Sunflowers grow bigger when moving with sun

Image Infrared imaging reveals changes in sunflower surface temperatures at different times of day. Credit Evan Brown University of Virginia

Every night for 100 nights, Harmer's post-doctorate researcher Hagop Atamian went into a field of sunflowers planted in pots and rotated them so they were facing west in the morning.

This behavior of these ethereal flowers remained a mystery until now when the scientists have answered this question, central to their charm. The lovely yellow flowers turn east in the morning to face the rising sun and then moves across the sky during the day before heading westward by dusk.

They also found, by placing ink dots on the stems and filming them, that the east side of the plant would grow faster when facing the sun.

"It's the first example of a plant's clock modulating growth in a natural environment, and having real repercussions for the plant", said Stacey Harmer, professor of plant biology at UC Davis and senior author on the paper to be published August 5 in the journal Science. That's a result of different sides of the stem elongating depending on what time of day it is.

To see if the plants had their own biological clocks that were reset daily by the Sun, similar to our own, or were simply moving according to a pre-determined schedule, the team performed a series of experiments.

"Just like people, plants rely on the daily rhythms of day and night to function", says Anne Sylvester, director of the National Science Foundation's Plant Genome Research Program.

Young sunflowers follow the sun, their still-green buds arcing from east to west across the summer sky as dawn turns to noon turns to dusk.

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Following the sun appeared to boost sunflower growth, as those that were staked and rendered motionless reached smaller sizes than those with freedom of movement. More sunlight equals better growth, and the plants know it.

To find out more about why the flowers do this, the researchers used staked plants in pots to prevent them from following the sun.

This shows that following the sun throughout the day is advantageous, helping the plants both grow in size and ensure pollination.

A report published in NY Times revealed, "At dawn, whole fields of sunflowers stand at attention, all facing east, and begin their romance with the rising sun". Again, the circadian clock helps lead to the eastward orientation of sunflower heads when they stop tracking as they bloom, Blackman said.

When plants were moved into an indoor growth chamber with immobile overhead light, they continued to swing back and forth for a few days - the kind of behavior you would expect from a mechanism driven by an internal clock. Second, the internal clock is set to track the direction of light, leading the stem to grow unevenly, and causing the sunflower to sway east and west throughout the day. A portable heater was enough to restore a large numbers of pollinators back to the west-facing ones.

In another experiment, the researchers prevented outdoor potted sunflowers from tracking the sun.

"We show that these roles impact how much the plants grow and how well they attract pollinators, thus demonstrating that this regulation of light-responsive growth by the clock is likely an (evolutionary) adaptation", said co-author Benjamin Blackman, an assistant professor of plant and microbial biology at UC Berkeley.