36.3 : Biological Clocks and Seasonal Responses

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.