As fall colors fade away, the perennial question arises: Why the brilliantly colored leaves? The old explanation, that the colors were always there, hidden by the green, has been discarded. Though possibly true for the yellows, plants actively produce more anthocyanins, the substances responsible for reds and oranges, in autumn. Two hypotheses now are battling it out. One holds that bright colors warn such insects as aphids that laying overwintering eggs on such trees is apt to be futile—that the tree's well protected against insects. The other hypothesis notes that leaves are a hotbed of activity in autumn as the photosynthetic apparatus is dismantled and its constituents moved out for storage. This view indicates that at this time, the leaves are susceptible to damage from sunlight, interrupting the salvage work. The red and orange pigments, but not the yellows, shade the machinery, preventing serious damage. There is experimental evidence for both ideas.
On the local scene, the brilliant yellows of our montane aspens might fit the insect protection theme, while red maple leaves may protect from the fall sun.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
