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Lecture Comments (2)

1 answer

Last reply by: Dr Carleen Eaton
Sun Apr 28, 2013 11:52 PM

Post by Zoe Loos on April 22, 2013

Is thigmotropism positive or negative?

Plant Hormones and Tropisms

  • Auxins are produced in the apical meristem of plants and stimulate stem elongation and phototropism as well as fruit development. Indoleacetic acid (IAA), an auxin, occurs naturally in plants.
  • Cytokinins are produced in the roots of plants and stimulate cell division and differentiation and play a role in apical dominance.
  • Gibberellins promote the elongation of plant stems and are responsible for the rapid growth of a floral stalk, which is known as bolting.
  • Ethylene is a hormone that is a gas. It stimulates fruit ripening, leaf abscission and promotes apoptosis of plant cells.
  • Abscisic Acid (ABA) promotes seed dormancy, preventing seeds from germinating until conditions are favorable. ABA also stimulates the closure of stomata in leaves when conditions are dry.
  • A tropism is a response in which a plant turns toward or away from a stimulus. Types of tropism include phototropism, gravitropism and thigmotropism.

Plant Hormones and Tropisms

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Plant Cell Signaling 0:17
    • Plant Cell Signaling Overview
    • Step 1: Reception
    • Step 2: Transduction
    • Step 3: Response
    • Second Messengers
    • Protein Kinases
  • Auxins 6:14
    • Auxins
    • Indoleacetic Acid (IAA)
  • Cytokinins and Gibberellins 11:10
    • Cytokinins: Apical Dominance & Delay of Aging
    • Gibberellins: 'Bolting'
  • Ethylene 15:33
    • Ethylene
    • Positive Feedback
    • Leaf Abscission
    • Mechanical Stress: Triple Response
  • Abscisic Acid 21:10
    • Abscisic Acid
  • Tropisms 23:11
    • Positive Tropism
    • Negative Tropism
    • Statoliths
  • Phytochromes and Photoperiodism 27:48
    • Phytochromes: PR and PFR
    • Circadian Rhythms
    • Photoperiod
    • Photoperiodism
    • Gerner & Allard
    • Short-Day Plant
    • Long-Day Plant
  • Example 1: Plant Hormones 41:28
  • Example 2: Cytokinins & Gibberellins 43:00
  • Example 3: Match the Following Terms to their Description 44:46
  • Example 4: Hormones & Cell Response 46:14

Transcription: Plant Hormones and Tropisms

Welcome to

In this final section in these series of lectures on plants, we are going to be talking about plant hormones and tropisms.0002

And what unites these two topics is that they both focus on the responses of plants to various stimuli.0010

We are going to start by talking about cell signaling in plants.0019

And this is really just a review of what we talked about earlier in the course because the cell signaling0022

in plants involves many of the same mechanisms that cell signaling in other cells, animal cells, involves.0028

So, there is a three-step process through which hormones and other stimuli - by other stimuli, I mean environmental stimuli like light or touch -0036

these all can trigger responses in plant cells through a certain process.0047

And the first phase of this process is the reception phase.0051

The second is transduction or signal transduction, and then, the final phase is the response phase.0056

In the first phase, in the reception phase, the plant receives the signal, and this stimulus can, it is a reception of the stimulus.0064

What happens here is that the stimulus interacts with the receptor.0085

The stimulus can be a molecule, a hormone, which is a chemical stimulus.0094

Or it can be a stimulus such as light that triggers the response in a photoreceptor or touch.0101

I am going to focus on hormones right now, but remember that plant cell signaling is not just limited to hormones.0109

Remember that the molecule that binds, in this case a hormone, if it binds to a receptor, it is called a ligand.0117

And then, here, we have the receptor, and that the receptor and the ligand are going to be complimentary0123

in shape just as an enzyme and a substrate are complimentary, which allows for specificity.0130

If the stimulus is something like light, then, there will be a photoreceptor.0138

And we are going to be talking about one type of photoreceptor that is specific or responds best to red light. It is activated by red light.0143

