Autumn colors are caused by senescence.
Senescence is the life stage of a plant or plant part when its metabolism slows prior to dying.
Our lovely fall colors are caused by a deciduous tree’s inability to maintain chlorophyll levels within its leaves. Chlorophyll, being green and abundant most of the time, masks the other colors that are alway present within a leaf. Shorter days and cooler temperatures trigger the tree to form a layer of cork at the base of each leaf, blocking the flow of water and nutrients and interfering with the leaf’s ability to produce chlorophyll. Eventually, the other colors can shine through. The veins of a leaf are the last part to turn color because it is the last place nutrients were available.
Autumn leaf color and the final days of a flower’s life are examples of developmental senescence. Developmental senescence occurs at the cellular level in all life stages. The seed leaves of a bean plant experience senescence when they wither and fall off. The mature leaves of the same plant will also exhibit senescence when they die. Ultimately, the metabolism of the entire plant will slow to the point of death. In each case, developmental senescence is triggered from within the plant.
Sometimes senescence is not developmental. It can also be induced for laboratory research, or as a result of injury or stress. Water stress can trigger senescence. A large plant with many leaves may be unable to maintain its canopy in a drought. Rather than risk death, it absorbs water and nutrients from outer leaves and then seals them off from the food supply, allowing them to fall away. Sunburn can also cause senescence. As an under-hydrated leaf receives direct sunlight, the cells most damaged slow their metabolism and relinquish their water and nutrients to the surrounding cells.
As seasons change and plants age, you will see many examples of senescence, but the colors of autumn are my favorite!
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