The Autumn season is upon us, and many trees in Manhattan are showing signs of changing color. Have you ever wondered how the trees know what season it is? Many trees and plants are photoperiodic, meaning they can detect the hours of darkness in a 24 hour period. In this way, trees and plants can detect the lengthening of nights into winter or shortening of nights heading into summer. This clever ability is achieved through pigments within the leaves called phytochromes, which can trigger a cascade of specific hormones and growth factors which regulate growth, flowering, and changes in leaf color during autumn. Leaves produce chlorophyll throughout most of the year, a green pigment critical for photosynthesis which allows trees and plants to absorb energy from light. Chlorophyll masks other pigments present within the leaf such as carotenes and xanthophyll, which are responsible for orange and yellow coloring, respectively. As the length of night increases during autumn, it triggers a cork-like membrane to form around the base of the leaf stalk called an abscission. The abscission slowly cuts off the supply of nutrients to the leaf, thus limiting the production of chlorophyll and allowing the orange and yellow colors to be visible. Anthocyanin is also produced in autumn, which gives leaves a red and purple coloring. Eventually nutrients to the leaves are completely halted causing the leaves to fall off. After accumulating a certain amount of time in the cold during winter months, which is referred to as the number of chill hours, trees can then respond to the increasingly warmer temperatures and shorter nights of Spring. During this time there is an upregulation of genes responsible for producing antioxidants and vitamin C to rid the tree of hydrogen peroxide which has built up during the winter dormancy. Trees are now able to produce the hormones and growth factors necessary to begin flowering and making new leaves again. Happy Autumn Everyone!

To see my other post on Autumn in New York, please click here.