Berkeley Talks transcript: Why do leaves change color in the fall?

Listen to Berkeley Talks episode #129: ‘Why do leaves change color in the fall?’

[Music: “Silver Lanyard” by Blue Dot Sessions]

Intro: This is Berkeley Talks, a Berkeley News podcast from the Office of Communications and Public Affairs that features lectures and conversations at UC Berkeley. You can subscribe on Spotify, Apple Podcasts, Acast or wherever you listen. Also, we have another podcast, Berkeley Voices, about the people who make UC Berkeley the world-changing place that it is.

[Music fades]

Christine Manoux: Hello, everyone. Welcome to the University of California Botanical Garden at Berkeley’s virtual conference room. I’m Christine Manoux, I’m the education director at the Botanical Garden. And it’s my great pleasure to have you all here joining us on this beautiful fall day for what is going to be a lovely talk about the season and the botany behind the season. If you’re not familiar with the UC Botanical Garden, we’re 34 acres just above the UC Berkeley campus. And we actually are one of the most plant diverse places in the world. We have plants from every continent, except one of course, Antarctica. And because of that, we actually have a rotating display that all year is quite fascinating. There’s always something in bloom or in seed or changing color. So it’s really a fantastic place to visit year round. Right now some of our favorite collections are the Eastern North American collection and the Asian collection. Behind me virtually is Asia with Strawberry Creek falling in the background.

We do hope you’ll be able to join us at the garden in-person sometimes soon. Membership is also something you might want to explore, if you’re not a member already, provides you with free access, including our early member’s hour 9 a.m. to 10 a.m. every morning. And it is the season of gift giving. So maybe if you are already a member, you might think about gifting membership this year.

We do have in December a number of exciting programs, we’re bringing back our wreath making workshops, which already have sold out. But if you’d like to join our wait list for those workshops, you can, we’re going to have a virtual tour of the conifers of our collection. So that has no limit. So feel free to sign up for that. And there’s a number of other things that we’ll be adding to the calendar soon. So do you check out all the things that are happening at the UC Botanical Garden.

With that I’m going to turn it over to our director of the garden. We’re so pleased to have him join us and share his expertise. He’s been a professor of plant biology on the UC Berkeley campus for over 20 years. And please join me in virtually welcoming our director of the UC Botanical Garden, Dr. Lew Feldman.

Lew Feldman: Thank you, Christine. Although you can only see me up in the corner, I really am sitting here in my office here on the campus at the garden. Today the lure to draw you to the talk is to try and present you some information on the background and the science, and maybe some of the phenomena associated with the development of fall color in plants. And what we’d like to do is, although fall colors are the reason that many of you are here. What I’d like to do is to set the coloring of the leaves and these events, which are associated with leaf coloring into the life cycle of the plant, into the overall lifetime of the plant, so that you can understand not only why the color comes about, but also have some idea of when it’s going to occur and what are the events which affect it.

Today, what I hope to do is to take you through a life history of plants and to insert in that life history the events which you’re associated with, what has drawn you here today, that is trying to understand, or to have some more background on why plants develop fall coloring and how the fall coloring develops. So if we look at the typical stages of plant development, we have at the upper corner of this, of the upper left hand corner, a tree, which is flowering, this would be in the springtime and the upper right hand corner is a tree which is fully leafed out. And it is now undergoing maximum photosynthesis. And at the end, or during the leafing out time events begin, which prepare the tree in this particular case for a period of dormancy, which is shown in the lower left picture. And then finally, dormancy is begun and the tree has lost its leaves.

What we’re going to do is to talk about events which are associated with the development of fall coloring, but to insert them into this cycle that you see here represented on the first slide. Now, the processes which are readying a plant for winter, they often involve the loss of leaves. And when we consider the processes, which are involved in the development of fall coloring, they are in a technical term grouped under the word senescence, which is something we do as well. But in this case, we’re talking about it in terms of a plant.

And senescence is a way in which plants prepare themselves for a change in the climate, a change in the climate which is frequently not good for the plant. And during this change in climate the plant reads itself in many ways so that it can withstand the winter. Part of the processes which we’re going to talk about is that the plant is going to try and recover many nutrients from its senescing organs, that is from its leaves. And to try to keep those nutrients from being lost as they would if the leaf just fell on the ground and was not able to return the nutrients to the plant.

Leaf senescence is an important process, which is involved in the development of fall coloring. And it results from really an orderly degradation of cells and leaves which go through a number of physiological and biochemical processes, we’re going to talk about what those are. And that ultimately results in the development of the color, but also in the resorption or the movement of minerals from the leaf back into the plant. And for those of you who are interested in this process of resorption of nutrients, I would like to say that the resorption is not absolute, not every nutrient, which is in the senescing leaf returns to the plant, but most do.

Here we have a leaf, which is in the process of getting ready to senesce. And at the senescence process, we have now materials which leave the leaf and enter into the parent plant. And the senescent processes actually begin with the departure of some of the nutrients. What actually controls the loss of coloring in the leaf and development of senescence. What science has found is that the defining character, which actually initiates the senescence process in the leaves is photoperiod. And photoperiod was originally defined as the number of hours of light. That is the length of the daytime. That is the primary push for the development of the senescent processes. And then temperature comes into also controlling senescent processes, and we’ll talk about that. And the question which we might want to put at the beginning is, why is photo period the single most important environmental parameter associated with senescence?

