Plants Keep Track of the Hours and the Seasons

Remember the tropical plant Mimosa pudica that quickly folds up when touched

(thigmotropism)? Well, every day at dawn, it unfolds its leaves and every day at dusk it folds

its leaves. These movements, seen in many other plants as well, form a regular daily pattern.

Vocabulary • You respond to changes in time……

• Your pulse rate, blood pressure, and body temperature change with the time of day

• Different time zones could affect you—what we call jet lag.

• An organism's "biological clock" is set by daily signals from the environment, especially light.

circadian rhythm: rhythmic pattern of behavior (in an animal) or biological cycle (in a plant) that follows an approximately 24-hour natural cycle

Day Length and SeasonsIn addition to maintaining a 24-hour cycle, plants also respond to changes in environmental cues

that come with the seasons.

Some plants flower in the spring, for instance, while others flower in the fall.

i.e. Tulips (spring) and Christmas cactus (fall)

Pictures

Christmas Cactus

Blooms in the fall usually close to Christmas hence the name…..flower in fall or winter when the dark period exceeds a certain length, called critical night length

Vocabularyphotoperiodism: plant response to cycles of light and darkness

short-day plant: plant that blooms when the dark period exceeds a critical length

long-day plant: plant that blooms when the dark period is shorter than a critical length

day-neutral plant: plant that blooms when it reaches a certain stage of maturity, regardless of day length

How do we use this information in real life….

Flower growers use information about photoperiodism to produce certain flowers out of

season.Chrysanthemums, for instance, are short-day plants

that normally bloom in autumn. But a florist can prevent a chrysanthemum from blooming until the

spring by exposing the plants to flashes of light during the long autumn and winter nights. The

plant thus detects two short nights, instead of one long night, and fails to bloom.

Chrysanthemums

How does a plant monitor day length as the seasons change?

•Sunrise and sunset are detected by pigmented proteins called phytochromes•phytochromes absorb the red light that is abundant at sunrise, they change shape to an active form that triggers a variety of plant responses.•After sunset, the phytochromes gradually change back to their inactive form