Puzzling Pigmentation

Here’s a question: why do the leaves of red maple (Acer rubrum L.) turn red in the fall?

Fall is in the air, and our red maples are determined to make a show of it this year! Though they may not be quite as famous for their fall color as their cousins the sugar maples, the reds are busy, busy living up to their name these days.

Stories about how this plant got its name vary, but they all come down to the fact that almost year-round it’s got something red to show for itself. Scarlet maple flowers are an early sign of spring. Minuscule and too high in the branches to get a close look, yes. But en masse they’re a colorful clue that spring is indeed on the way. The flowers are followed shortly by young leaves and samaras, also – you guessed it – cheery red in hue. Even in summertime the observant naturalist will notice this tree’s rosy leaf petioles. And in winter, the maroon-tipped branches offer a bit of color to that season’s muted palette.

Right now, as the trees are getting ready to shed their leaves, the leaves once again flush with this most lusty color. Red is not caused by one of the photosynthetic pigments, all of which do their work inside the leaf’s chloroplasts. Rather, it’s an anthocyanin, a pigment that is dissolved in the cells’ cytoplasm.

Red is a protective coloration – young leaves drooping at the ends of twigs to avoid the direct rays of the sun produce anthocyanins to shield themselves from UV radiation and cold temperatures while their photosynthetic pigments, including the chlorophylls, are being manufactured.

But what is the role of red leaves in the fall? In some cases it is thought that they function in “foliar fruit flagging,” bringing animals’ attention to ripe berries that may not be brightly colored. Blackgum, for instance, has dark – almost black — berries among its wine red leaves.  But that’s not the case for maples.

So why do red maples expend the energy to create this pigment so late in the leaf’s life?  What is this about? A sanguine assurance that spring will come again?  It’s a veritable Crimson Conundrum, Batman!


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