   Fall Leaves - October 27, 1999 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Chlorophyll, the green pigment in plant leaves, captures light energy then transfers and stores it as chemical energy in sugars and starches: the process of photosynthesis. Although chlorophyll is the best known of the plant pigments, several other pigments are present in plants. Carotenoid pigments are present in most plant leaves throughout the growing season and appear orange and yellow. Like the name, carotenoids are the orange pigment in carrots and other yellow/orange vegetables. Anthocyanins are purple and red plant pigments and found in red cabbage, chard, and turnips. What about the colors in fall leaves? As fall approaches, we observe leaves changing color from green to yellow, orange, red, and even purples. Fall leaf colors are produced when weather interacts with physiological plant processes to cause color changes. Initially, chlorophyll production begins to slow. A decrease in green pigment allows the yellow pigments to become more visible. Both chlorophylls and carotenoids are held within membrane-bound structures called plastids. Anthocyanins are produced by different processes and are found in the cell sap (cytoplasm). During summer, phosphate reaches high levels in leaves. Phosphate is an important compound used to add energy during the intermediate steps of photosynthesis. In deciduous trees, cool weather and shorter daylight hours signal leaves to decrease sugar production and prepare for winter. At this stage, phosphate is transported out of the leaves and into the stems. When this occurs, sugar breakdown processes change chemically which leads to the production of anthocyanins creating red to purplish fall colors. Different plant species have varying ratios of chlorophyll to other pigments. They also have widely varied physiological processes and leaf chemistry. This is the reason for the wide variation in fall color between deciduous tree species and even individuals within the same species. Aspens have little or no anthocyanins while Rocky Mountain maples have enough to make them pink to red. Purple leaf plum trees have abundant anthocyanins throughout the growing season. Landscape trees that provide good fall color are Liquidambar (many named varieties provide yellows, oranges, reds, and purples) and Chinese pistache (red). What happens to the deciduous leaves after the fall color show? Again, complex biochemistry is taking place within the plant. The technical term for leaf drop is abscission. In deciduous trees, the attachment of the leaf to the stem is designed to fail at the proper time. This abscission zone is characterized by two or more layers of cells: some with poorly developed cells walls to make it purposely weak and others that form a protective layer that can be walled off and waterproofed after the leaf drops. A leaf is an expensive investment for any plant and to just drop it of without any consideration for the materials contained therein would be a great waste. Consequently, many complex molecules, are broken down into smaller units and transported from leaves into stems, down the trunk, and into the roots. This allows deciduous forest trees to survive in nutrient poor environments by salvaging nutrients before leaves are dropped. After the salvage job is complete, other processes take over. Auxin (growth hormone) levels decrease in the leaf, ethylene production increases, and enzymes are secreted that weaken the abscission zone to the point of separation. The leaf drops to the ground and the remaining nutrients are stored on-site for use the following year when soil microbes release them again. The University of Arizona Cooperative Extension has publications and information on deciduous trees and composting. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |