Welcome to an introductory video tutorial on photoperiodic flowering.
This tutorial was narrated and produced by Clara Oh and Jay Dandekar, under the guidance
of Dr. Katia Bonaldi and Dr. Susan Golden.
Flowers are known for being aesthetically pleasing, but they serve a much greater purpose
for the plant.
Flowers are the reproductive structures of plants.
Once fertilized, they will produce fruits, containing the seeds, which are the offspring.
Plant species do not all flower at the same season.
For example, the iris flowers in the summer, the chrysanthemum flowers in the fall,
the snowdrop in the winter, and the tulip in the spring.
The season in which flowering occurs is crucial for the reproductive success and for the survival
of that species.
But, how do plants know when it’s time to flower?
Instead of reading a calendar as we do, plants collect information from environmental factors
that vary with seasons in a consistent manner.
Temperature is probably the seasonal factor that crosses your mind, right?
But in fact, the most predictable of all is the day-length, also called the photoperiod.
Indeed, the duration of daylight increases during spring and summer,
before decreasing in fall and winter.
This pattern depends on the relationship of Earth’s movement around the Sun, and happens
every year, the same way, right to the minute!
Because it’s so predictable, the photoperiod is the main signal that plants rely on to
keep track of the seasons and to flower at the right time...
a phenomenon that is called photoperiodic flowering.
This process was discovered in 1920 by two researchers, Garner and Allard, after they
observed that certain plant species always flower at the same time, no matter their age.
For instance, let’s consider the soybean variety Biloxi.
No matter whether it is planted in spring or in summer, if it is growing outdoor in
northern latitudes, it will always flower in september...
and this happens, every year, regardless of the varying temperature conditions or rainfalls.
That’s how they started suspecting that the relative length of day and night
may determine the flowering time...
And indeed, by artificially reducing the light hours to mimic a short “winter” day length,
Biloxi soybeans will flower and produce fruits without delay.
While conversely, plants kept growing under normal summer light conditions will keep growing,
and growing and growing until it’s finally time to flower.
A similar behavior is observed with the emblematic Maryland Mammoth variety of tobacco:
when artificially kept in summer light conditions, it will grow huge, without providing any flowers.
Further investigation revealed that there is an array of flowering behaviors among plant species:
We’ve seen that the iris flowers in the summer, and that is because it is responsive
to days that have more daylight than darkness, so we call it a long day plant.
and short-day plants, such as the snowdrop, flower in response to days in which there
is less daylight than darkness (like in fall or winter).
But there are also day-neutral plants that do not care about the photoperiod to determine
when they should flower.
You might be wondering how plants can measure day length.
Here’s some background:
For decades, researchers thought that a certain amount of light was necessary to trigger flowering
in long-day plants.
The idea was that a molecule would gradually accumulate during the day,
and be degraded during the night.
Under long-day conditions, that molecule would therefore be able to reach a critical threshold
above which flowering would be triggered.
This model, called the Hourglass model, could also work for short day plants, but in that case,
the molecule would be accumulating during the night, and would be degraded by light.
In the hourglass model, the photoperiodic accumulation is turned over every day,
and is dependent on an external signal: the light in the case of long day plants,
or the darkness in the case of short day plants.
Sounds good, but further experiments disproved this intuitive model.
Let’s consider a long-day plant, like the iris.
Such plants, placed under short-day conditions, do not flower.
Let’s now give a brief pulse of light during the night period.
If the Hourglass model were true,
sustained light for a long period would be required to accumulate the flowering molecule
above the crucial threshold, and a brief pulse of light would not make much difference...
but in fact, an acute stimulus in the night is sufficient to trigger flowering!
This finding suggests that the duration of light is less important
than the specific timing of the light.
Similarly, when short-day plants, here represented by the snowdrop, are placed under short-day conditions,
they normally trigger flowering.
However, they will not flower if a pulse of light is given to break up the night period.
Considering all of this information, there is no need for a certain amount of light or
a certain amount of dark to trigger flowering.
What really matters is not HOW MUCH light is perceived, but WHEN the light signal is perceived.
The current model that best explains photoperiodic flowering
is called the external coincidence model (ECM)
It was proposed by Pittendrigh and Minnis in 1964.
This model states that 2 types of information need to be integrated by the plant
for flowering to happen at the proper time.
The first piece of information is “external,” because it comes from the environment: that signal is light.
The second piece of information is “internal,” because it is provided by the circadian clock:
it’s a molecular signal that is provided at a specific time of day by the clock.
To be fair, the involvement of an internal timekeeper in photoperiodic flowering was
first anticipated by Erwin Bünning 30 years earlier (in 1936),
but the scientific community was not prepared for such a brilliant idea.
For flowering to be triggered, both signals need to happen at the same time; we say that they "coincide."
We understand this situation best in the case of a long-day plant, called
in which the “co-incidence” of light and clock signaling are important in the late afternoon/early evening.
Here is a scenario in which the “coincidence” happens and triggers flowering in long-day conditions.
The circadian clock causes the abundance of some proteins
to go up and down every day with a repeating rhythm.
Some of these proteins are critical for flowering.
In this example, we are in winter and the clock regulates the accumulation of a flowering-determinant molecule,
such that it accumulates during the day, and finally reaches a peak after the sun is already down.
While the sun is up, the molecule has not reached a critical threshold for flowering,
and it needs to be above that threshold and see light to “do its thing.”
In this case, it can’t induce flowering.
But as the seasons go by, the day-length increases,
until both the peak of accumulation and the daylight coincide:
At a specific time of the year, the flowering molecule is not only abundant, but also activated by light.
The “co-incidence” is achieved and therefore flowering is triggered!
The molecular mechanisms behind the external coincidence model are in fact more complicated,
and can be different from one plant species to another.
If you’re interested in learning more details about the molecular mechanisms involved in
the flowering of the long-day plant