Why have plants evolved to cue in as far as whether they undergo senescence onto photoperiod of all the possible things that are going on in the environment? And the answer is that photoperiod, which was originally defined as a period of numbers of hours of light that the plant measured. In fact, photoperiod, we now know the plant doesn’t measure the number of hours of light, it measures the length of the dark period. So from the original definition of photoperiod, meaning the plants were measuring the number of hours of light. We now know that as the days grow longer, the plants are measuring the amount of darkness. And as you know, when winter comes on the number of hours of light decrease, but importantly for the plant the number of hours of darkness increase. And the number of hours of darkness, when they reach a certain level, depending upon the genetics of the plant, indicate they are the cue for leaf senescence.

We now know that the number of hours of light that a plant measures is varied depending upon on what the species is. But what I want you to know about is that the number of hours of darkness is the important point. And for those of you who might go out on your street and find some plants growing next to street lights, even here in the bay area, we do have the leave senesce. You will notice that the leaves which are on the side of the tree which is bordering on the street lamp, are retained by the plant much longer than leaves on the opposite side of the tree away from the lamp. And those leaves then are measuring the number of hours of darkness.

Now, photoperiod exerts a strict control on leaf senescence in latitudes where winters are very severe, and that would be in most of the U.S. here in California, although photoperiod is important with regard to senescence in leaves, temperature gains and increased importance in also controlling the senescence processes. But because temperature can vary, as you know, some years temperature is warm, in the wintertime some years it’s cold. Plants do not depend only on temperature as the main key for inducing senescence. Photoperiod, or the number of hours of darkness is fixed, does not change. And so through evolution plants have keyed in on using the length of the dark period as the important predictor that the seasons are changing. And temperature, although important, it is not as reliable as far as the plant is concerned as is measuring the length of the dark period. So darkness is important. And for those of you are interested in controlling the processes which involve the development of the senescence, you need to consider how long the dark period is.

At lower latitudes, which would include here and as we go to the equator, the temperature exerts more of a control over the plant senescent processes. And we believe this is true because plants which are growing in warmer temperatures, it would be not beneficial for them to senesce, even though the days are getting shorter, because by senescing earlier, they are reducing the amount of photosynthesis that is carbon uptake, which they can still benefit from in areas where the autumn is warm. So leaf senescence proceeds in a sequential manner after the plant measures the amount of darkness. We begin first with the degradation of the molecule that’s important in photosynthesis, that is chlorophyll. This molecule gives the plant its characteristic green color and following the prescribed number, the genetically determined number of hours of darkness, chlorophyll begins to degrade.

It begins to turn over and begins to disappear. And this is important as we’ll talk about in a moment with regard to the development of the colors that you associate with the fall. When the chlorophyll begins to decay, this is associated with processes, which result in the recovery as I mentioned earlier, of many nutrients, which have been stored in the leaf. And the nutrients, which are most, are greatly coveted by the plant and are removed from the leaves are nitrogen, phosphorus, potassium, and sulfur. These elements are wanted by the parent plant and are withdrawn as much as they can be. Although some of them still remain in the leaf as the fall proceeds to winter. However, when the chlorophyll begins to degrade another signal, and those of you who are probably interested in plant growth and development might know this, another signal besides the lack of light begins to take effect.

And that is the change in the production of a growth regulator or hormone, which is known in as auxin. Normally Auxin is produced in leaves and it is then exported from the leaf into the parent plant. But during the fall, when the days get shorter, the amount of auxin produced in the leaf decreases. And this decrease in auxin is important in making the leaf begin the senescent processes. And with the development or the lack of the development of auxin production in the leaves, we then get the export from the leaves of the compounds, the elements that I talked to you about before. And the senescent process is speeded up as these elements are removed from the leaf.

While these events are associated with leaf senescence, for most of us, you know leaf senescence being mostly associated with the development of colors in leaves and the development of colors really depends upon pigments.

We’ve already talked about one pigment, which makes a plant green, that is chlorophyll, but there are many other pigments which contribute to the plant. And I’d like to introduce you to them now. Many of you may know of these pigments. We’re going to talk about them in a little more detail very shortly. We talked about chlorophyll, which is at the bottom of the slide. I’m sorry, my pointer doesn’t work. And chlorophyll, as we said, gives leaves its green color and is involved in photosynthesis. It is the important pigment of the plant. The other pigment, which is important is at the top. These are carotenoids and these produce yellow, orange and brown colors of plants. And you are probably familiar with carrots and bananas and daffodils. These are plants which take on the color because of carotenoids.

