2.3 Light and Flowering

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From the course by Tel Aviv University

Understanding Plants - Part I: What a Plant Knows

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Tel Aviv University

Understanding Plants - Part I: What a Plant Knows

422 оценки

For centuries we have collectively marveled at plant diversity and form—from Charles Darwin’s early fascination with stems and flowers to Seymour Krelborn’s distorted doting in Little Shop of Horrors. This course intends to present an intriguing and scientifically valid look at how plants themselves experience the world—from the colors they see to the sensations they feel. Highlighting the latest research in genetics and more, we will delve into the inner lives of plants and draw parallels with the human senses to reveal that we have much more in common with sunflowers and oak trees than we may realize. We’ll learn how plants know up from down, how they know when a neighbor has been infested by a group of hungry beetles, and whether they appreciate the music you’ve been playing for them or if they’re just deaf to the sounds around them. We’ll explore definitions of memory and consciousness as they relate to plants in asking whether we can say that plants might even be aware of their surroundings. This highly interdisciplinary course meshes historical studies with cutting edge modern research and will be relevant to all humans who seek their place in nature. This class has three main goals: 1. To introduce you to basic plant biology by exploring plant senses (sight, smell, hearing, touch, taste, balance). 2. To introduce you to biological research and the scientific method. 3. To get the student to question life in general and what defines us as humans. Once you've taken this course, if you are interested in a more in-depth study of plants, check out my follow-up course, Fundamentals of Plant Biology. In order to receive academic credit for this course you must successfully pass the academic exam on campus. For information on how to register for the academic exam – https://tauonline.tau.ac.il/registration Additionally, you can apply to certain degrees using the grades you received on the courses. Read more on this here – https://go.tau.ac.il/b.a/mooc-acceptance Teachers interested in teaching this course in their class rooms are invited to explore our Academic High school program here – https://tauonline.tau.ac.il/online-highschool

From the lesson

What a Plant Sees?

This week we start a systematic review of a plant's sensory systems by starting with plant responses to light. We will cover an overview of human vision, plant responses to light, Darwin's experiments showing plant responses to light, phototropism, phytochrome and flowering, and modern research on phototropism. In other words, this week we get into more advanced concepts in plant sensory biology. The last module is especially advanced, and will be clearer for those of you with a strong biology background. But do not fret, aside from very basic concepts, this module will NOT be included in the exam (you will not be responsible for understanding the intricacies of the experimental methods, etc.). If you have not already, please review the Course Syllabus for general information about this course.

2.1 Introduction5:21

2.2 Darwin's Experiment6:50

2.3 Light and Flowering7:06

2.4 Phytochrome: A Light-Activated Switch7:35

2.5 Arabidopsis Blind Mutant8:42

2.6 What a Plant Sees?5:55

2.7 Molecular Biology of Plant Vision8:53

Meet the Instructors

Professor Daniel Chamovitz, Ph.D.
Dean, George S. Wise Faculty of Life Sciences
Director, Manna Center for Plant Biosciences

I want to move now to talk about another response of plants to light and

that's when to flower.

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 could see in this pictures, this led to a huge,

huge plant which is called Maryland Mammoth.

And 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 lowered and only got killed in the winter,

they couldn't get seeds to propagate this plant.

This was actually a huge problem and

a very interesting question what caused this plant to stop flowering?

And this was a question that was taken up by scientists in

the United States Department of Agriculture.

And what they 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.

And this is how they discovered what we called photoperiodism.

Photoperiodism is the plants response to a changing day length so

that we know that there are plants, for

example, which will only flower when the length of day gets shorter.

We call these short-day plants, for example,

chrysanthemums, soybeans, or 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.

This is 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 middle of the 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 in do induce flowering.

And there wasn't the cold for it to die.

So each plant, for example here we have a plant called cocklebuer

which grows up in northern Canada 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?

And what type of experiment could we do in order to check this question?

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, for example, 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.

On the other hand, 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.

So 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.

So here's the experiment that was done in the same lab in

the United States Department of Agriculture and now we're talking for

the history buffs among you were talking about 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.

Blue light wouldn't work.

Any other color wouldn't work.

Only red light.

So already here we're seeing just like the weak in differentiate between blue and

red light, plants also differentiate between blue and red light,

but now here it gets even more interesting, because after you give

the flash of red light if it's followed by a flash of what's called far red light.

Far red light is about 730 nanometers.

This is the light that we barely see as the sun is going down.

It's very long wavelengths, we're basically blind to it.

That would inhibit the flash of the red light.

The far red light inhibits the action of the red light.

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.

Or 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.

We can continue this game, if after the red, you give far red, and

then again give red, you have either an Inhibited flowering in the short-day or

induced it in the long day plant.

Give again far red, you inhibit or you've canceled out the effect of the red light.

So in other words, we're actually seeing here a type of memory.

The plant remembers what the last color it seen.

If you're giving it red light, for a long day plant, you're activating flowering.

If you're giving it far red light, you're inhibiting the flower.

Give it red light again, activating.

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