Plants that do not turn red in autumn are generally more resistant to the effects of bright light during this time, and therefore do not need to produce red pigments. Protection from bright light during autumn is important because damage to the photosynthetic components during this time will reduce a plant’s capacity to recover nutrients from leaves. In many plants, the disassembly of leaf components in fall results in susceptibility to damage from bright light, particularly as temperatures decline. Red pigments are produced during autumn to help shade leaf photosynthetic systems from bright sunlight. Red leaves contain similar amounts of carotenoids as yellow leaves, but in red leaves, the yellow carotenoids are masked by the intensely colored anthocyanins. At first, the presence of both the red pigments and chlorophyll results in a dark burgundy appearance, but the brightly colored anthocyanins become increasingly visible as chlorophyll levels continue to decline. Unlike the yellow carotenoids, anthocyanins are made by plants midway through the autumn disassembly process, when a significant amount of chlorophyll is still present within the leaves. Without carotenoids, leaf photosynthetic components literally destroy themselves.Īnthocyanins are responsible for many of the reds, oranges, pinks, purples and blues in flowers and fruits, but also produce the brilliant red and orange hues of autumn foliage.
These pigments protect leaves from a variety of harmful compounds that are normal by-products of photosynthesis. Carotenoids are present in leaves throughout the growing season, and are visible only when chlorophyll levels become very low in autumn. There are two types of pigments that give leaves their bright autumn colors: carotenoids and anthocyanins.Ĭarotenoids are a class of pigments responsible for the bright yellow colors of fall leaves. Throughout autumn, plants are actively breaking down chlorophyll along with the many other photosynthetic components, and as levels of chlorophyll decline, the brightly colored pigments we associate with autumn leaves become visible. It is only during the latter stages of this process that the showy colors of autumn appear.Ĭhlorophyll is the pigment that gives plants their green color and is used by plants to capture the sun’s energy. However, much of this process takes place before any visible color change in leaves, and thus this surge of activity goes largely unnoticed. The disassembly of leaf components in autumn is highly organized and complex. The shortening days and cooler temperatures of autumn signal plants to begin a process by which a leaf’s photosynthetic components are disassembled and the nutrients contained within them are moved to stems and roots for storage and re-use the following year. Thus, it is important for plants to recover as many of these nutrients as possible before the leaves are discarded in autumn.
These nutrients are valuable resources in nature, and the amount of nutrients a plant possesses directly affects the plant’s capacity to grow and reproduce. These nutrients, in particular nitrogen and phosphorus, are components of proteins, pigments and other compounds necessary for photosynthesis, the process by which plants use the sun’s energy to manufacture sugars from water and carbon dioxide.
Recent research has done much to explain the functions of this phenomenon.ĭuring summer, the majority of a plant’s nutrients are contained within the leaves. Although a familiar annual event, much of our understanding of autumn leaf coloration has, until recently, been based primarily on random observation and speculation. The bright yellow, orange and red colors of autumn leaves are one of nature’s most beautiful spectacles. William Hoch, formerly of UW-Madison, Horticulture Dept. Carotenoids and anthocyanins give leaves their bright fall colors.
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