Why Do Leaves Change Color? No, Seriously!

The secrets of why the leaves of trees change yellow or red in the autumn are slowly being revealed. Those eye-seducing hues, it appears, are a lot more than pretty. They're the result of trees doing their utmost to survive. They're the shades of evolutionary success. 

Let's start with the green leaf: We all learned in school that it's the result of the most abundant pigment in the leaf being the green chlorophyll. When the cool air and shorter days of autumn arrive, leaves change to red or yellow not because the leaves are dying, but because of a series of clever processes underway.

Not so surprisingly, yellow leaves undergo one sort of color changing process and red leaves another. As the chlorophyll is being turned off, most leaves turn yellow, which is just a color that's already in the leaves but is usually flooded by green the rest of the growing season. 

But over the last decade or so, researchers have discovered something very different goes on in red leaves. As their chlorophyll drops, they would also turn yellow if not for the sudden rapid production of a brand new red pigment call anthocyanin, which was not previously present in the leaves.

This surprising revelation has led to a surge of interest by scientists who are trying to explain why a leaf with only a week or so to live would bother producing an entirely new pigment.

One theory for explaining red leaves is that they are the result of 35 million years of trees battling insects looking for places to get a last meal and lay their eggs in the fall. Red leaves are harder for insects like aphids to see, for instance, so they tend to go for the yellow leaves.

Some evidence for this theory can be found in the differences of fall colors between North America and Europe. There are few native European trees that turn red in fall, but they are mostly yellow. In North America, however, there are ample red-turning trees, as well as yellow. 

The reason for this may be that in North America, as well as in East Asia, North-South running mountain ranges allowed forests to shift their ranges North and South with climate changes, carrying their insects -- and their long-standing battles -- along the way.

In Europe, however, the major mountain ranges run east-west. So as climate warms or cools, trees have nowhere to go and simply die out -- along with the insects that live off of them. So in Europe, the insect-tree battles have a much shorter history, and so less time to evolve strategies like anthocyanins to fend them off. 

This theory was put forward by Simcha Lev-Yadun of the Department of Science Education- Biology at the University of Haifa-Oranim and Jarmo Holopainen of the University of Kuopio in Finland, and published in the journal New Phytologist.

Another theory suggests that the variation of red anthocyanin in leaves of trees that live in the same area might have more to do with the richness or poverty of the soil in which a tree grows and so it reflects the lengths trees must go to hang onto the nutrients they have invested in their leaves.

In a preliminary study by a student in Charlotte, N.C., it was found that fall leaf colors and the soils under sweet gum and red maple trees show a significant nutrient difference that matches autumn tree color patterns. The richer lowland soils corresponded to more yellow leaves and poorer highland soils correlated to redder leaves.

"It's very clear that there's a correlation," said plant physiologist Bill Hoch of Montana State University in Bozeman. What's more, it matches what he has discovered about the function of that stunning red anthocyanin.

Experiments make a pretty strong case for anthocyanin serving as a protective pigment that helps trees in nutrient-poor or stressed places to maximize the nutrients they can draw from the leaves before they are dropped to the ground, Hoch told Discovery News in an October 2007 article.

"They pull as many of the nutrients back into the plants as possible," said Hoch.

The red pigment protects any remaining green, food-making chloroplasts in the leaves from damage. This is especially valuable for trees in nutrient-poor soils or stressful situations because this "photo-protection" allows the leaves to keep making sugars in their leaves longer.

This, in turn, is vital for pulling nutrients out of the leaves because the only way the nutrients can be extracted from leaf to trunk is by hitching a ride on the trunk-bound sugars.

The bottom line, Hoch explained, is that the longer photosynthesis can continue on an autumnal, coloring leaf, the more nutrients can be drawn out of it for re-use in the spring. So where every drop of nutrient counts the most -- like perhaps on some nutrient-poor hillsides of North Carolina -- red is the color of autumn.

by "environment clean generations"

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Posted in: autumn,earth,europe,leaves,mountain,nutrient,photosynthesis,Seriously,Why Do Leaves Change Color No,yellow

Can Plants Think?

