With the winter solstice upon us, we're now experiencing the shortest
daylight periods of the year. Some of us like the change, and some of us
lament it. Plants and animals are affected too. It's these variations in
day length that help them set their internal clocks.
Migratory birds are a prime example. A Slate-colored Junco nesting in
northern Canada responds to the first shortening days of summer with a
series of physical changes: its reproductive organs become inactive and
shrink in size, hormones stimulate the rapid growth of a new set of
feathers (its non-breeding plumage), and fat deposits develop to provide
fuel for the long migratory flight ahead.
Thus the preparation for migration starts as soon as the days begin
to shorten. And the process must operate in reverse when the bird is in
its winter habitat in the United States.
As soon as days begin to lengthen, the Slate-colored Junco must gear
up physically for the flight north and breeding season. If it fails to
do so, it likely won't survive a long-distance migration.
Plants in temperate zones must also set their calendars accurately in
order to flower and, for deciduous species, develop and drop leaves at
the optimal time. Plants set their internal calendars using several
attributes from the sunlight they receive.
In fact, the angle of the sun may be more important to a plant than
day length. That's because plant cells produce compounds called
phytochromes in response to different portions of the light spectrum.
Direct sunlight is higher in red light, while indirect sunlight
contains more far-red light. During late fall and early winter, when the
sun remains low in the southern sky, the indirect light produces an
increase in far-red phytochromes. As spring approaches and the arc of
the sun rises in the sky, direct sunlight triggers the production of red
phytochromes.
The ratio of these two compounds mediates the hormones involved in
flowering, leaf drop, and bud development. Even seeds below the soil are
affected. It's true: the amount of red and far-red light that penetrate
the soil is sufficient to govern germination.
We wish to thank wildlife photographer Gay
Bumgarner for the image used in this story.