Young red leaves

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One of the first phenomena I noticed after moving to the tropics was the appearance in the rainy season of red-colored young leaves. As the leaves aged, the red disappeared and the leaves became green. Here, for instance, is a young avocado mango plant putting out new leaves:

Why? I wondered.

My first thoughts turned to seaweeds. Why seaweeds and not autumn leaves? Well, mostly because of my ignorance. I have no idea why some autumn leaves are red, some yellow, and some brown. But I do know why – or at least I’ve read why – seaweeds may be red or green or brown. So I decided to start from some knowledge rather than from complete ignorance.

The color of seaweeds is related to the intensity of light they are able to use or withstand. Green seaweeds are green from chlorophyll, require good levels of light, and therefore live high in the intertidal zone. The pigments in red seaweeds can use light from a specific part of the spectrum to enhance photosynthesis, thus enabling the reds to survive in deeper and darker waters than the green seaweeds. A different pigment improves photosynthesis in the brown seaweeds deep in the intertidal zone. So you can make a pretty good guess of where a seaweed lives if you simply know its color: green in shallow water, red a little deeper, and brown (such as kelp) in the deepest parts of the intertidal zone.

It seemed logical to me that the young red leaves in tropical plants, usually emerging in the rainy season, might be needing some help with photosynthesis on those overcast days. I thought they might be using the same pigment that red seaweeds do to tap additional light for use in photosynthesis.

I was wrong.

The red pigments in red seaweeds are phycobilin pigments (phycoerythrin and phycocyanin, each absorbing different colors of the spectrum).

The red pigments in young leaves in tropical plants are anthocyanins – the pigments that give cranberries their color.

Anthocyanins may be found in fruits, flowers, leaves, stems, and roots. Anthocyanins may act as a “sunscreen,” by protecting cells from UV or blue-green light. If this is the reason young leaves have red pigments, then it contrasts with the reason seaweeds have red pigments. Anthocyanins would protect young leaves from too much light whereas the phycobilins enable red seaweeds to survive at lower light intensities than they would otherwise.

The Wikipedia article mentions two other possible roles for anthocyanins in plants: 1) the red color may hide leaves from red-color-blind herbivores, which would be especially important in young leaves, and 2) the red color may signal unpalatability to herbivores, since phenols are often produced along with the anthocyanins.

I found a scientific paper that tested three hypotheses regarding the role of anthocyanins in young leaves:

1) the anthocyanins may deter or kill fungus (an idea presented in a 1989 paper)

2) the pigments may protect against a lessening of photosynthesis due to UV (presented in a 1995 paper)

3) protection from red-color-blind herbivores (presented in 1979 and 1993)

Other hypotheses were considered, but these three seemed the most plausible to the authors. They concluded that none of the hypotheses are completely convincing but that the third one – crypsis, they called it – offered the most promise for further work.

It’s certainly true that herbivores are brutal on young, tender leaves. It’s also been said that “herbivory rates are higher in tropical forests than in temperate ones and that, in contrast to leaves in temperate forests, most of the damage to tropical leaves occurs when they are young and expanding.”

I’d like to think that the red leaves I saw earlier in the rainy season are now green, hale, hearty, and tough!

Oh, and those autumn leaves of temperate climates? Well, I learned that carotenoid pigments turn leaves yellow, orange, and brown. Anthocyanins turn leaves red. Carotenoids are present in leaves all the time but are masked by chlorophyll. Anthocyanins develop in the fall.

So the tree I saw during my recent visit to Missouri was painted with the same pigment as the young leaves here in Panama. Pretty neat.

Update: 17 August 2007. A new study, sited at El Gentraso, has suggested that, since insect herbivores are red-colorblind, they would not be able to tell the difference between red and green leaves. Yellow leaves, by contrast, would potentially attract herbivores.

Aphids (and all other herbivorous insects looked at so far) don’t have good vision in the red — so red leaves wouldn’t look anything special to them. And yellow is powerfully attractive — it tickles their green receptors even more so than green itself, and brings insects flocking: ‘If trees wanted to deter herbivorous insects using color, yellow leaf coloration is about the worst strategy they could pick,’ they write.