There are five classic types of hormones—auxins, cytokinins, gibberellins, ethylene and abscisic acid—and new classes are still being discovered and synthesized. All hormones work together as a signaling network to regulate a plant’s defense responses. This is the final installment in a three-part series on plant hormones, this time focusing on ethylene.
Ethylene has an interesting past. In Ancient Greece, the Oracle at Delphi was a woman who would communicate with the gods and foretell prophecies. New evidence shows that the location of the sanctuary was built on a fault that consistently leaked ethylene gas from the innards of Gaia.
As an anesthetic and an asphyxiant, the gas could have contributed to her trance-like euphoric states of eternal wisdom. Elsewhere, Ancient Egyptians would gash unripe figs to ripen them for eating, which is evidence that they were at least conscious of this scientific principle, and in Germany in the 1800s, observers noticed that plants grown next to leaking gas street lamps would age quicker than normal. Since this was not desirable, some great minds set out to discover the chemical responsible: ethylene.
Ethylene is a simple gaseous phytohormone that can both inhibit and stimulate a plant’s growth, depending on how the plant responds to environmental stimuli. This is executed by affecting the rate of photosynthesis and other metabolic processes.
A plant is capable of synthesizing ethylene in its roots, shoots, flowers, fruits, leaves and seeds—pretty much everywhere. Plants synthesize ethylene from methionine, one of 20 essential amino acids, with the help of some crucial enzymes and coenzymes. In the presence of ethylene, plants shed leaves, fruits and flowers in a process called abscission. Ageing occurs with the changing of the seasons, called senescence, which is hastened by ethylene.
Ethylene plays many roles in plants. In low light situations, ethylene contributes to stretching to get the plant to reach out for more lumens. In low water situations, ethylene prompts the plant to stretch out taproots to search for more water. In the presence of too much water, ethylene promotes stretching so a plant can be taller than the waterline of the flood.
This stretching also occurs in crowded, competitive gardens. Seed germination and root hair formation is also enhanced by ethylene. There are many factors that enhance a plant’s ability to produce ethylene, including:
- Iron in the fertilizer or soil
- Plant tissue wounds or gashes
- Supplying auxins, particularly IAA
- Extreme cold
- Lack of watering (dry soil)
- Air pollution
Ethylene can be used both naturally and commercially to ripen fruits and flowers. As listed above, production of ethylene in plants is triggered by the presence of iron in soil. Growers want may wish to add an iron source to the soil at the onset of autumn.
In the case of indoor gardening, soluble iron can be added to the nutrient regimen in the middle week of the flowering cycle to initiate ethylene production. Vertical growth will slow and the plant’s focus with switch from vegetative growth to final fruit and flower formation. This is an important step of commercial flower forcing.
Operators of commercial indoor gardens and greenhouses can supply industrial ethylene gas to force fruit and flower ripening. The gas is inexpensive and easy to find. Fruits readily ripen at ethylene concentrations of 1,000-2,000 ppm, but crops treated with ethylene can have a slightly shorter shelf life, depending on the concentration.
The effect of ethylene on ripening can best be seen in tomatoes. Commercial tomatoes are picked green because they are easier to transport and manhandle. Red, ripe tomatoes will be smashed somewhere on the way from California to Michigan. When the green, unripe tomatoes arrive at their destination, they are exposed to ethylene gas and the tomatoes magically turn bright red.
While the fruit might not taste as good as a sun-ripened tomato right off the vine, using ethylene for ripening is a money saver across the board and enables tomatoes to be on dinner plates anywhere in the world in any given season. Ethylene also loosens ripe fruits from the vine, so they become easier to pick.
Finally, ethylene has been shown to thicken the cambium layer of stalks and stems, a process called radial swelling. This increases the volume of water, sugar and nutrients transported throughout the plant.
In essence, this hormone upgrades the plant’s plumbing system. It should be noted that this is different than the effect that silica has on stalks and stems. Silica strengthens the epidermal cell walls, while ethylene and particular enzymes will actually increase the size of the cambium layers. The late stages of flowering is the perfect time for this extra volume of fluids because so much of it is required at this time.