In a new study, scientists have found a cabbage relative capable of remembering and responding to information. The Persistence Of Memory A Polish study showed plants send electrochemical signals in a way that can be likened to an animal nervous system. This image shows chemical reactions in leaves that were not exposed to light; they are reacting to a chemical signal from a leaf that was exposed.

Plants are able to remember information and react to it, thanks to an internal communications system that can be likened to a central nervous system in animals, according to a new study by a Polish plant biologist.

Plants "remember" information about light, and a certain type of cell transmits that information, much like nerves do in animals. 

In the study, which was published in the early online version of the journal Plant Cell July 16, the researchers found that light shone on one leaf of an Arabidopsis thaliana plant caused the whole plant to respond.

The response lasted even after the light source was taken away, suggesting the plant remembered the light input.

"The signaling continiues after the light is off; it is building short-term memory," said the lead author, Stanislaw Karpinski, in an e-mail message. "The leaves are able to physiologically 'memorize' different excess light episodes and use this stored information, for example, for improving their acclimation and immune defenses."

The leaves remember light quality as well as quantity, Karpinski added -- different wavelengths of light produce a different response, suggesting the plants use the information to generate protective chemical reactions like pathogen defense or food production.

 Scientists found that light shining on a leaf cell triggered a cascade of events that was immediately signaled to the rest of the plant via a type of cell called a bundle sheath cell. Those cells exist in every part of a plant. Karpinski, of the Warsaw University of Life Sciences in Poland, measured the electrical signals from those cells, and compared it to finding a central nervous system for plants.

Terence Murphy, a plant biology professor at the University of California-Davis who was not involved in the research, said shining light on that first leaf could have any number of effects. 

"The leaf would be loaded up with starch, maybe; that's going to have a real effect on how it communicates through the phloem (vascular system) to other leaves. It's not unreasonable that you could illuminate one leaf and affect the other leaves," he said. 

The trick is finding out how the other leaves are informed -- and that's what appears to have been done in the Polish study. Bundle sheath cells surround the veins in leaves, stems and roots, so it's reasonable to think they transmit the electrical impulse, Murphy said.

Biologists have long known that plants can remember -- they need to know whether they've gone through a cold season before they can germinate in the spring, for instance. It's not memory as we know it, but a prolonged change in plant internal systems that causes effects later.

What's more, scientists already know plants transmit electrical signals in response to a stimulus, just as nerves do. This is easily measured using a basic electrode setup, according to Murphy. 

Karpinski said the light memory represents a new way for plants to respond to pathogens or disease -- normally, they respond by direct contact with an invader. 

"This information would not be a revelation untill we find that plant leaves can remember it for several days and process this memorized information to (bolster) their defense mechanisms against seasonal diseases," he wrote.

 

Karpinski is well-known among plant biologists for earlier work on how plants respond to light stress. In a previous study, he also showed chemical signals can be passed throughout whole plants, allowing them to respond to and survive environmental changes. Understanding the mechanisms that cause those signals is a new step, however.

William John Lucas, distinguished professor of plant biology at UC-Davis and chair of the plant biology department, said an internal communication system would provide a wealth of information to different parts of the plant.

"A particular tissue within a plant needs to be able to signal to the rest of the plant in terms of what are its conditions, what should you expect," he said. "If a young leaf is emerging out of a plant, it would be nice for that leaf to know about the conditions in which it is going to emerge."

Lucas studies how plants pick up non-biological information, such as water and light, and how they transmit that information so the entire plant knows under which constraints it will grow. Plants can't move to a sunnier, wetter spot, so they need to make the most of their environment. 

Tapping into their "nervous system" would help scientists understand how they do that, Lucas said. That knowledge could lead to optimized food crops or hardier trees.

"There are no neurons in plants, but there is a communication network that we don't fully understand," he said. "There are important implications for these kinds of studies."

 by "environment clean generations"

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