Evolutionary Mysteries 2: Why Some Young Leaves Are Red

This week’s evolution video is about why some young leaves are red. You may have noticed that the unfolding leaves on plants as diverse as redbuds and blackjack oaks are reddish—in some cases, as with the blackjack, a brilliant scarlet. This is not true of all plants, but true of enough of them to invite an evolutionary answer.

The leading contender for an explanation is that young leaves are pretending to be dead. A young leaf has its whole life before it. Its entire contribution to the growth of the plant can be threatened by a herbivore eating it. When a herbivore eats an older leaf, however, the plant has less to lose in terms of future growth. The reddish color of a young leaf is a camouflage that makes a healthy young leaf look dead, thus diverting herbivores to the older leaves. This becomes more believable when you consider that many insects would not distinguish between red and brown, the color of plant death. The red color is caused by anthocyanins. They will reduce the amount of sunlight that gets to the chloroplasts, but by the time the leaf is fully grown and ready to produce food, the anthocyanins fade.

While this explanation is the most likely, it is not the only possibility. It could be that the young leaf is vulnerable to harmful ultraviolet radiation, which can cause damage but is outside the range of photosynthetically useful light. The red color, therefore, might be protection from ultraviolet damage.

And there is no reason why both answers could not be true at the same time.

To prove which of these explanations, or any other, might be true would be difficult. It might require breeding plants that produce more or less anthocyanin: young leaves without it, older leaves with it. Actually, some horticultural variants (such as red-leaved redbud trees) have adult leaves that are red (and green at the same time, resulting in a purplish color). Do these reddish adult leaves have less herbivore damage than green leaves? I do not know, but it is possible. Another way would be to compare herbivore damage in red vs. green patches on the same leaf; one study demonstrated that herbivores avoided the red areas ofvariegated leaves.

Keep your eyes open and there are mysteries of nature all around you that cry out for an evolutionary explanation.