptors are in regards to determining day-length. It has been found that both cryptochrome and phytochrome abundance relies on light and the amount of cryptochrome can change depending on day-length. Cryptochromes entrain the circadian clock to light. Cryptochromes absorb blue light and UV-A. Ĭryptochromes are another type of photoreceptor that is important in photoperiodism. A short-day plant will not flower if light is turned on for a few minutes in the middle of the night and a long-day plant can flower if exposed to more red-light in the middle of the night. Other experiments have proven this by exposing plants to extra red-light in the middle of the night. They discovered that plants tend to flower later when exposed to more red light, proving that red light is inhibitory to flowering. showed that manipulations of the red-to far-red ratio in Arabidopsis can alter flowering. This is important in regards to plant flowering. P fr can also be converted back to P r by a process known as dark reversion, where long periods of darkness trigger the conversion of P fr. This system of P fr to P r conversion allows the plant to sense when it is night and when it is day. In comparison, plants receive more far-red in the shade, and this converts phytochrome from P fr to its inactive form, P r, inhibiting germination. Red light (which is present during the day) converts phytochrome to its active form (P fr) which then stimulates various processes such as germination, flowering or branching. Phytochrome comes in two forms: P r and P fr.

In a further subdivision, obligate photoperiodic plants absolutely require a long or short enough night before flowering, whereas facultative photoperiodic plants are more likely to flower under one condition. Many flowering plants (angiosperms) use a photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in night length, or photoperiod, which they take as signals to flower.

When nights are short, an excess amount of Pfr remains in the day time and during long nights, most of the Pfr is reverted to Pr. Pr converts to Pfr during the day time and Pfr slowly reverts to Pr during the night time.