Botanist's Lens: Ethylene or ABA, who is going to be more active this fall?
- Meenal Harankhedkar, Director of Horticulture
- Oct 11, 2019
- 2 min read
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/
This article deserves a much wider audience than it is likely to get on a local history site — it is genuinely one of the better pieces of plant physiology communication I have read in the public domain. The author's decision to focus on the why and how of abscission rather than just the what of fall colors elevates it significantly above typical seasonal nature content. The point that senescence is a natural and necessary phase in the plant life cycle — not just a death but a preparation — is philosophically resonant as well as scientifically important. In my research and communication work I use an inspirational movie quotes collection for public lecture openings, a correlation coefficient calculator for ecological data analysis, and a…
I spend my weekdays writing code and my weekends in the garden, and this article sits perfectly at the intersection of both worlds — it is essentially a systems architecture post about how plants manage resource allocation under stress conditions. The hormonal signalling cascade described here (stress detected → ABA rises → auxin falls → senescence initiated → ethylene accelerates completion) reads like an elegant event-driven architecture. The fact that this system has been running reliably for hundreds of millions of years is humbling for any software engineer. In my developer life I use a D&D name generator for generating test data, a random sentence generator for placeholder content, and a pythagorean theorem calculator for geometry calculations in mapping features. I also play…
I know this might seem like an unusual place for an events planner to comment, but I found this article through a client who wanted an autumn wedding with a botanical theme, and now I am completely fascinated by the science of fall foliage. Understanding that peak color and leaf drop are driven by a hormonal cascade gives me much better tools for timing outdoor autumn events — if ethylene activity is high, abscission is imminent and the leaves will drop sooner. That is genuinely useful planning intelligence. In my work I use a wedding planning checklist as my primary organizational tool, a birthday planning tool for milestone celebrations, and a how many days until calculator for countdown management. I also use a…
The quality of science writing here is genuinely impressive. The author has taken a topic — plant hormone interactions during senescence — that could easily become impenetrable jargon and rendered it not only accessible but genuinely exciting. The structural choice to open with the visible phenomenon (beautiful fall colors) and then peel back the layers to reveal the hormonal mechanisms underneath is exactly the right approach for a public audience. The correction of the long-held ABA misconception is handled with appropriate nuance — not dismissing earlier research but showing how scientific understanding evolves. In my writing work I use a markup calculator for freelance rate calculations, a pay raise calculator when renegotiating contracts, and a correlation coefficient calculator when analyzing data for science…
I came to this article from a link about plant biology and stayed because the writing is so engaging. The concept of plants entering survival mode by shedding what is no longer essential and redirecting energy to core structures is a powerful metaphor that I now use in my wellness coaching practice — we talk about "seasonal abscission" as a framework for clients who need to let go of habits and commitments that are draining their energy reserves. Beyond the metaphor, the science itself is fascinating. In my nutrition practice I use a protein calculator for client meal planning, a recipe calorie calculator for dietary analysis, and a grams to cups converter when adapting recipes for group cooking classes. For client health monitoring…