36.3: Biological Clocks and Seasonal Responses

The circadian—or biological—clock is an intrinsic, timekeeping, molecular mechanism that allows plants to coordinate physiological activities over 24-hour cycles called circadian rhythms. Photoperiodism is a collective term for the biological responses of plants to variations in the relative lengths of dark and light periods. The period of light-exposure is called the photoperiod.

One example of photoperiodism in plants is seasonal flowering. Scientists believe that plants are cued to flower by the correspondence of their circadian clocks to changes in the photoperiod. They detect these changes using light-sensitive photoreceptor systems.

Phytochromes are a group of photoreceptors involved in flowering and other light-mediated processes. The phytochrome system enables plants to compare the duration of dark periods over several days.

Short-day (long-night) plants flower after a minimum number of consecutive long nights. Long-day (short-night) plants, by contrast, initiate flowering following a minimum number of consecutive short nights.

Phytochromes exist as two interconvertible forms: Pr and Pfr. Pr is converted into Pfr during the day, so Pfr is more abundant in daylight hours. Pfr is converted into Pr at night, so there is more Pr at nighttime. Therefore, plants can determine the length of the day-night cycle by measuring the Pr/Pfr ratio at dawn. The long nights of winter reduce Pfr levels at dawn, while the shorter nights of spring result in higher Pfr levels at sunrise.

The flowering of most plants is synchronized with seasonal changes, allowing plants to germinate during optimal conditions. How do plants sense time and seasonal changes? 

Internal timekeepers called biological clocks sense environmental variations, such as changing light levels. Biological clocks allow plants to regularly follow circadian rhythms, daily, 24-hour behavioral cycles. 

Plants also use biological clocks to respond to seasonal changes. One of the mechanisms through which plants respond to varying seasons is the phytochrome system.

Phytochromes are light-sensitive receptors. One of their many functions in plants is to detect changing seasons. Phytochromes do this by measuring the day length or photoperiod. The ability of phytochromes to regulate photoperiodism, biological responses to the photoperiod, depends on their light-stimulated transition between two interconvertible forms⁠—the inactive Pr and the active Pfr.

Phytochromes are synthesized in the dark, in their inactive Pr form, within the plant cytoplasm. During the day, Pr absorbs red light from sunlight and quickly converts into its biologically active Pfr form. Pfr can activate cytoplasmic molecules, or translocate to the nucleus and regulate gene expression. 

At night, the Pfr levels in plant cells decline, due to the slow darkness reversion of  Pfr to Pr or the destruction of Pfr by enzymes.

During the long nights of winter, the levels of Pfr in plant cells may completely drop by sunrise. If the nights are shorter, as they are in the springtime, a considerable amount of Pfr may remain at sunrise.

The ratio of Pr to Pfr at dawn allows plants to determine the length of the day-night cycle. Because Pfr levels fluctuate with seasons, higher levels of Pfr may activate plants that flower during seasons with long days while lower levels of Pfr are necessary to activate plants that flower during short days. 

Interactions between the phytochrome system and the biological clock enable plants to measure the relative lengths of nights and days throughout the year and synchronize their activities with the seasons.