The mystery of why leaves change colour in autumn
Although the chemistry of autumn colours is well understood, there is an ongoing and lively debate among scientists about why trees evolved their vivid autumn colours in the first place.
Driving north through Duchess County, New York, in mid-October, I'm (35) ________ by the resplendent golden yellows and fiery red leaves. The pops of colour break open like fireworks when the sunlight (36) ________ across the leaves. It's a harbinger that colder months lie ahead.
Yet, despite receiving so much attention, we still don't really know why trees (37) ________ this transformation in the first place. Leaves that turn yellow were actually always yellow – the colour comes from a set of biochemical pigments known as carotenoids. These are simply revealed once the green chlorophyll in the leaves breaks down and the nutrients are reabsorbed by the plants as they prepare to (38) ________ for the harsh winter weather.
One leading theory proposes that red leaf colour may have co-evolved alongside pests, acting as a (39) ________ signal to ward off insects.
It would be easy to assume that the pattern of red-leaf (40) ________ perhaps follows simple differences in temperature. But the idea is still contested. “There is no one photoprotection study that shows the answer unambiguously,” says Wilkinson. “Our understanding is instead based on multiple studies tending to suggest the same thing.”
The insect co-evolution theory, by comparison, suggests that trees have evolved to change their leaf colours as a warning to deciduous leaf-eating insects, such as aphids.
Aphids are destructive creatures, and an infestation could do significant harm to a tree, so the idea of it using colour to ward off these insects seems (41) ________. But several academic papers over the years have contested this theory.
A major crack in the argument is how aphids see and respond to colour. Aphids, along with other leaf-eating insects such as locusts and caterpillars, do not possess receptors in their eyes that enable them to perceive the red colour the way humans do. To them, it looks dull, grey, or even black.
Considering all this, Renner does not believe the co-evolution theory (42) ________. Gallinat, however, isn't so sure. “Some of those findings that support that hypothesis, I think, are still pretty reasonable.”