I did a search and didn’t find anything previously, so here goes.
Why do plum trees have purple leaves? What is the purpose? Are there other plants with unusual colored leaves? Again what is the purpose?
Why do plum trees have purple leaves? BECAUSE THEY’RE PLUM TREES!
But seriously, ask me why leaves change color in the fall, and the answer would be long, but quite simple. All I can tell you is that a group of pigments called anthocyanins causes the purple color found in many types of flowers, fruit skin, and even some leaves. In a plum tree, there is either a lack of chlorophyll (The pigment that gives most leaves their green color), or there is an over abundance of anthocyanins. I’d bet it’s the latter. The “purpose” of this purple color is another matter. The vast majority of things in nature do in fact have a specific purpose. Flowers display bright colors (some colors are even invisible to the naked eye), to attract insects for polination. Fruits are usually colored to attract animals to eat them.
I’m sorry to say that I don’t really know exactly what purpose the purple leaves serve. Most likely some kind of attractant for some kind of animal or insect. Could also simply be a bi-product of something happening inside the leaf itself. Sorry I couldn’t be of more help.
Ozone, is that the same pigment that causes plums (the fruit) to be purple? Maybe the tree just produces too much pigment, or can’t control where it goes very well, so the leaves get some extra purple, too. Of course, this begs the question of why the fruits are purple, too…
Interesting…
My parents have a plum tree. It has green leaves.
Assuming their tree is just a weird varient, I’d say it has something to do with the same pigmentation as the fruit.
-niggle
Yes, the anthocyanins would cause the purple color in both the leaves and the skin of the fruit, but it’s doubtful that it’s from the tree not knowing what to do with extra pigment.
There are very few instances of things existing in nature that aren’t needed, but there are a few. Usually these are things that were needed at one time, but aren’t anymore. A good example in the human body is the appendix. No longer needed, but still there. You might also class wisdom teeth into this category.
This said, it’s possible that the plum tree with the purple leaves, evolved to have purple leaves at one time, when it needed them, and even though it doesn’t need them anymore, they stay.
Of course I’m just pitching out ideas here. There may have been studies done on the purpose of trees with purple leaves, but I’m not aware of them. I spoke with a plant specialist that I work with. He’s a micro-biologist, specializing in plants, and knows just about everything there is to know about plants. He had no real answer as to why the leaves are purple. Just one of those great mysteries of life I guess… 
I know that plants have many photosynthetic pigments. The standard one that reacts with the most common wavelengths of light happens to be green. But there are others, they are the ones you see in the fall after the green chlorophyll has been removed from the leaves. These collect photons from other parts of the spectrum. The sun doesn’t put out as much energy at these wavelengths, so there isn’t as many of these pigments, so most plants are green.
But some plants have different levels of the pigments, so they might appear different colors. And of course, there must be non-photosynthetic pigments as well. I imagine that purple plum trees aren’t as effective at photosynthesis as green plum trees, it’s just that arborists think they’re cool and keep planting them.
This is what I remember from college botany some years ago, so take it with a grain of salt.
Interesting. So are purple plum trees a recent development? If we allowed them to grow wild would they be replaced by green ones via natural selection? Or is there some unknown purpose. Also, why is it only plums. You’d think we could get nectarines with yellowish red leaves, oranges with orange leaves, and so on.
Actually, chlorophyll has its lowest efficiency at the peak emission of the Sun. The Sun’s peak emission is in the green range of the spectrum, and that’s the part that chlorophyll reflects most efficiently-- if it’s reflecting it for us to see, it isn’t absorbing it. Don’t ask me why this is so.
It’s not only plums. I’m a hardcore gardener–even used to take money from people to design their gardeners–and one of my “specialties” is non-green foliage. There’s almost as wide a range of foliage colors as there is flower colors. The perceived distinction, I think, is because foliage is USUALLY green, and flowers are USUALLY not green. I have plants with green flowers, and plants with red flowers and red leaves, etc. There’s no reason to expect parallels between the two: orange leaves on an orange tree are no more likely than any other combination.
FWIW, I’ve never seen a truly blue leaf, though there are leaves that definitely have a bluish cast to the them. But that’s about the only gap I can think of in either floral or foliage colors.
It’s not only plums. I’m a hardcore gardener–even used to take money from people to design their gardeners–and one of my “specialties” is non-green foliage. There’s almost as wide a range of foliage colors as there is flower colors. The perceived distinction, I think, is because foliage is USUALLY green, and flowers are USUALLY not green. I have plants with green flowers, and plants with red flowers and red leaves, etc. There’s no reason to expect parallels between the two: orange leaves on an orange tree are no more likely than any other combination.
FWIW, I’ve never seen a truly blue leaf, though there are leaves that definitely have a bluish cast to the them. But that’s about the only gap I can think of in either floral or foliage colors.
Sorry for the double post; SDMB’s being weirdish today.
Anyway, also in response to oldscratch, non-green foliage also exists in nature. I believe there’s one species of plum that is naturally purple, so wild plantings wouldn’t necessarily “revert” to green.
There are, however, a great number of commercial hybrids whose foliage color has been manipulated by hybridizers. These, theoretically, would revert; some new hybrids are not very successful because an individual plant may revert after a few seasons. (Ajuga “Burgundy Glow” loses its magenta tones in a couple years, and reverts to green and white, which in itself would probably revert in nature.)
In any case, I’m not sure of the purpose, but I can tell you from practice that red-leaved plants are more likely(with a few exceptions) to require hot sunlight to do well; you rarely seen red in the leaf of a shade-tolerant plant (Heucheras excepted). So I’m sure it has something to do with light absorbtion. Also, it’s not unusual for an otherwise green plant to have red or purple undersides on its leaves (this is especially common in tropical plants), which may be another clue.
But wait. I thought the sun put out most of it’s energy in yellow, that’s why sunlight is yellowish. And we think the color “yellow” is bright because our eyes are the most sensitive to yellow wavelenghts, because that’s one of the most common ones. I am not a physicist.