In the latest development in the ongoing debate about why some leaves turn bright red in the fall, a new study suggests that the color is a signal to insect pests to stay away. Harvard biologist Marco Archetti sought to prove the theory, first put forth in 2001 by the late evolutionary biologist William Hamilton, that the red pigments, or anthocyanins, serve as a plant’s chemical defense. Archetti studied aphids’ survival rates in wild apple trees, which turn more red, compared with farmed trees, which produce more green and yellow leaves. He found that aphids don’t show up as frequently on apple trees that turn red in the fall [ScienceNews]. He also reports in the study, published in Proceedings of the Royal Society B, that once spring came, young aphids who had fed on red trees in the fall were less likely to grow to maturity than aphids placed in the green or yellow trees.
Archetti chose aphids for the study because fall is their mating season: They leave their summer plants to find a good tree for mating and egg laying. Aphids can damage trees in two ways, especially when the new generation hatches in the spring. The insects steal the sap and also spread diseases with their piercing mouthparts that end up as entomological dirty needles. So trees would do well to dodge aphids [ScienceNews]. To test whether the red signals a threat to the insects, Archetti placed nesting aphids in both red- and green-leaved apple trees in the fall of 2007, and found that the next spring, 60 percent of those in green trees had survived, compared with 29 percent in red trees. The reason behind this disparity is unclear, but Archetti’s and other studies suggest that the red leaves either have toxic chemical defenses or hold fewer nutrients for young aphids [ScienceNow Daily News].
Not everyone buys Archetti’s answer to the long-debated question. Environmental scientist David Wilkinson believes that leaves turn red for a different reason: “I think the most likely explanation is that these [anthocyanins] are effectively sunscreens that allow the photosynthesis to continue as the machinery of photosynthesis is broken apart in the autumn” [BBC], he says, while plant geneticist Andrew Flavell thinks that a genetic link between leaf color and fruit taste may be the cause. Florida scientist David Lee said, “The nice thing about this article is that it collects and reports important new data on the phenomenon.” … But he still questions some of the interpretations. And says the paper hasn’t wooed him away from his contention that anthocyanins protect leaf chemistry from light damage at cool temperatures [ScienceNews].
Clearly, the debate is far from over, but researchers all agree that trees must have a really good reason for producing the red pigment. The pigments that produce yellow and orange leaves in the fall are present year-round, and help protect chlorophyll, the molecule at the heart of photosynthesis, from sunlight damage; when chlorophyll is broken down in the autumn those yellows and oranges become visible. In contrast, the red anthocyanins are produced only in the fall. It is a costly job of molecule building for the plant and an enigma to scientists, since the leaves will at that point soon be dropped entirely [BBC News].
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