Slowly but surely, many leaves have started to show off their fall colors. The pigments in the leaves that are responsible for the beautiful warm colors are anthocyanins, carotenoids or xanthophyll. As the primary pigment, chlorophyll fades and the green color diminishes, other pretty fall colors start coming through. There is ample information and research on plant pigments responsible for fall colors. But there is little information on the anatomical and physiological changes and what actually happens inside the plants as the leaves start falling form the tree.
Ethylene (plant hormone responsible for ripening and senescence), Abscisic acid -ABA (stress hormone), and Auxin (growth hormone) all play a symphony and cause 'Abscission- falling/separation of leaf tissue' from the plant. What is commonly seen as leaves falling from the tree, is really abscission of a leaf tissue at the base that makes the leaf drop. When Abscission happens, an abscission zone is developed at the base of the petiole (stem of leaf). But why and how is this abscission layer formed?
Here is a simple way to understand it: when plants are stressed, they go from full growth stage into a survival mode. When the plants undergo stress, the stress hormone (ABA) levels rise and the growth hormone (auxin) level that promote cell elongation, reduces. When cell elongation reduces, meaning the plant gets a signal that growth has reduced/stopped in certain parts, the plant gets a signal for senescence (death of tissue) and an abscission layer starts forming. That's when Ethylene levels increase to fasten senescence.
For a long time, abscission was connected with Abscisic acid-ABA, which was thought to be solely responsible for abscission (breaking of leaf from petiole), but research shows that Ethylene is actually responsible for completing senescence. Ethylene acting as a hormone in plants, exists as a gas at trace levels and causes not only abscission, but also fruit ripening and opening of flowers.
"Ethylene has been used in practice since the ancient Egyptians, who would gas figs in order to stimulate ripening. The ancient Chinese would burn incense in closed rooms to enhance the ripening of pears". (https://pages.wustl.edu/ipgsa/ethylene). “Commercial ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Farmers in Florida would commonly get their crops to ripen in sheds by lighting kerosene lamps, which was originally thought to induce ripening from the heat”. (https://en.m.wikipedia.org/wiki/Ethylene).
Fall colors of leaves are a treat to the senses. It is also a reminder for a seasonal change. What is happening inside the plant is also a change and preparation for a stressful cold period. Plants dropping all their leaves, is a survival mode for plants wherein it is conserving its energy and sending it to the roots and branches instead of the leaves.
It is important to note that senescence is also a natural phase in the life cycle of a plant. With fall colors currently at the brim, Ethylene at trace levels is surely going to be very active in millions of leaves instead of ABA which was originally thought to be leading senescence.
Enjoy the vibrant warm colors of leaves before abscission starts and leaves actually drop down. Here is a good link for fall color prediction map for this year: https://smokymountains.com/fall-foliage-map/