April 4, 2003
WASHINGTON -- Plants appear still and silent, but inside a clock is ticking.
Scientists in Israel and the U.S. Agriculture Department have discovered that plants, like animals, have a 24-hour biological clock.
Like the body clock that tells humans to wake up, plants have one that tells them to prepare for the sun.
The plant clock is set so it goes off around the same time every morning, usually a few hours before noon. The late morning alarm tells plants to prepare for intense sunlight, triggering processes that help the plants make food, said Autar K. Mattoo, a plant physiologist in the department's Agricultural Research Services lab.
The clock controls an enzyme that modifies a protein called D1, Mattoo said. This protein is critical for photosynthesis, the process whereby plants extract light and convert it to food.
When D1 binds with phosphorus, it creates a modified protein found in chloroplast -- a special structure in the cell that's made of carbohydrates, fat and proteins.
Mattoo says scientists believe the modified protein tells the plant to adjust its metabolism so it will protect itself from high light.
"It cannot run away. Their roots are stuck in the soil, so they have devised and perfected processes that allow them to survive in the harshest extreme environments," he said.
If the plants are exposed to excessive ultraviolet radiation, "plants produce molecules called flavonoids, which act as the sunscreen," Mattoo said.
The clock shuts off in the evening as the sun goes down.