And finally, the color which we will talk about briefly now is anthocyanin. And this is the color which begins to appear more prominently in the fall and which ultimately gives us the beautiful reds and shades of red as we go into the fall, coloring. Looking a little more closely at the pigments. You can see their general coloring, chlorophyll is green, xanthophylls are yellow, carotenoids are orange. And then the anthocyanins, which we talked about before, are the reds. These are the important pigments, which are involved in the development or in the expression of fall coloring. And again, I just want to spend some time talking about the pigments so that you understand how they interact with each other. Chlorophyll, as we said, is involved in photosynthesis and is present in the leaf, but begins to break down in the fall, begins to disappear. The other pigments carotenoids and xanthophylls at the bottom of the page are pigments which are present in leaves during most of their lifecycle that are masked by the green of the chlorophyll.

Anthocyanins are the pigments which then begin to develop. Although the leaf may have them and they may be masked, they develop in the fall and they become clearer as the chlorophyll begins to break down. So here are the four classes of pigments. The last one is really not a pigment. Fall coloring, where we have a chlorophyll on the left. And then we have the different colors of the leaf with the anthocyanins toward the right. And finally, when the leaf falls to the ground, it’s brown or dead, and this brown color is because of the cell walls, not because of any pigments, which are still present. For those of you who are interested in the structures. I’d like to point out that the, and this is important for what we’re going to talk about in a, in a few minutes that the chlorophylls the carotenoids and what the carotenoids are [inaudible] and organic solvents.

And this will come, reason it’ll come clear in a moment. And anthocyanins are soluble in water. And so we’re going to talk about them. You can see their chemical formulas below, and I just want to mention to you that for those of you who think that carotenoids only occur in plants, they also occur in eggs. For example, they’re what give eggs its yellow color. So let’s look at a plant which is undergoing senescence now. And if we look at a single cell in the leaf of a plant, we notice that it has a nucleus, and it has a large structure in the middle of this cell, which actually gives it its shape. It’s full of water basically. It’s called a vacuole. And this is important in the moment, I’ll tell you why.

And then we have the chloroplasts, the chloroplasts are organelles. They are bodies within the cell which are green. And they are the places where photosynthesis occurs. When senescence occurs, and again, we said, senescence is a programmed process, it’s not random. It occurs because of a sequential series of events. We begin to see importantly changes inside the cell. So at the far right, what you notice in the cell is that the chloroplasts, the places where the green color was being produced have now basically disappeared, have been eliminated as far as being alive in the cell as it dies. And if we look carefully at a cell, and I’d like you to look at the lower level of pictures here.

On the lower left side you can see the chloroplasts around the edge of the cell they’re green, and in the center of the cell is the vacuole. So as fall approaches and as photo period has done its job of initiating senescence in the development of fall coloring, the chloroplasts begin to break down. And then in the vacuole, that’s that structure which was watery in the center of the cell, anthocyanins, the red pigments begin to develop. So notice that anthocyanins in generally are not present in the plant in great amounts when it is in the spring or summer, and that they appear in the fall with the breakdown or the turnover of the chlorophyll. So this chlorophyll breakdown is a very orderly process. And when it breaks down, when the chloroplast break down, the anthocyanins begin to be formed and they are revealed because the green color is no longer present.

Three factors influence the leaf color. We’ve already talked about the length of the night, which is important. The other aspect in some environments is weather, we’ve talked about that. And clearly what gives the leaf its color is, or are the leaf pigments. And here we see the slide that I began the talk with, where we have a range of colors and these range of colors from the left to the right represent a disappearance of chlorophyll, a revealing of the carotenoids in the yellow colors and a synthesis then of the anthocyanins. So you’re going to see a range of colors when you look at leaves, and these range of colors result from pigments being unmasked, or being synthesized as a result, also our chlorophyll breaking down and disappearing.

Here’s a very interesting picture of the leaf here. You can actually see a number of the processes occurring. In the very center of the leaf the chlorophyll is still present and the leaf is green in the center, but toward the periphery of the leafs you can see the red of anthocyanins being produced. So the periphery has died or has undergone senescence first. Then you can see the yellow, which has been unmasked by the disappearance of chlorophyll, representing the carotenoids. And then toward the center of the leaf you can see the green color, which persists because of the chlorophyll, and that green color is masking the carotenoids. And at this point then the green color being present, there may be anthocyanins present, but they probably aren’t present to a very great extent. And if they were, they would be masked by the green of the chlorophyll.

Here is a closeup of a leaf, which is undergoing senescence. And what I want you to notice is that the veins, the large prominent structures running throughout the leaf, the veins are associated most closely with green cells. That is cells, which are still undergoing photosynthesis. Whereas the cells that are more distant from the veins have begun to turn red and have begun to express anthocyanins. These cells, which are furthest from the veins have lost their nutrients first and the cells which are still undergoing photosynthesis and retain the nutrients are closest to the vein. So when you look at leaves, look at the way they lose their color and also take on the anthocyanin color. It is an expression of which parts of the leaf die first, which parts of the leaf take out their nutrients first.

Now let’s look at how you would look at the nutrients or the pigments, I should say, in a leaf. And you can do this at home with some isopropyl alcohol, or if you have some strong liquor, you could also do it, like vodka, by taking the leaf tissue and mashing it up in a mortar and pestle, and in the organic solvent of the vodka or of the isopropyl alcohol, the pigments will come out. And you can take these pigments and you can put them in what’s known as a separatory funnel, and you can concentrate the pigments, you can get them into a small volume. And then if you take an aliquot, a small amount of that liquid and you put it on a piece of paper, what we’re going to do is a technique which is known as paper chromatography.

