Photoperiodism
Name:____________________________________P____
Photoperiodism:
Plant type
Long-Day
Short-Day
Day-Neutral
Phytochrome:
P:
r
P :
fr
Other notes:
Flowering and light
Examples
Pfr form
Flash of light
Photoperiodism
The term "photoperiodism" was coined to describe a plant's ability to flower in response to changes in the photoperiod:
the relative lengths of day and night. Because flowers produce seeds, flowering is crucially important for the plant to
complete its life cycle. Although people had long known that plants such as tulips flower in the spring and
chrysanthemums flower in the fall, until the early 1900s little was known about what actually caused flowering.
Beginning in 1910, Wightman Garner and Henry Allard conducted experiments to test the effect of day length on
flowering. They discovered that plants such as barley flowered when the day length was longer than a certain critical
length. These plants, which they named long-day plants (LDPs), flower mainly in the summer as the days are getting
longer. Others, such as soybeans, flower when the day length is shorter than a certain critical length. These short-day
plants (SDPs) flower in the fall as the days are getting shorter. Still others are not sensitive to the photoperiod and are
called day-neutral plants.
Photoperiodism is responsible for the distribution of many plants worldwide. For example, ragweed (a SDP) is not found
in northern Maine because the plant flowers only when the day length is shorter than 14.5 hours. In northern Maine, days
do not shorten to this length until August. This is so late in the growing season that the first frost arrives before the
resulting seeds are mature enough to resist the low temperatures, and so the species cannot survive there. By
contrast, spinach (a LDP) is not found in the tropics because there the days are never long enough to stimulate the
flowering process.
To investigate photoperiodism, plants can be grown in growth chambers, in which timers are used to control the length of
the light and dark periods. Such research has shown that the dark period is more important than the light period. For
example, if SDPs are grown under short-day conditions but the dark period is interrupted by a flash of light, the SDPs will
not flower. The long night that normally accompanies a short day is interrupted by the flash. An interruption of the light
period with dark has no effect. Thus, SDPs should more accurately be called long-night plants; and LDPs should be called
short-night plants to emphasize the key role played by darkness in photoperiodism. Most plants require several weeks of
the appropriate long-night or short-night cycle before they will flower.
Red light having a wavelength of 660 nanometers was found to be the most effective for interrupting the dark period, and
this effect can be reversed by a subsequent exposure to far-red light (730 nanometers). These observations led to the
discovery of phytochrome, the pigment responsible for absorbing those wavelengths and apparently the light sensor in
photoperiodism. It has been suggested that photoperiodism results from an interaction between phytochrome and the
plant's biological clock, which measures the time between successive dawns (rich in red light) and successive dusks (rich
in far-red light). Under the appropriate conditions, these interactions are thought to activate the genes for flowering.
Many other processes in plants and animals are now known to be affected by the photoperiod.
Robert C. Evans
http://www.biologyreference.com/Ph-Po/Photoperiodism.html
Photoperiodism and Phytochrome
Plants can also respond very specifically to changes in day length. This response is called photoperiodism. If you live in
the northern hemisphere, you will notice that some plants flower at the same time every spring, regardless of the
temperature. On the Penn State campus at University Park, there are shrubs with small yellow flowers called forsythia. On
March 20th, day and night are equal length. After that day, the day length exceeds the period of darkness at night and
keeps getting longer until about June 20th. The forsythia respond to this increase in day length and flower, always at the
same time every year, the last week in March, because the day length at that time triggers their flowering response. These
plants are called long-day (or short-night) plants.
There are other plants that are called short-day (or longnight) plants. They flower when the day length falls below a
certain level. In the northern hemisphere, these plants
flower in the fall. If you go out hiking in the woods in
central Pennsylvania in the fall, you may see a tree that has
small yellow flowers on it--these are witch hazel trees. At
this point in time, many of the trees have lost their leaves so
these trees are fairly striking. These trees flower in the fall,
responding to this decrease in day length. Some plants are
day-neutral plants. Day length has no effect on these.
Snapdragons are day neutral plants because the length of
the day does not affect their ability to flower.
How does the plant "know" how long the day is? This seems to be a fairly sophisticated
thing for a plant to be able to sense, the relative length of light and darkness during a day.
Experiments were done to determine how this response works and to see if the mechanism
could be uncovered. These were done in an experimental chamber, where the plant's
exposure to light could be carefully controlled. This was done for both a long-day plant
(spring flowering) and a short-day plant (fall flowering). As expected, when the plants
were exposed to longer day length, in this case about 16 hours, the long-day plant flowered
and the short-day plant did not (Figure 9.23, Panel 1). Also, when exposed to shorter day
length, about eight hours, the long-day plant did not flower and the short-day plant did
(Figure 9.23, Panel 2). However, what would happen if the period of darkness were
interrupted by a flash of light? When this was done under the conditions that caused the
short-day plant to flower (a long night), interrupting that night, the long-day plants reacted
as though it was a short night and flowered, while the short-day plants did not flower
(Figure 9.23, Panel 3). How could this one flash of light so profoundly affect these plants?
It turns out that this process is governed by a pigment.
Remember, pigments absorb light, just as the pigments in our retina absorb light allowing us to see. The pigment involved
in photoperiodism is phytochrome.
There are two forms of this pigment: Pr is the inactive form and Pfr is the active form. It is the Pfr form that triggers the
plant response. When this Pfr form is present, it triggers flowering or triggers a seed to germinate. Remember, we
learned above that abscisic acid can help seeds to go dormant. One of the triggers that the seeds use to break dormancy is
the amount of light that occurs in the spring as the light levels increase during the day.
A flash of red light converts the inactive form, Pr, to the active form. Pfr.
In those plants that were exposed to that flash of light during the dark period, that flash converted the inactive form to the
active form. This signals to the short-day plants that there is too much light, so they will not flower. However, it signals to
the long-day plants that there is enough light, so they flower even though the actual day length is too short to trigger that
response.
This response happens because the two forms of phytochrome respond to different wavelengths of red light. Pr is
converted to Pfr by red light in the visible spectrum, about 660 nm. Under normal conditions with no flash of light during
the dark period, the Pfr will gradually convert back to Pr. However that flash during the night reconverts all of the Pr to Pfr,
making the plant respond as if it were under long-day conditions. Pfr also can be converted to Pr by far-red light, about 730
nm, at the end of the visible spectrum. This finding was very exciting because it was the first time that we realized that a
pigment can be used to control a plant's "behavior," the time at which it flowers. The plants are using pigments to sense
environmental conditions, day length.
https://online.science.psu.edu/biol011_sandbox_7239/node/7272