TPO28听力文本文档5
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Lecture3-Botany
Narrator
Listen to part of a lecture in a botany class.
Professor
OK. Last time we talked about photosynthesis, the process by which plants use light to convert
carbon dioxide and water into food. Today I want to talk about another way light affects plants.
I am sure you all know from physics class about how light moves in microscopic ways and that we
can only see light when the wavelength of that light is in a specific range. Plus, depending on
the wavelengths, we see different colors.
Well, plants are also capable of distinguishing between different wavelengths of light. Now,
I don’t want to confuse you. It is not like plants have eyes. Plants don’t see in the sense
that humans or animals do, but they do have photoreceptors.
Photoreceptors are cells that respond to light by sending out a chemical signal. And the
organism, the plant, reacts to this signal. In fact, the signals that plants get from their
photoreceptors sometimes cause significant reactions.
And many plants are seasonal. And one way they know when winter is ending and spring is
beginning is by sensing the change in light. The time when an adult plant flowers is based on
the amount of light the plant senses. Certain plant species won’t flower if they sense too much
light and some plants will only flower if they sense a specific amount of light. Of course, these
aren’t conscious reactions. These plants just automatically respond to light in certain ways.
Plants are also able to distinguish between specific wavelengths of light that the human eye
cannot even see! Specifically there’s a wavelength called far-red. Although why they call it
far-red … I mean, it is not red at all. It lies in the infrared range of the spectrum. We can’t
see it, but plants can sense it as a different wavelength.
OK. Now I need to mention another thing about photosynthesis. I didn’t explain how different
wavelengths of light affect photosynthesis. When a plant absorbs light for performing
photosynthesis, it only absorbs some wavelengths of light and reflects others. Plants absorb most
of the red light that hits them, but plants only absorb some of the far-red light that hits them.
They reflect the rest. Remember this, because it’s going to be relevant in an experiment I want
to discuss.
This fascinating experiment showed that plants not only detect and react to specific
wavelengths of light, plants can also
detect and react to changes in the ratio of one wavelength to another. This experiment was
called the Pampas experiment.
The idea behind the Pampas experiment had to do with the response of plants to changes in
the ratio of red light to far-red
light that the plants sense with their photoreceptors. Some biologists hypothesize that a
plant will stop growing if it’s in the
shade of another plant, a reaction that’s triggered when it senses an unusual ratio of red
light to far-red light. OK.
Imagine there are two plants. One below the other. The plant on top would absorb most of the
red light for photosynthesis,
but reflect most of the far-red light. That would lead to the plant in its shade sensing an
unusual ratio. There will be less red
light and more far-red light than normal.
What that ratio signifies is important. A ratio of less red and to more far-red light would
cause a reaction from the plant. It
would stop growing taller, because that plant would sense that it wasn’t going to get enough
sunlight to provide the energy
to grow large.
To test their hypothesis, researchers took some electrical lights, um… actually, they were
light-emitting diodes, or LEDs.
These light-emitting diodes could simulate red light. So they put these LEDs around some plants
that were in the shade. The
LEDs produce light that the plants sensed as red. But, unlike sunlight, the light from these
LEDs did not support
photosynthesis. So the plants sensed the proper ratio of red light to far-red light and reacted
by continuing to grow taller,
while in reality these plants were not getting enough energy from photosynthesis to support
all of that growth. And because
they weren’t getting enough energy to support their growth, most of the shaded plants died
after a short time.
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