A drop of the extract of the leaf is put onto the piece of paper. That piece of paper is then put into a small tube. And at the bottom of the tube is a solvent. And the solvent is usually alcohol of some kind and the alcohol then migrates up the piece of paper, it’s a chromatograph. And as it migrates up the different pigments, which were present in our extract, begin to separate from each other. They begin to separate based on their chemical nature. And you can begin to see now in this leaf here the various pigments, which make up the coloring of the leaf. This is a leaf which you can see has chlorophyll, it has xanthophyll, it has other carotenoids, but it doesn’t yet have any anthocyanin. So this is a leaf which is coming from a plant or which has not yet entered the senescence process.

Here is another way of looking at the chromatogram. It’s a very simple thing to do. You can actually do it at home if you have some alcohol with some newspaper. And you can see that the pigment, which is at the top of the chromatogram is carotene or carotenoid, it’s the orange pigment of a plant, and then xanthophylls, and then the chlorophylls are at the bottom. Remember that the chlorophylls are masking the xanthophyll in the carotenoids or the carotene. It’s present, but we don’t see them.

Here is a contrast now of two paper chromatograms, one from plants that are growing in the spring and the fall, and another from plants which are into the fall already. And what I want you to notice is that first of all, compared to the left, which is from the spring and summer, that when we do that same chromatogram the chlorophylls disappear. They are almost gone, although we know where they would be on the chromatogram. The xanthophylls and the carotenes have now been unmasked. And they tend to increase in their amount, although not greatly, they’re fairly stable molecules. But importantly, what I want you to notice is that the anthocyanins now appear. These are molecules, which are water soluble. That is, they were in the vacuole of the plant. And as a result, when you put the paper chromatogram in an organic solvent, the anthocyanins are not soluble, they don’t move very much. But we see them, because the other pigments have been moved away from. And because the chlorophylls have also degraded, they’ve disappeared.

You can do this at home. It’s a very simple thing to do if you have some alcohol or some vodka, something that will allow you to separate the pigments. So if we were to summarize in a sort of timescale the events, the important events involved in the production of the color, and in the senescent process, we have this sort of cartoon here and what I want to draw your attention to, our events, which we talked about before the chlorophyll begins to degrade. The green color begins to disappear. In this particular plant they have the carotenoids degrading, although it isn’t always the case. And there’s also in this particular case carotenoid accumulation. And then we see the accumulation of the anthocyanins. This is really an important aspect of it.

While these events are going on with regard to the pigments, you can see below some of the other events which go on in this leaf as its undergoing senescence. And notice that sugars change in the leaf because photosynthesis affected proteins decay. And we talked about the movement of phosphate and other substances out of the leaf, into the plant, because the plant wants to covet these very important nutrients. And then finally, as you can see in phase four, the leaf has lost its cellular contents and the leaf is dead and drops off the plant. So notice, this particular process is less than a month long in this particular plant that is being illustrated here. And so it can happen relatively quickly.

So, what are the conditions that favor the senescent process, and more importantly, that favor the formation of the bright red colors that we associate with the fall? And in most cases, the best fall color, the brightest fall color occurs when we have warm sunny days followed by cool, but not freezing nights. This is an important aspect. If you have rainy or cloudy days as frequently occurs in areas which can have colored leaves, this rain or cloudy days actually limits photosynthesis and limits the amount of sugar formation in the leaves. And that is substances which would be used for making the anthocyanins. That is those pigments that bring on the brilliant, beautiful colors of fall that we know about. So these rainier cloudy days tend to diminish or to tamp down the bright colors that we typically associate with fall.

Now, carotenoids are always present in the leaf, as I said before. And so the yellow and gold colors of the leaf remain relatively constant from year to year. But again, they are usually masked by the chlorophyll, and are only revealed when the chlorophyll degrades. Other factors can also affect color. And those factors that we said could be the weather, number of warm days. But then also we can have moisture in the soil affecting the development of autumn colors. And most importantly, a late spring or severe summer drought, which occurs way before the fall can delay the onset of fall color. And also, a warm period during fall will also lower the intensity of the fall colors. So there are a number of events during the year, as far as weather is concerned, which can affect the development of color. It is not simply the length of the dark period which brings us our bright colors.

A typical map that you might find for coloring. As you can see here, in this particular case, in the upper right hand corner and in the upper corner is the Wisconsin and Minnesota, you can see that the color begins in late September. And as you go further south, as it takes longer for the photo period to change, and as weather becomes more important, we delay the development of fall color as the season goes on. So that in California, in most cases we may not even get much fall coloring, simply because we never get cool nights and we never really get freezing temperatures. And that’s the same thing for Southern Florida. You might ask, “Is it adaptive to have coloring?”

