Why sunflowers face east

Date:
August 11, 2021
Source:
University of California - Davis
Summary:
Sunflowers face the rising sun because increased morning warmth
attracts more bees and also helps the plants reproduce more
efficiently, according to a new study.



FULL STORY ========================================================================== Sunflowers face the rising sun because increased morning warmth attracts
more bees and also helps the plants reproduce more efficiently, according
to a study by researchers at the University of California, Davis. The
results were published Aug. 9 in New Phytologist.


========================================================================== "It's quite striking that they face east," said Stacey Harmer, professor
of plant biology in the UC Davis College of Biological Sciences and senior author on the paper. "It's better for them to face east, as they produce
more offspring." While sunflowers are growing, their heads turn back
and forth to track the sun during the day. Previous work from Harmer's
lab showed that this tracking is controlled by the plant's internal
circadian clock.

But as the flower heads, or capitula, mature and their stems become
stiff and woody, this movement decreases until the heads are all facing
the morning sun.

When postdoctoral researcher Nicky Creux changed the orientation of
sunflowers by turning their pots around, she noticed that east-facing
flower heads attracted a lot more bees, especially in the morning,
than plants facing west.

In a series of experiments, Creux, Harmer and colleagues found that
the east- facing heads were significantly warmer in the morning than west-facing flower heads. That warmth brings an energy benefit to foraging
bees early in the morning, Harmer said. Direct sunlight also lights up ultraviolet markings on the flower petals that are visible to bees but
not to human eyes.

Orientation affects pollen release and flower development A sunflower
is actually a composite of hundreds, sometimes thousands, of individual flowers. These individual florets develop first at the outer edge of
the flower head, forming characteristic spiral patterns.

The orientation of the plants also affected flower development and
reproductive success. East-facing plants tended to produce larger
and heavier seeds. They also released pollen earlier in the morning,
coinciding with the times when bees visit.

These effects seemed to be controlled by the temperature at the flower
head.

When researchers used a portable heater to warm up west-facing heads,
they were able to get similar results to east-facing flower heads.

Finally, Evan Brown, an undergraduate student supervised by Ben Blackman
at the University of Virginia, took sterile male plants, which could
produce seeds but not make pollen, and surrounded them with normal plants facing east or west.

Using genotyping, they were able to distinguish whether the male-sterile
plants were pollinated by east- or west-facing plants. The team found
that pollen from the east-facing plants was responsible for more offspring
than that from west- facing plants.

The work was supported by grants from NSF and the U.S. Department of Agriculture-National Institute of Food and Agriculture.

========================================================================== Story Source: Materials provided by
University_of_California_-_Davis. Original written by Andy Fell. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Nicky M. Creux, Evan A. Brown, Austin G. Garner, Sana Saeed, C. Lane
Scher, Srinidhi V. Holalu, Daniel Yang, Julin N. Maloof, Benjamin K.

Blackman, Stacey L. Harmer. Flower orientation influences floral
temperature, pollinator visits and plant fitness. New Phytologist,
2021; DOI: 10.1111/nph.17627 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210811113133.htm

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