May 21, 2000
Each year millions of tourists travel to the woods of New England, upstate New York, Wisconsin, and Canada to witness the appearance of brilliant autumn colors. Sugar maples, red oaks, sumac, birch and other trees and shrubs turn from green to bright red, orange, and yellow. The short, cool days of autumn bring an end to the production of chlorophyll (the green light-gathering pigment). As chlorophyll gradually breaks down, the colors of the more stable carotenoid (yellow/orange) and anthocyanin (red/blue/purple) pigments become visible. Some years the leaves of a particular species are bright yellow, while other years they may appear redder or even purple. Although no one can accurately predict the timing or coloration of each year's show, we do know that there is a chemical basis for it. Anthocyanins (Figure I) are responsible not only for the red and purplish colors of autumn, but also for similar colors in various summer leaves (red cabbage, red lettuce, red plum tree), flowers (roses, hydrangeas, geraniums, bachelor buttons, dark pansies), fruit (cherries, red apples, grapes, tomato, blackberry, blueberry, plum), roots (beets, radishes), bulbs (red onions), and petioles (rhubarb). Anthocyanins are water-soluble and are dissolved in the cell sap rather than bound to the membranes as chlorophyll is. If cell sap in a leaf is very acidic, then anthocyanins make the leaves a bright red color, but if it is less acidic the color may appear purple. The color of anthocyanin depends on acidity, and thus it may serve as a pH (acid/base) indicator (Figure II). pH is a quantitative measure of the acidity or basicity of solutions. Numerically, the pH of a solution is defined as the negative logarithm (base 10) of the hydronium ion (H3O+) concentration. If the pH is less than 7 the solution is acidic, if it is greater than 7 the solution is basic, and if it is 7 the solution is neutral. The...