The final step in the senescence process after the colors have been produced and after we’ve experienced the beauty of the fall is really the separation, the falling of the leaf from the plant. And this is known as abscission when the leaf separates itself from the plant. And if you look carefully at a stem, which has had entered fall, you’ll notice a place where the leaf was attached. This is the abscission zone and the leaf leaves a scar where it falls off of the stem. This abscission process is an important process, which goes on because what has to happen is that first of all, chemicals have to be produced to weaken the joining of the leaf with the stem. And we know something about what those chemicals are. But as importantly, once the leaf separates from the stem, there’s a wound that’s created and that wound has to be sealed to protect the living tissue, which is underneath the point of attachment of the leaf. And that wound is frequently covered by a layer of cork, which then protects the scar to prevent pathogens from entering the plant.

Here is another picture. This is a cartoon. It’s sometimes called a closing scar, but it really is cork, which is forming over this cut there. Now, in addition to the leaves becoming either dormant or lost from the plant, the buds, which are the renewal shoots for the spring also become dormant. And they become dormant because in the scales, which enclose the buds on the stem and these scales as you will, these buds, as you’ll notice are sitting above leaf scars, there used to be leaves associated with these scales. These buds are now also dormant and they develop dormancy because in the scales, that is the brown structures covering the buds, we develop chemicals which are known as inhibitors, which actually prevent the buds from growing out. They actually induce dormancy in the bud. So that associated with the development of the fall colors, the buds are made to become dormant by the development of inhibitory compounds in these scales that surround the buds.

And in this particular case, the bud scales can prevent the twig from allowing the new shoots to grow out. And the way in which a plant normally gets ready to release the buds from dormancy or inhibition is that the compound, which has inhibited the growth of the buds has to disappear. And as it turns out, what makes that compound disappear is cold. So by extending the period of cold that a plant is subjected to, this is a way in which the inhibitory compound is reduced. And it is a signal to the bud that it is time to grow out.

Now, just to contrast this with light, of course, this is not a perfect mechanism, because it is possible that the inhibitor might develop to a certain level to allow the bud scales to grow out, the buds to grow out. And then maybe another period of cold would come and kill the new shoots. So this depends upon average years in which the number of hours of cold are enough to reduce the inhibitor. And then the number of hours of cold are followed by warm conducive conditions for the new shoots to grow out. But that isn’t always the case.

There’s also something known as cold hardiness. And this is genetically determined by plants and intolerance of cold temperatures is a major factor, which limits native plant distributions. And this is important again, in the decision of which plants to put in a particular area. And also, as we mentioned before, in getting the plants to know when to grow when winter is over and when it is to develop and to release their shoots. Plants which are poorly adapted with regard to their cold heartiness, as we said early, come out of dormancy too early and then be injured if there’s another period of cold.

So, how can plants avoid freezing? Well, one way they avoid freezing is by developing multiple mechanisms to allow them to measure cold. In other ways though plants have developed something of an antifreeze, and this antifreeze really is a relatively simple compound. They put sugars into the cells and these sugars have an effect of lowering the freezing point of the liquid, which is still found in the living cells and thereby preventing damage to the cells. In spruce plants, for example, they don’t use sugar, they have a protein or protein molecules, which actually prevent the development of ice crystals, which would be detrimental and which would kill the cell by destroying the insides of it. So cold is important with regard to ending dormancy, but if it’s too cold and if we don’t have a way of regulating the effects of the cold, we can damage the plant, maybe even kill the plant.

So, now I wanted to ask you a question, which I posed to other people and maybe something which has occurred you. Why develop color? What’s the adaptive advantage to a plant of producing these beautiful fall colors. This happened long before humans evolved, so it’s not for our benefit that the plant produces these beautiful fall colors. There are a number of hypotheses for the evolution of autumn and colors, and I’d like to touch on them and then let you know what we think is the most likely hypothesis.

And I’ll just go through these relatively quickly. We can spend more time talking about them in a moment. One is a hypothesis is called photo protection. And in this hypothesis it is believed that the red pigments, which form allow the plant to more efficiently recover the nutrients in the leaves, prevents damage. It is believed that the red pigments allow plants to tolerate water stress, and also leaf forming if forming should occur at the wrong time.

Another one, which I’d like to move down to, is co-evolution. And this is one that I’m going to talk about a little more and just briefly, co-evolution is that the red color of the leaves indicates to an insect that it’s not a suitable host, that the tree is not a suitable host. There’s a possibility that the red color camouflages the tree to insect attack. And there’s also the possibility that it’s anti-camouflage, that the color enhances the suitability of the tree for certain parasites to come and live on the tree. And then there are some other possible hypotheses for the development of the red color. And you can read about them. If you’re interested, I list them here.

Let’s just talk about a few of the hypotheses, which are really the most interesting ones. One is photo protection. And this is one which seems to be very likely with regard to science right now that the pigments do protect the plants from highlight and highlight will allow a process known as photo oxidation to occur. For those of you who are here in California, but more importantly in the tropics, if you look at newly produced leaves, which come out on plants, they’re often red, and that red coloring believed to protect those leaves from damage by light. And the same thing is going on here it’s believed in the fall leaves through the production of the red pigments. We also mentioned that the red pigments may induce drought resistance, and this is an important aspect. Although, this particular hypothesis has not received a lot of evidence.

