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shedding their leaves – or, as they call it, abscission – was abscisic acid. Despite its name, it turns out this substance is not that chief in abscission. Two different plant hormones, ethylene and auxins, play a much more significant role.
Auxins- a group of plant hormones that play a big part in trees’ autumnal extravaganza
Abscisic acid – despite its name – plays only a small role in abscission or the loss of leaves
Ethylene, the simplest alkene, is a gas that quickens fruit ripening, but reduces leaf growth and encourages abscission. Ethylene is produced from the amino acid methionine via a long chain of biochemical reactions. As darkness excites the biosynthesis of this gas, plants increase their ethylene production in autumn and winter, initiating their leaves to shred. Auxins are a family of indole derivatives that control cell growth. A healthy, green leaf harvests ample number of auxins and so keeps growing. It also carries auxins to the rest of the plant through its petiole – the stalk that links the leaf to the tree. However, when autumn comes and chlorophyll levels decrease, so do auxins. The tree no longer gets chemical signals from the leaves that they are green and healthy, and so the abscission commences.