So, there can be specificity in that way, as well.0150

The second phase is the transduction or transmission of the signal, transduction of the signal to the interior of the cell.0154

And I am going to go into more detail on this. This is a very important step.0169

And it involves some of the concepts like second messengers that we talked about earlier on in the course.0174

Third is the response, and the response can involve transcribing, activating transcription.0181

It can involve turning off a gene, so turning a gene on or off, or it can involve activating a protein or inactivating it.0194

So, the response can be at the level of a DNA or the level of a protein.0206

The response can result in growth. It can result in growth inhibition, manufacturing certain proteins.0214

It can also involve triggering apoptosis. Recall that apoptosis is programmed cell death, so the response could actually be that the cell dies.0223

Looking more closely at this second step,0233

transmission of the cell from the surface to the interior of the cell is accomplished via signal transduction pathways.0236

And signal transduction pathways require second messengers. They often use second messengers.0244

Second messengers are small molecules such as in a plant, cyclic GMP is one. Calcium is another.0251

And binding of the hormone to its receptor can trigger a conformational change in the receptor, which may activate the receptor,0260

which will activate another molecule, which in turn, may either allow calcium to enter the cell or result in increase in cyclic GMP levels.0269

And then, those second messengers can go on and activate other proteins.0279

An example of another molecule that might be activated by a second messenger is a protein kinase.0284

Remember that protein kinases phosphorylate other proteins. They phosphorylate proteins.0289

This second messenger may activate a protein kinase, which will phosphorylate a protein, which could activate that protein and so on.0304

This cascade is extremely powerful because one thing it does is to amply the signal.0312

Binding of a hormone to a receptor or light hitting the receptor can end up increasing the level of cyclic GMP by a lot.0317

And all those molecules, in turn, can activate an enzyme. That enzyme can activate many other proteins, and those can activate many other proteins.0329

So, just binding of one hormone to a receptor can cause a strong response in the cell.0339

And that is why even low levels of a hormone in some cases can elicit a response.0345

So, then, finally, in this response phase, what will happen is maybe transcription is activated.0352

Maybe there is a transcription factor involved, and transcription is activated, so more of a protein will be made.0360

Or transcription is turned off, or a protein is activated and turned off; and then, the cell will respond based on what has occurred at that final step.0365

We are going to start out by talking about some groups of plant hormones.0374

Auxins are produced in the apical meristem of plants, and they have multiple functions.0378

They stimulate stem elongation. They stimulate phototropism, and also fruit development.0386

Now, we are going to talk about tropisms in just a bit, so you will know what it is.0396

But just for right now, this is the response by which plants turn toward the light.0401

So, if you have a plant in the window, it is getting good light from that one side, you will see the plant bend towards the light.0406

And then, if you turn the plant the other way, the plant will grow, so that is going towards the light again. That is phototropism.0412

As you consider these hormones, you should keep in mind that it is not the absolute level of the hormone that is most important.0419

Usually, it is the relative levels, so it is not just how high is auxin. It is how high is auxin relative to, say, cytokinin or ethylene, which is a growth inhibitor.0426

So, it is not the absolute level. It is what is going on with this hormone compared with the other hormones.0437

There are both natural and synthetic auxins. Indoleacetic acid, IAA is a natural auxin found in plants.0445

As I mentioned, it stimulates stem elongation, and I am going to talk about the mechanism through which IAA stimulates stem elongation.0454

What it actually does is it stimulates proton pumps in the plant cell wall.0463

The result is going to be an increase in the hydrogen ion concentration and, therefore, a decrease in pH.0475

Acidic conditions like this activate enzymes that breakdown the cell wall.0487

So, when the cell wall is very rigid and strong, only a certain amount of water can get in because of the limitation by this cell wall.0501

It will not expand. It is not flexible.0512

However, if this cell wall is broken down, becomes more flexible, and water can enter the cell; so they will all be in in its vacuole.0514

