Why do leaves change color in the Fall?

October 7, 2009 Sarada Krishnan , Director of Horticulture & Center for Global Initiatives

Malus 'Beverly' Have you ever wondered what really happens within plants to help them carry on through the winter? As autumn approaches, the sun begins to set closer to the horizon, leading to cooler nights and shorter day lengths. This sends a warning signal to plants that winter is soon approaching and that they need to get ready to go into dormancy, which is similar to hibernation in animals. During dormancy, the plants stop growing, fluids within cells stop flowing and the tender growing tips are enclosed in a tight, frost resistant bud. Before entering this dormancy, the leaves of the plants turn from green to yellow, orange, or red before dropping to the ground. Quercus texana So, what causes the change in leaf color? In leaves, both the green chlorophyll and the yellow-orange carotenoid pigments occur within the chloroplasts. Since there are more chlorophyll pigments than the carotenoid pigments, the leaves appear green. In the fall, the chlorophyll pigments decompose allowing the caretenoids to express themselves as orange and yellow. In some plants, the leaf cells produce red pigments, the anthocyanins. In these leaves, once the chlorophyll has decomposed, the anthocyanins mask the caretenoids, thus turning the leaves red. The change in color and dropping of leaves are the plants' way of avoiding freezing damage to themselves. During freezing, the water within the cells of leaves turn to ice, causing disruption of the tissues. These damaged tissues become inviting sites for fungi and bacteria to invade the plant. In order to prevent microbial invasion, plants drop their leaves and form a protective seal over the areas where the leaves were once attached before the freeze damage can occur. Even though plants look lifeless in the winter, be assured that they are ever sensitive of their surroundings, keeping tabs on the temperature and day length before getting ready to spring back to life!


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