One hypothesis, which has received a lot of evidence is co-evolution. And that is that the red color of the leaf somehow signals insects or other organisms that they do or do not want to be on the plant. Camouflage for some plants, in some plants may be a way of indicating to a herbivore who might want to eat the leaves that it’s not a suitable place for the herbivore, or it could be for an insect as a way of making the insect believe that the leaf is not a good place for it to eat. Camouflage could also make the leaf less detectable for herbivores.

There’s also something known as anti-camouflage, and anti-camouflage is kind of an interesting idea. Because in this view, what happens is that the red color actually draws parasites to the leaf to feed upon other insects, which might be damaging the leaf. These two ideas in co-evolution have some support in science and they are ones which are currently being explored. There are some other ideas, as we mentioned before, the red pigment may make the leaf have less palatability for animals that want to feed on the leaves. They may, again, with the yellow color, they also have that same thing.

And then the other idea is that it is possible that the red color or the coloring of leaves in the fall may attract insects, such as this case, aphids, which then attract ants that defend the trees from other insects. And many of you have seen on your plants at home ants, which patrol the leaf and actually protect the aphids, which are on the leaves because the ants receive some food from the aphids. So these are some ideas with regard to the evolutionary reason for why plants have developed red color, what possible role the development of red color might serve.

The co-evolution hypothesis, as I mentioned, is the one where there’s a lot of evidence for, and it has been found that in the fall insects move to trees in the autumn that were preferentially colonized green rather than red leaves, and that red leaves reduce insect load. So the co-evolution hypothesis seems to be one in which people are very enthusiastic about as is the photo protection process we talked about before. And those are the two leading hypotheses right now with regard to why leaves develop red color.

Finally, I want to end by talking briefly about climate change and what climate change is going to mean for the development of the fall color that many of us are so familiar with. The accumulated evidence, which now exists, suggests that climatic warming will delay the onset of leaf senescence and ultimately leave fall. And that the contributions of photo period and temperature to the control of leaf senescence were going to have to resolve how these two different environmental parameters affect leaf senescence and how warming is going to affect the development of leaf senescence. Right now there is the idea that senescence will be delayed, but because there are so many factors, some of which we touched upon here, which affect the development of color, this is an area which is going to have to be highly and greatly explored, and especially in relation to climate change.

The likely effects of climate change are, as we said before, a delay of leaf senescence, and it’s been estimated that a delay of six to eight days will occur for each degree Celsius that the temperature persists. So that for plants, which are growing in higher temperature, for each six to eight days of higher temperature that we will have a delay if the temperature warms up one degree. And this is really an important point, something which is going to influence what kinds of trees which are planted.

And therefore, warming temperatures extend the growing season by advancing not only autumn leaf color change, but also ultimately by affecting when the leaves come out from the new buds, very important consideration, it’s going to be increasingly important when it comes to deciding what trees to plant in areas.

Finally, drought has an effect on the development of the coloring of leaves. And drought is going to be also increasingly important with climate change and drought will probably speed up, it speed up leaf senescence in many ways, and will also affect the absorption, the resorption of nutrients from the leaves, because they’re going to be senescing prematurely.

So, the overall effects of climate change and nutrient absorption are going to depend upon both the contrasting effects of warming and drought and on the ability of plants to adapt. And this is going to be important, be because when nutrients are absorbed, those nutrients are used the following year when the plants leaf out and begin to grow again. And so climate change is not going to affect the plant simply one year with regard to coloring, but the effect of climate change is going to persist for the next year and perhaps for many years thereafter.

In this particular case, we can conclude that climate warming is going to most likely delay leaf senescence and fall. And in many species sensitive to temperature going to have a weak, if any effect, it’s going to have a weak effect, if any, on species which are not sensitive to climate change. And this is an important aspect that climatic warming is going to affect mostly those species which are growing in areas that typically get cold, probably will have less effect in areas like ours or in areas in Florida where there’s not a period of cold. But also, we have to figure into this equation drought. And that’s something as you know, with climate change, which is going to increase greatly.

Finally, I just want to talk about before we end the talk today about when dormancy ends and the new leaves are produced. We’ve already said that the buds grow out because the inhibitor to the growth of the buds decreases in amount, new leaves are produced. And then the materials which have been stored in the stem during the winter, that is the elements that were removed from the leaves that fell down, they now are going to be located in the sap of the trunk and in the roots. And those new nutrients have to be moved up to the new developing buds.

If we look at this cartoon here in the fall on the right, the nutrients, which were in the green leaves and in the photosynthesizing parts of the plant in the autumn are moved down into the stem and into the roots where they’re stored. And in the springtime, when new growth is to appear those nutrients, which have sequestered are now moved up the stem into the buds and are used to nourish the new twigs or the new leaves which grow out of the plant.

And here are some of the buds. I just want to point out to you. These are the dormant buds. They have essentially a tiny little shoot in a very compact area. They’re surrounded by the scales, which are preventing them from growing out. And when the winter cold releases the amount of inhibitor or looses it, the buds germinate, the buds scales, now don’t protect the bud anymore. And they have now lost their inhibitor and the buds begin to grow out and leaf out. And then we have the leaves and the beginning of the cycle again. Notice on this particular picture that these leaves are red, and these leaves are red because they’re new. And it’s a point which I mentioned earlier in the talk, the red color is produced by the anthocyanins, the same material that makes the leaves red in the fall.