And that is going to make the cell more turgid and, therefore, larger.0524

This is only a temporary increase in the size of the cell.0529

Therefore, what auxin also does is it stimulates the production of cytoplasm, cell wall components, organelles,0532

so the various parts that the cell would need to become larger, not just temporarily through water coming in but on a permanent basis.0541

OK, so that is stimulation of the growth of stems.0552

Another function of auxin is to stimulate secondary growth, so it stimulates secondary growth in non-herbaceous plants.0557

Remember when we talked about plant structure and growth, I talked about primary versus secondary growth,0569

and that wooded plants undergo a secondary growth.0574

Auxin also stimulates the growth of fruit, and some of these qualities account for the commercial use of auxin.0580

So, this is a natural form of auxin. However, let me talk about one synthetic form, and that is 2,4-D.0587

2,4-D is a synthetic auxin that is an herbicide, and I said that this stimulates growth; so how does it work as an herbicide?0596

Well, too high of a hormone concentration can actually be toxic to cells, so in certain crops, say, corn, corn can tolerate a higher level of auxin.0603

So, there is a lot of auxin around, it will still continue to grow.0615

However, other plants like weeds that are more sensitive to auxin will have their growth stopped.0618

They will overdose from the auxin and then, die, so this can be used as an herbicide.0626

Another use is that tomatoes that are grown in green houses can be sprayed with synthetic auxin to induce the production of fruit.0630

In a green house, there are not a bunch of pollinators flying around.0639

So, the fruit would not develop because the plant does not want to waste all its energy making fruit when fertilization has not occurred.0643

We can get around this by spraying the plants with auxin.0651

Finally, auxins are found in a type of rooting powder.0655

So, if you have a plant cutting, and you want it to grow better, you can treat it with rooting powder.0659

And it will develop more roots more quickly than if you did not treat it.0663

So, this first class of hormone is the auxins. Another couple of hormones here, cytokinins and gibberellins, also stimulate growth.0668

We will talk about cytokinins first. They are produced in the roots of plants, and their name helps to tell you their function.0677

They stimulate cell division and differentiation.0684

So, just remember cytokinesis, which is part of cell division and cytokinins.0687

They are produced in the roots, and then, they are transported to other areas of the plants.0694

They also play a role in what is called apical dominance, so they influence it.0699

Apical dominance is when the central stem is the dominant stem.0706

So, what happens is the plant will grow up towards the light instead of just getting thicker and bushier and growing outward.0715

There is a terminal bud on the central stem, and it prevents the growth of lateral buds.0725

Instead of making all these in the lateral branches out to the side, the plants are going to mostly grow up.0732

And it is the terminal bud on the central stem that prevents these axillary buds from growing.0740

Apical dominance is maintained by auxin, so cytokinin plays a role in this balance.0749

But apical dominance is actually maintained by auxin, which is produced in the apical bud, and it suppresses the growth of these lateral buds.0756

So, apical dominance is maintained by or due to auxin.0766

Cytokinins do the opposite. They stimulate growth of lateral branches.0777

It is the balance or the interpolate between auxins and cytokinins that determines what the outcome is.0791

Is the plant going to grow up, or is it going to grow outward?0797

Another function of cytokinins, so one was its role in apical dominance. The other is delay of aging.0800

Cytokinins actually inhibit the breakdown of proteins and actually stimulate protein synthesis, which results in a delay of senescence or aging.0814

Forests spray flowers with cytokinins in order to keep cut flowers fresh. It prevents the flowers from getting old and aging.0824

So, that is cytokinins. The second type of hormone that I am going to talk about are gibberellins.0832

Along with auxins, gibberellins promote the elongation of stems, so they stimulate the elongation of stems.0842

There have been over a hundred types of gibberellins discovered, and they are produced in the roots of plants.0851

They are also produced in young leaves.0857

And experiments I have shown that gibberellins have a very dramatic effect on0859

plant growth especially in plants that are dwarf plants that are much smaller than normal.0863

And then, if you treat them with gibberellins, they will grow to a normal height.0867

Gibberellins are also responsible for bolting, and this is a particular type of stem growth.0871

