In autumn, our days shorten noticeably and frosty dawns become the norm across most of Utah. Now leafy plants must be preparing for winter. Their summer of intense metabolic activities must gradually give way to winter’s dormancy. Photosynthesis and respiration are gradually shut down as nutrients and sugars are withdrawn from leaves, to be shunted to the stem and roots for storage.
The brilliant autumn yellows of our aspens, ash trees and cottonwoods, as well as the crimsons of our maples and sumacs, are all indicative of leafy plants frugality with their valuable nutrient stores. The foliar pigment phytochrome first registers the lengthening nights, initiating the cascade of physiological events that prepare a tree for the icy blasts of winter. Before discarding their leaves, deciduous trees and shrubs rescue and store what they can of sugars and nutrients found in their leaves.
The key photosynthetic green pigment, chlorophyll, and its attendant enzymes are all broken down, their components moved to storage for recycling next spring. Essential nutrients, such as nitrogen and phosphorus, are likewise extracted from foliage for later reuse. With chlorophyll gone, the other colorful leaf pigments are revealed in all their glory. These accessory pigments have been there all along, they just have been masked by the dominant green of chlorophyll.
These accessory pigments serve several functional purposes for the leaf. Some pigments protect the leaf from sunburn, some scavenge free radicals, but most capture energy from wavelengths of light missed by chlorophyll. The multi-hued spectrum of sunlight, as revealed by a prism or a rainbow, not only allows us to see splashy fall foliage colors, it is the reason for their existence.
For the plant physiologist and chemist, then, the palette of colorful leaf pigments have complex functional explanations. More mysterious psychological stirrings accompany the aching beauty of our autumn foliage, but it gives an undeniable tug at my heart. Standing before a blazing yellow stand of aspens, I smile to think that recycling can be so beautiful.
Photo: Courtesy www.bridgerlandaudubon.org
Text: Bridgerland Audubon Society – Jim Cane, Linda Kervin