What makes the cherry trees bloom when they do?

 Brian Palmer, Published: April 4, Washington Post

Washingtonians have been flocking to the Mall for 99 years to watch their beloved cherry trees blossom. Thousands of tourists flood the city to picnic beneath the low-hanging branches. Their children frolic in the fallen pink petals, and National Park Service employees scold the naughty ones who pluck a fresh souvenir. And yet with all the merriment, probably very few wonder just how the floral extravaganza reproduces itself biologically year after year.

For the scientifically inclined, research by Ove Nilsson of Sweden’s Umea Plant Science Center has revealed a genetic basis for when a plant blooms; he notes that, in a sense, the blossom festival started way back last June.

 “The buds that open in the spring and produce these beautiful flowers were actually initiated the year before,” says Nilsson. “The flower initiation takes place in the summer, and the flowers develop through the fall.”

Here’s how it works. Every morning when the sun breaks over the horizon — no matter what time of year it is — a clock starts ticking inside the trees. After a specific number of hours, the plants’ cells start producing high levels of a molecule known as the FT protein. This protein is responsible for initiating processes that help the plant grow.

But the FT protein has a curious property: In the absence of sunlight, it curls up and dies. So when the sun sets, it degrades rather quickly and becomes useless to the plant.

Nilsson thinks this peculiarity is the key to the seasonality of certain flowering plants, including most trees that blossom in the spring.

If a plant is genetically programmed to produce lots of FT protein starting, say, 13 hours after dawn, the molecule will be abundant during the last few hours of daylight in the longer days of summer. And those few hours are enough to kick-start crucial growth processes. So the cherry trees do most of the heavy lifting for the spring festival — bud and flower formation — way back in the summer and early fall.

As autumn progresses and the days shorten, that same 13-hour clock — the number of hours will vary by species and even by individual plant — means that the FT protein will be produced in darkness, and it will degrade before it can work its magic. The trees take this as a signal to drop their leaves and stop generating new buds.

When winter arrives, day length and temperatures both reach their annual nadir. At this point, the plant goes through vernalization, a dormancy period that has enormous importance in protecting the trees, for reasons explained below.

In springtime, one might imagine that the FT protein process would reverse: The days get longer, the FT protein is produced during daylight hours, and the plant kicks off its growth process. Surprisingly, that’s not what happens.

While the cherry trees recognize the arrival of winter by sensing a decline in daylight, they detect the arrival of spring based on temperature. They wait for what’s called a “temperature sum”— basically a bunch of warm days in a row. (The precise temperatures and durations required vary by plant.) When the temperature sum is reached, the blossoms open. Of course, since the National Park Service can’t control the weather, peak blossoming doesn’t always coincide with the Cherry Blossom Festival’s lighting of the stone lantern and the Japanese taiko drummers.