It is a type of growth in which there is the rapid growth of a floral stalk.0876

This occurs in plants that are low-lying, for example, onions and lettuce. When they go into their reproductive phase, bolting occurs.0884

So, a plant that is low to the ground like lettuce gets ready to reproduce.0893

It will develop relatively quickly these long stalks, and at the end of them is flowers.0898

So, the flowers at the tips of long stems are a result of bolting.0905

Along with auxins, gibberellin also stimulates the development of fruit.0911

Plants may be treated with synthetic gibberellins to promote fruit growth, which will result in larger fruits.0919

These were two more of the hormones that stimulate growth. Now, we are going to talk about a hormone that has some different effects.0927

Ethylene is a hormone that is actually a gas, and it stimulates the ripening of fruit, leaf abscissions; and it promotes apoptosis.0936

One interesting thing about ethylene is that it involves positive feedback.0948

Let's talk about the example of ripening of fruit.0955

Ethylene actually helps to trigger the breakdown of cell walls, and you know that unripe fruit is firmer.0962

When ethylene stimulates the breakdown of cell walls, that is going to cause the fruit to soften.0968

And what happens is the ethylene causes the fruit to ripen. That causes the fruit to produce more ethylene, which causes more ripening.0973

So, this is actually an example of positive feedback.0993

We have talked about negative feedback before. This is an example of positive feedback.0995

Ethylene is used commercially to cause fruit to ripen. Fruits are often picked before they are ripe because then, they do not spoil as easily.1001

They do not bruise as easily when they are transported to where they are going to be sold.1007

And then, part of being sold, they can be treated with ethylene, and that helps them to ripen.1011

And you might have done this at home. You buy pears or something, and they are too hard.1016

They are not ripe, so one trick that people know is to put them in a bag, close the bag up, leave them a couple days, take the pears out.1021

And what you are trying to do is keep that ethylene in and allow the concentration of that gas to build up to induce ripening.1027

Conversely, if you want to slow down fruit from ripening, you want to make sure it has good air circulation around it,1036

so the ethylene levels do not build up, and in fact, certain fruits, if they are not ready to be sold,1041

they will make sure that there is good air circulation and even exposure to CO2 to decrease the ethylene levels.1048

Fruit ripening, that is one function. Another function of ethylene is to stimulate leaf abscission.1056

What is leaf abscission? It is the death of leaves, and then, their subsequent dropping off from a plant.1062

This is an example of a deciduous tree, and these are the trees that lose their leaves in the fall.1069

They do not have leaves in the winter, when it is dry out, and they minimize water loss that way; and then, the leaves will grow back in the spring.1076

Abscission is a result of an abscission layer that forms near the base of the leaf.1086

There is something called an abscission layer. It forms near the base of the leaf.1093

Then, the leaf falls off, and where the leaf was on the tree, a scar will form to protect it, prevent infection at that spot.1100

And what ethylene does is it actually causes the breakdown of the cell walls in this layer.1108

So, ethylene causes the breakdown of cell walls in the abscission layer.1116

Stimulates fruit ripening, that is one, leaf abscission, and finally, ethylene promotes apoptosis.1131

As I mentioned, apoptosis is programmed cell death. It is cell suicide.1137

Sometimes, an apoptosis is just a part of a developmental process in shaping an animal or a plant.1143

Certain cells need to die, kind of, prune back. Other times, apoptosis is a result of infection.1151

If a cell is infected, it will send out signals, and it will end up dying instead of infecting other cells.1160

So, it is a plant's way of saving the overall organism at the expense of that one cell or if a cell is damaged.1168

Just briefly to mention one more response that ethylene can trigger is a response to mechanical stress.1177

If there is a mechanical stress, there is something called a triple response that occurs.1185

A triple response involves growth slowing. The growth of the stem slows, so the stem growth slows.1195

The stem becomes thicker, stem thickens, and it curves.1206

For example, let's say a plant is trying to grow, and it turns out that it is growing up, it is growing up, and then, it hits something.1215

Let's say it is growing under a rock, and the stem hits this rock.1223

Well, it cannot go through the rock, so what happens is this mechanical stimulus, this pressure, causes the growth to slow.1228

