「植物看得見你」公開課筆記／2.3 Light & Flowering

In the early 20th century the tobacco farmers in Maryland discovered a new strain which they called Maryland Mammoth, which rather than flowering in late summer and giving seeds, continued to produce more and more and more leaves. You would think that the farmers would be happy with such a plant because you'd get more leaves per yield, but unfortunately because it never flowered and only got killed in the winter, they couldn't get seeds to propagate this plant.

Scientists in the United States Department of Agriculture discovered was that if they would take the Maryland Mammoth's plants and put them in a shed early in the afternoon, they can induce them to start flowering. This is how they discovered what we called photoperiodism -- the plants response to a changing day length.

For example, which will only flower when the length of day gets shorter. We call these short day plants, such as chrysanthemums, soybeans and the Maryland Mammoth. On the other hand, there are other plants which we call long day plants which only flower as the days get longer, such as irises or wheat. These are usually plants that flower in the spring or early summer. There's a actually a third class of plants which don't really show photoperiodism. They will flower when their plant gets to a certain body size, such as tomatoes or dandelions. We called these day neutral plants.

So photoperiodism helps a plant know what is the length of day, and when is the right time of the season in order to flower. Now plants adapted photoperiodism according to where they grew wildly. For example, in the mid-summer in Canada, the days are much longer than what they would be in Florida. Whereas in the winter the days are longer in Florida than they are in Canada and it's also warmer. So for example, if you would take Maryland Mammoth which dies in the winter in Maryland when there might be snow on the ground and put it in Florida. There the short days induce flowering, and there wasn't the cold for it to die.

Cocklebuer which grows up in northern Canada and northern United States. It will only flower under a very long day. If cocklebuer was grown in the southern United States or southern Europe it would never flower.

So if we look at photoperiodism the question becomes what's important? Is it the length of the day? Or is it the length of the night?

So the experiment that was done by the scientists in the United States Department of Agriculture, was to turn on the lights in the middle of the night. So if you take a short day plant, such as chrysanthemum, and put it in conditions where it should flower with a short-day. But turn on the lights in the middle of the night, you inhibit the flowering, and it's enough to turn on the lights for just a few minutes.

If you take a long-day plant and keep it in conditions of short-day, these are conditions which would inhibit flowering. But you turn on the lights in the middle of the night, again, just for a few minutes, you can induce a small day plant to flower under conditions of short-day.

So we learned from this is that the plants are not really sensing how long the day is, but how long the night is.

Now from an agricultural point of view this is very important. Because this meant that for example that flower growers can induce their plants to flower whenever they want irregardless of the conditions outside by changing the lights in their greenhouses. So we could get chrysanthemums flowering all year long just by manipulating the light conditions.

Now we saw earlier that plants bend to blue light. They differentiate between the colors. What color of light are the plants seen here to induce flowering? So the obvious experiment would be to turn on different colors of light in the middle of the night and see if we could inhibit flowering in the short-day plant or induce flowering in a long day plant.

The experiment that was done in the same lab in the United States Department of Agriculture in the middle 20th century. What they did is turned on different colors of light, and what they found was that only one color could either inhibit or induce flowering. A flash of red light in the middle of the night would inhibit flowering in a short day plant, and it would induce flowering in a long day plant.

After you give the flash of red light, if it's followed by a flash of far red light (730 nanometers). That would inhibit the action of the red light. It is the light that we barely see as the sun is going down. It's very long wavelengths, we're basically blind to it.

So if you gave a red light, you inhibit the flowering of the short day plant. If you give red light and then far red light, it's as if you had never turned on the lights to begin with, it won't flower.

If we're looking at the long day plant, you give the red light it would induce the flowering. But if you give red and then the far red, you inhibit it.

In other words, we're actually seeing here a type of memory. The plant remembers what the last color it seen.