How plants tell time

Gardeners often marvel at how plants of the same species tend to flower, fruit and go to seed with remarkable synchronicity, as if on cue. Some flowers even follow a strict daily routine, opening their petals at dawn’s light and closing at dusk. How are plants able to keep such rigid schedules? The secret lies in a kind of vegetative internal clock that has fascinated CALS plant scientists for decades. Here’s what they’ve learned about how it works:

See the light. The primary component of plants’ internal clock is a light-absorbing molecule known as phytochrome. A turquoise-colored pigment that can exist in two different forms, phytochrome inter-converts between a “Pr” form, which absorbs red light, and a “Pfr” form, which absorbs far-red light.

The sun winds the clock. In full sunlight, plants’ leaf cells contain equal amounts of phytochrome’s two forms—50 percent Pr and 50 percent Pfr.

The clock counts down at night. When it’s dark, phytochrome molecules that are in the Pfr form start converting to Pr at a slow, steady rate. Because this rate is constant, the amount of Pfr remaining in the leaf cells at any given time is a direct reflection of the number of hours a plant has sat in darkness. In this way, plants use phytochrome to count the hours of the night—they don’t directly measure day length—and use that information to dictate when they sprout, flower, fruit and go to seed during the growing season.

Plants got rhythm. The phytochrome clock enables plants to establish a 24-hour circadian rhythm. For plants that open their flower petals at dawn and close them at dusk, the phytochrome clock allows them to anticipate the sun’s rise and set. In fact, when placed in a dark closet, these plants will continue to open and close their flowers right on schedule for a number of days, until the circadian rhythm finally dampens.