So, this seed germinates. The plant is growing up.1239

It hits this rock. Then, the release of ethylene triggered by this pressure here will cause the growth to slow and to curve and is thicker.1245

So, it is thicker, and it is stronger, and now, it is growing sideways; and then, hopefully, it gets past the rock and then, is able to grow up.1259

So, it is a way of getting around a barrier.1267

The final hormone we are going to talk about is abscisic acid. This is a growth inhibitor.1272

And you might recall me mentioning this when I talked about stomates, stomata.1277

Abscisic acid promotes seed dormancy, and therefore, prevents seeds from germinating when conditions are unfavorable.1284

Abscisic acid or ABA also stimulates the closure of stomata when conditions are dry.1294

We will start out talking about seed dormancy. Seeds remain dormant when conditions are not favorable to growth.1301

So, if it is too cold, if there is not enough moisture, the seed will not germinate because if it germinates under bad conditions,1308

the plant probably is not going to survive.1314

Keep in mind, though, that hormones do not act alone. It is often the ratio of hormones that matters.1317

If ABA increases relative to other hormones that might stimulate germination, then, the decision will be made for dormancy.1324

One mechanism to which this can all work is rain.1333

A seed falls on the ground. Let's say it is dry.1336

So, a dry month, there is not rain. The seed just, sort of, stays there in the dirt.1339

The ABA levels are relatively high. It does not germinate, and then, there is some good rains.1345

The rain can wash the ABA out of the seed, so that allows the seed to germinate.1351

It goes out of its dormant state, and it knows that it is a good time to germinate because it is wet enough that it washed the ABA away.1357

I also mentioned that ABA stimulates the closure of stomata in the leaves.1366

Recall that plants produce ABA when they are dehydrated, when they are dried out.1372

And that causes the guard cells that are surrounding/flanking the stoma to close.1378

Alright, so we discussed various classes of hormones. Next, we are going to talk about tropisms.1388

I already mentioned phototropism, but let's broaden out our discussion of tropisms.1394

In general, a tropism is a response in which a plant turns toward or away from a stimulus.1399

Light is a very important stimulus in plants, but we are going to talk about some others, as well.1407

Phototropism has to do with turning towards the light. Gravitropism, the stimulus here is gravity, and thigmotropism, the stimulus is touch.1413

In a positive tropism, the plant grows toward a stimulus. In a negative tropism, the plant grows away from the stimulus.1431

In phototropism, what occurs is that cells on the dark side of the plant grow more quickly. They elongate.1467

And the result is that if the cells on the light side, so let's say here is the sun,1486

and it is hitting the plant on this side, cells on the far side are going to grow longer.1499

And then, cells on this other side are not going to elongate as much, and the result will be that it curves, that the plant bends towards the light.1508

One theory is that higher levels of auxin are present in the side of the plant away from the sunlight, which stimulates the growth in those cells.1525

And phototropism allows plants to maximize their exposure to the light they need for photosynthesis. Again, this is an example of a positive tropism.1535

Now, gravitropism has to do with the influence of gravitational pull on plant growth.1547

In shoots, the stem, the leaves, are going to want to grow away from the direction of gravity.1555

So, if there is a seedling, and here, it shows just a little seedling, the seeds in the ground, if the roots do not go down, and the shoot does not grow up,1562

the plant is not going to survive, but how does it know which way is up, which way is down?1574

Well, there are plastids called stagnalis, and these are special plastids that contain grains of starch.1580

And these grains of starch sink to the part of the cell that is the lowest.1595

Think of it as like if you put some sand in a glass, and there was water in the glass, and then, the grains of sand would sink to the bottom.1600

If you tip the glass to the side, the grains would, then, sink again to the lowest part of the glass.1607

So, it is believed that that is the mechanism through which the plant can detect gravity, and the shoot will grow away from the pull of gravity.1613

So, that is a negative tropism.1621

The roots need to grow into the ground to get to water and minerals into the soil, so they are going to grow in the direction of gravity.1624

