Plants exposed to high concentration (> 0.05 ppm) ethylene exhibit a range of familiar symptoms, such as leaf yellowing, epinasty or abscission, and flower senescence (see Figure 1) and abscission. Usually, low concentrations (< 0.05 ppm) of ethylene precede high concentrations. Imagine a slightly malfunctioning heater becoming progressively worse.
So, plant symptoms of low concentration ethylene can serve as an early warning sign for bigger trouble to come. Figure 1 shows the effects of short- and long-term exposure to low concentrations of ethylene on petunia.
For more information on plant responses to low concentration chronic ethylene, visit GreenhouseGrower.com to access last month’s article titled, “Ethylene In The Greenhouse.”
If you suspect ethylene is present, the first thing to do is verify there is indeed a problem. One of the easiest ways to detect ethylene is by using indicator plants. These are plants so sensitive that they respond dramatically, even at concentrations as low as 0.01 ppm.
Cuphea and tomato both make excellent indicator plants. Cuphea will abscise all of its flowers (Figure 2), and tomato will bend its leaves downward as if they’re wilted–but they will remain turgid (epinasty) when ethylene is present even in very low concentrations. Such responses usually show up within 24 hours of exposure, although lower greenhouse temperatures can slow the response.
Use indicator plants that have not previously been exposed to ethylene and place them throughout the greenhouse, including any areas you suspect ethylene is originating.
A second and more reliable method to detect ethylene is by sending an air sample to a commercial lab or a university lab such as the one at North Carolina State University’s Plant Disease and Insect Clinic. Contact the lab directly to find out about the details of the program, obtain the necessary instructions and materials for collecting an air sample before sending one in. When taking samples to send for testing, choose from areas you suspect ethylene may be coming. These could be inside, near a unit heater or outside the greenhouse near a loading dock.
Also, check out sample areas where there’s no ethylene problem, as a basis for comparison. Commercial kits for detecting ethylene are also available, but are not sensitive enough to detect low concentrations.
Finding The Source
If you find ethylene is present in your greenhouse, the next step is to find the source. Ethylene can come from several sources, such as ripening fruit, decomposing plant material, plant growth regulators that release ethylene and as a byproduct of incomplete combustion.
One class of plant growth regulators (including Florel and Pistill) works by producing low concentrations of ethylene when the active ingredient breaks down. If applied near or on sensitive species, the same brief-exposure, low-concentration ethylene responses will be seen. Florel’s ethylene is largely dissipated within 72 hours.
The most common source of ethylene in a greenhouse is from combustion of fuel. Any type of combustion, be it an engine or a heater, can give off ethylene. Loading docks near the greenhouse and heavy equipment used inside the greenhouse can both be sources. Avoid using gasoline or propane-powered equipment at times when the houses are not ventilated.
Of course, the most common type of combustion in the greenhouse is the heating system itself. A poorly operated or maintained heating system can rapidly ruin a crop.
Historically, as fuel prices increase, so do reports of ethylene damage. When fuel prices are low and greenhouses are not as tight or thoroughly insulated, ethylene from existing sources is easily ventilated away through leaks. But when fuel is expensive, growers tighten and insulate houses, and ethylene from pre-existing sources builds up and becomes a problem. So, ethylene that was not a problem before becomes a problem when fuel becomes dear and the greenhouse is tightened.
Preventing Ethylene Damage
If you discover you do have an ethylene problem, the easiest and most immediate intervention is periodic ventilation of the greenhouse with clean outside air. Of course, this is only a short-term fix and shouldn’t be considered a long-term solution, particularly when heating costs are high.
The best approach to stop ethylene damage is the time-honored ounce of prevention. Careful and regular attention to the plant growing environment, shipping areas and post-harvest conditions can go a long way. Ethylene can be present in any of these areas. Be sure to keep the potential sources mentioned above away from highly sensitive crops.
Also, pay special attention to when and where heavy equipment is used. If heavy equipment is used near the greenhouse, be sure the house is well ventilated so the exhaust can rapidly escape. Wherever possible, seal off the house from loading docks or similar areas. If growing areas cannot be sealed off from loading docks, consider using battery-operated equipment whenever possible. When designing a new greenhouse, orient intake vents so they face away from areas where vehicles frequently operate.
By far, the most frequent source of ethylene is a malfunctioning heating system. Diligent heating system maintenance not only prevents ethylene problems but also improves your system’s efficiency. Of all the heating systems available, gas-fired unit heaters typically require the most attention in terms of ethylene. Key issues with unit heaters are maintenance, distribution tubes, vent stacks, ventilation and fuel lines.
An annual maintenance program should start with the heat exchanger. Check the exchanger for cracks by operating the unit while looking for light through seams and connections. Gas lines should be checked for leaks by painting them with soapy water and looking for bubbles. Vent stacks should be examined for blockages or leaks between the seams. The stacks should be high enough so there is no risk of exhaust making its way back into the greenhouse. The pilot light and orifice should also be inspected and cleaned. The furnace flame normally burns light blue. If it is orange or yellow, combustion is incomplete and service is needed.
Outside of maintenance, the installation of gas-fired unit heaters must be done carefully to avoid problems from a heater that is capable of operating perfectly well. A distribution tube should be installed to mix the hot air coming from the heater with ambient air and distribute it efficiently throughout the greenhouse. The mixing of hot air with ambient air dilutes any potential source of ethylene and eliminates temperature gradients.
Distribution tubes are especially important for unvented heaters that release the products of combustion into the greenhouse. Unvented heaters are very popular when heating costs are high because of their efficiency. When unvented heaters are used it is critical to periodically check that they are firing correctly and that adequate ventilation of outside air is available.
Air intake required for combustion (make-up air) is also important for unit heaters. If the heater has insufficient make-up air, oxygen levels within the greenhouse can drop and pollutants can build up quickly. Such a condition is not only dangerous to the crop but to personnel as well. Generally, 1 square inch of vent cross-sectional area per 2,000 Btu capacity is enough. You will find specific information in the installation and operation manual for your furnace.
The Bottom Line
The key to preventing ethylene damage is being proactive. Ensure that heaters and equipment are well maintained and reduce plant exposure to potential ethylene sources. Keep an eye on your plants for irregular growth and if potential ethylene damage is suspected, be ready to take steps to identify the source of the problem and eliminate the cause.