But in this particular case, this red color probably is associated with preventing damage from highlight to the chlorophyll. So this supports the hypothesis that red color, at least in early produced leaves, is important with preventing the damaging effects of light, which are also known as photo bleaching or photo oxidation.

So finally, I’ll just end with this slide, which shows typical picture from probably New England of a pond reflecting the lovely fall anthocyanin colors, the carotenoids and the greens, which are found mostly in the conifers, that is the gymnosperms, which don’t change color or don’t change color in this dramatic a way. So with that, I want to thank you for listening to this talk. I hope you’ve come away with a better understanding of the events which are associated with the development of the beautiful color in leaves. Realize it’s not here for us. Those coloring of leaves has a dramatic effect on the lifetime of the plant. And importantly, that this development of coloring is not only going to change life for us when climate change occurs, but is also going to change how plants adapt, or if they can adapt to the new environments in which they find themselves. So thanks a lot.

Christine Manoux: Thank you, Lew, so much. So informative, never dived so deep into a leaf. So thank you. And thank you for leaving some time for questions. We definitely have a lot of fun questions in our chat box that I would love to field to you. And if others want to put some in, we’ll see how many we can get to.

One question we have is about leaf senescence that happens here with some of our California native plants during the summertime. How is that similar or different?

Lew Feldman: That’s great. That’s a really good one. California natives, they actually leaf out earlier in the spring to take advantage of the rain. And they also undergo dormancy preparation earlier in the time than plants that are growing in a different environment. So that the horse Chestnut, for example, the Buckeye, California Buckeye, it begins to undergo senescence really in August. And it is undergoing senescence at the expense of probably growth, because at that particular time, it is probably keying in on the lack of water, although it’s possible that it’s also measuring the photo period. But most likely it is the way of its responding to a water stress and, or it is a way of it predicting that a water stress is going to come probably in September. And so the senescence process begins early, either early fall or late summer before typically the weather, that is the weather has gotten cold. But the photo period is changing. And clearly it’s associated with a lack of water in the Mediterranean climate in the late summer.

Christine Manoux: And then this question is about evergreen trees that do not lose its leaves. What are the comparative benefits or detriments to being evergreen versus deciduous?

Lew Feldman: Yeah, so that’s a really good point that the evergreen trees, they have the problem of having to persist, their leaves have to persist during the cold of the winter. And during the cold of the winter in many places it’s usually a very dry period. It’s the humidity is quite low and those plants have to deal with losing water. And so what they frequently do is develop waxy layers to protect the leaves from losing water. And as I said earlier, many of the evergreen trees develop these proteins, which are kind of antifreeze proteins, which actually allow cells to survive having water and not having the water freeze. So rather than having a way of getting rid of their leaves, what they do is develop protections for their leaves. And those protections are mostly involved with making sure the water doesn’t freeze or what water they do have to make sure it’s not lost.

Christine Manoux: This might be a related question. Someone had noticed on the map where you were showing the different months of leaf color change, that Alaska was not appearing. They were surprised by that because of course it has the cold. But is the reason maybe because those types of trees are not the kind that change color, but rather more likely to be the evergreen, or?

Lew Feldman: Yeah. I think what it is, is you might recall the best color is developed when you have cold nights, but not freezing nights. And I think that Alaska probably gets freezing nights and that’s what really retards color production.

Christine Manoux: Great. So someone asked a question about a leave that seem to go straight from green to brown. Is something traumatic happening they were asking, but maybe you could explain that?

Lew Feldman: Right, I think in those circumstances where leaves go quickly from green to brown it is a stress, probably a drought stress, or it could be because the plant is somehow being attacked by a disease. But normally the leaves won’t turn so quickly because this does not give the plant an opportunity to recover the nutrients that it is already put into the leaves. So if a leaf turns brown very quickly, it’s probably because of lack of water.

Christine Manoux: Okay. We have a little bit more time. Can you comment on what color various varieties of trees display? What trees are known for their reds versus their yellows, or?

Lew Feldman: I mean, I don’t really know. They certainly, around here, the ginkgos are known for their lovely yellows, they don’t turn red. And you have aspens, which you see up in the mountains. But I suppose around here, the trees which are most colorful are the liquid amber trees, which develop the red color. And those leaves are really all over the place right now and they’re quite beautiful. In other areas I’m less familiar, so I’ll probably be better not to say anything.

Christine Manoux: Great. I have two questions related to light. Does artificial light affect the pigment degradation?

Lew Feldman: Yes. Artificial light, again, going back to that example I gave earlier, which is if you have a tree growing by a street light, the leaves which are closest to the street light, they will eventually lose their color. They will lose their green color, but it’ll be delayed. And the loss of the green color will probably be pushed by a change in temperature and not by a change in photo period because the lights are always on at night. And in those particular cases, those leaves probably don’t develop the red color or the very beautiful color. So that, yes, the answer is, artificial light will probably affect the development of color as we know in this particular last slide that can develop in leaves, yeah.