So, that is a positive tropism.1631

Finally, thigmotropism is the response of a plant to touch. Plants may change their direction of growth because of touch.1635

An example is a plant like ivy that climbs. When ivy comes into contact with the surface, that contact, that touch of the surface, will cause the ivy to curl.1644

It will curl around the tree or grab onto the walls so that it can grow up it, grow in a vertical direction.1656

These are three of the major tropisms you should be familiar with.1663

The next topic we are going to talk about is photoperiodism, and we have to start out by learning some things about phytochromes.1668

Remember that one type of signal a plant can receive is a chemical signal from a hormone like auxin.1676

Though plants also receive the other signals we talked about, signals from light, signals from gravity, signals from touch.1683

If you will recall back when we talked about stomata opening and closing, I discussed blue light receptors.1691

So, the plasma membranes of blue light receptors are found in guard cells. They are found in the plasma membranes of guard cells that flank the stoma.1701

And when there is light present, these receptors are stimulated. Potassium is absorbed into the guard cells.1712

Water follows the potassium in, and the stomata open.1722

Therefore, when light is present, the stomata are going to be open, which makes sense because when light is present, photosynthesis can occur.1727

And gas exchange needs to occur.1735

The stomata is synchronized. The opening in the stomata is synchronized with the availability of the light.1737

These blue light receptors also detect light in the process of phototropism.1744

There is a second type of receptor that you should be familiar with, and that is mainly what we are going to talk about; and these are phytochromes.1750

These are photoreceptors that mainly absorb red light. They exist in two forms.1757

They convert back and forth between these two forms: PR and PFR.1763

PR is so named because it is best or most efficient at absorbing red light, whereas, PFR is best at absorbing far red light.1770

Here is what happens. If PR absorbs red light, so red light hits it, that is going to cause it to convert to PFR.1790

Then, if there is far red light that hits this PFR, that will cause it to convert back to PR, so absorption of red light causes PR to convert to PFR.1811

The absorption of far red light causes PFR to convert to PR.1828

In the dark, if there is just at night, there is no sun out, what is going to happen in the darkness is that PFR will just spontaneously convert back to PR.1834

Another piece of information you need to know is that the conversion from PR to PFR is faster than this conversion from PFR back to PR.1850

So, when there is light even though the sunlight can contain both red and far red light,1861

the balance is going to be shifted towards PFR because this conversion is faster than the other way.1868

And the balance of PR to PFR can trigger or affect the responses of the plant.1877

For example, seed germination is triggered by a build-up of PFR.1884

Remember that moisture is important for plant growth but so is light.1904

So, these phytochromes allow the plant to detect when there are good light conditions that are favorable for survival, and germination can occur.1908

Phytochromes are also responsible for the fact that plants have specific responses based on the time of year that it is.1919

Let's start just talking about, before we get into that, the whole idea of circadian rhythms.1928

Plants, animals, humans, many organisms, have what is called circadian rhythms.1936

And these are responses - physiological responses, biochemical responses - that are dictated or controlled by a 24-hour cycle.1943

Now, these responses can be affected by light, but light does not completely account for these responses.1960

People, plants, have an internal clock, so even if someone is in the dark, an animal, a person, they will still secrete hormones in a certain 24-hour cycle.1967

Or a plant will open its stoma and close the stoma on a 24-hour cycle.1978

So, circadian rhythms can be affected by light and other factors, but they are also just an internal clock.1984

Now, a photoperiod does have to do with light, and what a photoperiod is, is it refers to the relative length of night and day.1993

Another way to think of it is the number of hours of daylight in a 24-hour period.2008

Photoperiodism refers to physiological responses due to a photoperiod.2019

So, it is the physiological response an organism has due to the photoperiod or influenced by the photoperiod.2032

An example is flowering. Plants flower at a certain time of year every year depending on the species.2044

Many plants do, and these plants have a way to detect when it is a certain a month, when it is a certain week.2055

How do they know? Well, it has to do with the photoperiod, the number of hours that it is dark per day and the number of hours it is light per day.2063

In the early 20th century, there were two individuals working for the US Department of Agriculture, and their names were Garner and Allard.2072

And they worked for the US Department of Agriculture, and they discovered a variant of a tobacco plant.2083

And the tobacco plant was called Maryland Mammoth, and normally, this type of tobacco plant flowers in the summer.2088

However, they ran across a variant that flowered in December.2097

And they did some experiments to figure out why this plant was flowering in a different time of year.2103

They used artificial light, and they discovered that it was the photoperiod that accounted for the blooming in December.2113

This variant only bloomed when the day - and by day I mean the day created by artificial light - was shorter than a certain number of hours.2123

It was shorter than a critical number of hours, and they coined a phrase to describe this plant that was short day, that this was called a short-day plant.2133

It does not mean that they bloom only when the day is really short.2150

What it means is they only bloom when day or daylight is shorter than a critical number of hours.2153

You might hear the phrase "critical day length" when it is shorter than a critical day length, that will be a short-day plant that is influenced by that.2171

For example, this variant of Maryland Mammoth would only bloom if it was exposed to light for 14 hours or less per day.2179

So, it bloomed when exposed to 14 hours or less or light per day. Therefore, there is a maximum number of hours of light per day.2189

If there is more than that number, it will not bloom, so it needs a day shorter than 14 hours.2209

Poinsettias are short day plants. Chrysanthemums are short day plants.2216

As you can imagine, the other type of plant is long-day plants.2220

They only flower when days are at least, so they are longer than a critical number of hours.2226

For example, there might be a plant that only blooms when the days are 12 hours or longer, when there are at least 12 hours of daylight per day.2249

This is a minimum number of hours of daylight, whereas, short-day plants, there is a maximum number that they will tolerate.2259

Lettuce, radishes, those are both long-day plants.2267

Now, here is where things get a bit confusing because experiments done in the 1940s showed that short day and long day are misnomers.2270

It is not the hours of daylight that affect when the plant is going to bloom, when it is going to flower. It is actually the number of hours in the night.2281

Of course, those two are completely related. Nonetheless, short-day plants are actually long-night plants.2290

So, short-day plants equal long-night plants.2301

So, what the plant is detecting is not that the day is shorter than 14 hours or maximum 14 hours.2307

What it is detecting is that night is a minimum of 10 hours, that night is at least 10 hours long.2315

By the same measure, long-day plants are actually short-night plants.2327

So, if there is a plant that only flowers when day is at least 12 hours, what it is really doing is looking at a night that is shorter than 12 hours.2334

It is the night length that matters, but these names were given a long time ago; and they are just used for traditional reasons.2354

The idea, though, is that there is a critical number or hours of daylight or dark.2364

And that is how the plant can detect what time of year it is and when to flower.2370

Not all plants are affected by this. In fact, some plants are called day-neutral.2378

These plants do not respond to the changes in the photoperiod.2385

They flower because they just got into a certain point in their development. It is time for them to flower.2388

Or, they will flower when the temperature has been above a certain point for a certain length of time.2393

So, it is not just light that dictates things, but day-neutral plants have other signals that they look at.2400

Now, we finally get back to what phytochromes have to do with all this.2409

Well, they are the plants way of detecting how many hours of darkness there are, how many hours of daylight there are.2412

Remember that the PR to PFR conversion has to do with light, the absorption of red light versus the absorption of far red light,2418

and in darkness, PFR converting automatically back to PR.2427

Therefore, this provides the plant with a way of detecting how many hours of night there are and how many hours of day there are.2432

And when there is a certain level of PFR built up versus PR, then, the plant would bloom.2439

And plants can be very, very specific to this literally blooming.2446

Even a slight change in the amount of day versus night, even a couple minutes, can make a plant bloom or not bloom, so it is a very finely tuned mechanism.2452

OK, so a lot of information, but photoperiod is the relative lengths of night and day. Photoperiodism is the physiological response to a photoperiod.2464

And photoperiodism is largely controlled by the balance of PR versus PFR, these two forms of phytochromes.2476