Christine Manoux: And then similarly, someone was wondering about nighttime light pollution — if that affects leaf color?

Lew Feldman: Yeah, I guess my best answer is the experience I’ve had is that the leaves of the plant have to be pretty close to the light. It has to be similar to what you’d have in at least in weak daylight for the plants to really be affected. If it’s just a pollution from general lighting, but not very bright light, it likely would not affect the coloring. That’s my guess.

Christine Manoux: Okay. This question is, “First off, thank you so much for the interesting talk,” they say. “Why do some deciduous trees such as maple and Oak have stronger fall colors than others? Do leaves of all deciduous trees have all of those pigments?”

Lew Feldman: Yeah. Well, again, one of the things you want to ask yourself is that humans have moved a lot of these plants out of their natural environments. So we talked about cold hardiness with the genetic adaptation of a plant for a particular area. It could be that some of the plants we’re talking about are no longer in the environments that normally were involved in the natural production of the colors that we know about.

But more likely, it is just simply because the plants don’t have the pigments to make the color or for whatever reason, anthocyanins, because the production of all these pigments is genetically regulated. It could be, for example, if let’s say the co-evolution hypothesis is correct, it could be that the red color was produced because there were herbivores or some kind of predator on the leaves. And in another environment those herbivores or predators are not present. And therefore it’s not necessary for the leaf, if in fact, co-evolution is an important reason, necessary for the leaf to develop the red color. So I think there are lots of ways of looking at it as to why some plants produce red colors and others don’t.

Christine Manoux: And then someone asks, “These colors that are masked during most of the life of the leaf, do we know if they have any other purpose?”

Lew Feldman: Yes, yes they do. So these other pigments are … So chlorophyll absorb certain wavelengths of light, and the other pigments, although they’re masked by chlorophyll, they actually still receive light. And those other pigments are actually funnels. They actually funnel light at wavelengths, which are different from what chlorophyll absorbs at. Funnels, wavelengths of light to provide additional energy for the photosynthesis process. So they actually do capture sunlight and that sunlight is used, but it is not used from the chloroplasts, it is used from energy from these other pigments, which is then funneled to the chloroplasts to acquire carbon.

Christine Manoux: Great. Somebody is wondering if perhaps the falling leaves also serve a purpose in soil health?

Lew Feldman: Right, so the answer is, partly what I said before is that we know that all of the nutrients in a leaf are not resort back into the parent plant. And it is very likely if you want to look at it in an evolutionary sense that as the leaf decays that the plant itself actually determines how much nutrient is to remain in the leaf. And those nutrients, when the leaf falls to the ground, are used to encourage and support a microbial environment or an environment of insects, for example, or worms beneath the tree. And that enriches the soil around the roots. And so the plant is maybe making a judgment as to what nutrients it wants to take back in, but at the same time wants to release and make sure that it falls to the soil so that its root system develops in a wonderful environment. And there would be maybe symbiotic relationships, which depend upon nutrients being added to the soil or possibly there could even be substances which prevent pathogens from growing in the leaves which fall into the soil.

Christine Manoux: Fascinating. A couple people are wondering if you know what species the leaf was that had the whole rainbow of color on it?

Lew Feldman: I do know. And tell them to write me and I will tell them what it is, yeah.

Christine Manoux: Okay.

Lew Feldman: I don’t know what it is now.

Christine Manoux: Feel free to email [email protected] with the question and we’ll try to get that answered for you. Thank you. Someone’s asking about the recent atmospheric river. Would that also affect senescence, because it seems to be affecting leafing out?

Lew Feldman: Yeah. Yeah, I think that’s a really good point, because in that particular case, I would actually say assuming the leaves were not blown off the plant, that the atmospheric river probably might delay senescence from a point of view that the plant does not feel the tug of drought. And if it doesn’t feel the tug of drought, it would say, “Hey, I’m going to try to keep my leaves as long as possible so that I can maximize the amount of photosynthesis I can perform and maximize the amount of carbon that I can capture and fix into sugars.” So that’s my guess, if it’s going to have any effect, if the leaves are not pulled off, that it might delay the development of the leaf color.

Christine Manoux: Great. Well, that is all we have time for today. If you have a pressing question that didn’t get answered, you can always email us at [email protected]. Also, our membership department is offering everyone who registered for this program a discount on new membership or upgrades or gift memberships. I’ve just put into the chat box a special discount code. It’s Fall20, to get 20% off any gift membership or upgrade or membership purchase. Thank you so much, everyone for joining us today. It was a pleasure to have you. We hope you come see the beautiful leaf color change at the UC Botanical Garden, and hope to see you at another program sometime soon. So thank you everyone, and enjoy the rest of this beautiful afternoon.

[Music: “Silver Lanyard” by Blue Dot Sessions]

Outro: You’ve been listening to Berkeley Talks, a Berkeley News podcast from the Office of Communications and Public Affairs that features lectures and conversations at UC Berkeley. You can subscribe on Spotify, Apple Podcasts, Acast or wherever you listen. You can find all of our podcast episodes with transcripts and photos on Berkeley News at news.berkeley.edu/podcasts.