Alright, so we are going to go ahead and do some examples now.2488

Example one: a seed lands in the soil during a dry month. The seed does not germinate until after a series of rain showers in the spring.2492

Which hormone is responsible for the seed dormancy during the dry season?2500

Well, recall that abscisic acid is the hormone that would point to, that would maintain the seed dormancy.2505

Although, it is the balance of abscisic acid to growth hormones that really matters.2516

And what would happen in the spring is that the water wash the abscisic acid out of the seed, and germination can occur.2521

A plant is placed near a window that receives a good light. Later, the plant is noted to curve towards the window.2528

What is the name of the hormone that stimulates this response?2534

This is phototropism that the plant is growing towards the light. It is a positive tropism, and auxin is the hormone that is responsible for this response.2538

A fruit seller receives a delivery of fruit that is not yet ripe. To ripen the fruit more quickly, she places them in a closed container together.2553

What hormone is the fruit seller trying to increase the concentration of?2560

Well, recall that ethylene is a gas, and it promotes fruit ripening and that if you place fruit in a closed container,2564

the ethylene concentration will rise promoting fruit ripening, promoting more ethylene through positive feedback.2573

Example two: list two effects of cytokinins on plants.2581

Well, there are more than two, but in general, what cytokinins do is they stimulate cell division, and they stimulate differentiation.2588

Because of this...cell division, that is one effect. Differentiation, that is another effect, and the result is that they influence apical dominance.2599

Auxin also influences apical dominance. Auxin promotes dominance by the central stem and suppresses growth of the axillary buds.2618

Whereas, cytokinins actually stimulate growth of the lateral branches.2629

Another effect is the delay of aging. Cytokinins inhibit the breakdown of protein and actually stimulate protein synthesis thus delaying the aging process.2642

List two effects of gibberellins on plants. Well, they stimulate stem elongation along with auxin.2655

They are responsible for bolting, which is the rapid growth of a floral stalk.2666

They are important in the development of fruit, as well, and are used commercially to promote the growth of fruit.2674

Example three: this is a matching question. The first is no. 1, photoperiod: a group of hormones that stimulates cell division and differentiation.2687

Well, photoperiod is not a group of hormones.2700

The relative lengths of night and day, a change in the direction of growth triggered by touch or a red light sensitive2702

receptor that plays a key role in photoperiodism- well, photo, light and then, periods, so that would be the length of time.2711

The relative lengths of night and day- that is the photoperiod.2721

Two, phytochromes: phytochromes are not hormones. They are not a change in the direction of the growth.2726

A red light sensitive receptor, these are receptors, and they are sensitive to red light.2735

And they are important in the responses due to a photoperiod, so photoperiodism, so D is correct. Two is D.2740

Cytokinins: a group of hormones that stimulates cell division and differentiation.2749

Remember I said remember cytokinesis is cell division? And that helps to remember what cytokinins do.2754

OK, finally thigmotropism is a change in the direction of the growth triggered by touch, so this means touch; and then, tropism is bending towards.2761

OK, Example four: what are two mechanisms by which a hormone can induce a response in a cell?2772

Well, it can act at the level of the gene, or it can act at the level of a protein.2782

In other words, it could induce transcription. It could turn a gene on.2793

It could turn a gene off. It could activate a protein, or it could inactivate a protein.2800

And so, the overall two mechanisms are: that it can affect gene function or transcription. It can affect transcription.2811

Or it can affect the protein that has already been transcribed and translated- one and two.2823

Why only small amounts of a hormone needed to elicit a response in a cell?2836

Recall the second step is transduction in cell signaling, and what occurs during transduction is amplification.2840

So, the signal is the hormone is the signal, and it is amplified by the signal cascade that one receptor can activate many proteins.2849

Or it can greatly increase the level of cyclic GMP or calcium or another second messenger.2870

Each of those second messengers is going to activate proteins and so on, so there is amplification in that second step of cell signaling.2876

That concludes this lecture on plant hormones and tropisms at

Thank you for visiting.2889