Better crops

Ethylene (C₂H₄) is an odorless, colorless gas that can have both positive and negative impacts on greenhouse crops. It is a plant hormone, which means that it is produced by plants and affects growth and development at very low concentrations. Ethylene plays a role in seed germination, adventitious rooting, fruit ripening, leaf shedding, leaf yellowing and flower death (i.e. senescence). Growers can apply ethylene-based plant growth regulators, to increase lateral branching, remove flowers, or prevent flower initiation. Ethylene applications are also used to initiate flowering in bromeliads. Ethylene is often called a stress hormone because plants produce ethylene in response to stress. This is why you must never apply these products to plants that are under stress from high temperatures, water deficits, or disease or insect pressures. These plants already produce elevated levels of ethylene, and a normal ethylene application will cause toxic side effects including accelerated leaf yellowing and senescence.

Figure 1.  Ethylene dose response in impatiens. The severity of ethylene damage is determined by the sensitivity of the specific crop but also the concentration of ethylene and the time that the plant is exposed. Impatiens are considered to be highly sensitive to ethylene. After one day of exposure to 2 ppm ethylene, most open flowers are shed. After two days, all of the open flowers have been lost and buds also begin to abscise. After three days the plants do not have any flowers or buds.

Sources of ethylene in the greenhouse
Ethylene gas may be produced by both biological (i.e. plants) and non-biological sources. Because it is a gas, ethylene can easily move throughout the greenhouse. The most common source of ethylene contamination in the greenhouse is malfunctioning heating units. Incomplete combustion of organic fuels can result in the production of carbon monoxide, ethylene and other byproducts. CO₂ burners used for CO₂ enrichment can also produce ethylene. Ethylene from heaters, CO₂ burners or any combustion engines can accumulate to damaging (i.e. biologically active) concentrations in greenhouses, especially in tight structures and during the winter months when there is little ventilation. Any equipment including blowers, trimmers, and carts that use combustion engines should therefore be avoided in a production environment. Water heaters used to heat irrigation water should not be located in the production or propagation areas. Leaky gas lines can also result in plant damage in the greenhouse. Both propane and natural gas may contain small hydrocarbon contaminants that can cause ethylene like damage. Burning cigarettes also produce ethylene, so smoking should never be allowed in the greenhouse or near air intakes.

Plants themselves produce ethylene gas that is released into the greenhouse environment. Old or dying (i.e. senescing) flowers and ripening fruit produce more ethylene than leaves and young plants. When plants are wounded due to sleeving, mechanical, or insect damage, they also produce elevated levels of ethylene. Dead plant material that is decaying gives off ethylene. Proper greenhouse sanitation is not just important for preventing disease and insect problems, but it plays an important role in reducing ethylene damage as well. For example, damage to individual plants can occur when dying petals that have fallen from hanging baskets accumulate and decay on the plug trays or young plants below.

Figure 2.  Petunias ‘Sanguna Pastel Yellow’ grown at low levels of ethylene (10 or 50 ppb) for six weeks were generally stunted with malformed growth and small leaves. Flowering was delayed and flowers were smaller in the ethylene contaminated greenhouses. This experiment was conducted at Cornell University. Photos are courtesy of Neil Mattson and Roland Leatherwood.

Symptoms of ethylene damage
The severity of ethylene damage depends on the sensitivity of the plant species to ethylene, the concentration of ethylene, the amount of time the plant is exposed, and the temperature. High temperature stress causes plants to produce more ethylene, but plants are also more sensitive to damage from ethylene contamination at higher temperatures. In general, young flower buds are less sensitive to ethylene damage than open flowers, and leaves are less sensitive than flowers. Figure 1 shows how the dose of ethylene (concentration over time) affects impatiens.

In a greenhouse production environment plants may be exposed to lower concentrations, such as 25 to 200 parts per billion (ppb),of ethylene over weeks or even months during the colder times of the year. This type of chronic ethylene exposure results in stunted or malformed growth, flowers that do not initiate, and flower bud abortion. If grown to flowering under these conditions, the flowers become smaller and show accelerated death (Figure 2). Leaves start to prematurely yellow (i.e. chlorosis) and eventually they will become brown and dry (i.e. necrotic). This type of ethylene damage can be hard to diagnose, because similar symptoms may result from nutrient or pathogen disorders. In many instances where constant low levels of ethylene are present in the production environment, you may first observe damage on ethylene sensitive crops grown in hanging baskets (like geraniums). This is because ethylene is lighter than air and it will accumulate to higher concentrations up in the peak of the greenhouse.

If exposed to higher levels of ethylene, in the parts per million (ppm) range, ethylene damage occurs much more quickly. At 1 ppm, ethylene is considered to have full biological activity and most plants will show some signs of damage within hours or a few days. This damage is much easier to attribute to ethylene. In these instances, it is likely that plants will be growing well and you will come in one morning to find that all the open flowers on your geranium baskets have been shed. Symptoms of acute ethylene damage include shedding or shattering of leaves, buds or petals, rapid flower aging and wilting or leaf yellowing (Figure 3). A classic symptom of ethylene damage is epinasty. This is the characteristic downward growth of the leaf petiole. Plants look like they are experiencing drought stress, but they are well hydrated and fully turgid. While many different plants show epinasty, tomatoes consistently show the most severe epinasty at the lowest concentrations of ethylene.
 

Figure 3.  Symptoms of exposure to 3 ppm ethylene for two days. Torenia (left) shed flowers and showed accelerated petal senescence. Portulaca (middle) retained most of their buds, but all open flowers wilted. Alyssum (right) lose most of their open flowers and showed significant leaf yellowing.

Figure 4.  The geranium hanging basket (left) produced in a greenhouse that contained ethylene contamination at 800 ppb suffered extreme damage. Plants resumed vegetative growth after removal from the ethylene (right plant in right photo), but when compared to plants produced in a non-contaminated environment (left plant in right photo), they are delayed. One month after removal from the ethylene-contaminated greenhouse, these geraniums still did not have any flower buds.

Recovery of ethylene-damaged plants
In most instances, ethylene damage will not kill your crop, but it will often impact the marketability of the crop by delaying flowering or by removing all open flowers. Ethylene damage at the young seedling stage can result in plant death and entire crops may need to be replanted. The geranium hanging basket shown in Figure 4 was produced in a greenhouse that contained 800 ppb ethylene over a period of multiple weeks (or longer). Extensive damage like this essentially results in a crop that is not saleable. When removed from the ethylene environment, new growth resumed on these baskets but flowering was delayed by over one month (Figure 4). Most plants with moderate damage will resume normal vegetative growth and flowering within 1 to 2 weeks. In contrast, plants like tulips and Easter lilies, that flower only once, will not recover when buds and open flowers have been damaged by ethylene. Plants experiencing epinasty will recover quite quickly and these crops are the most likely to remain of marketable quality.

How to prevent ethylene damage
The proper maintenance and installation of heating units is the best way to prevent ethylene damage in the greenhouse. Regular maintenance should include inspection of heat exchangers, exhaust vent stacks and fuel lines. Leaks in gas lines can be identified by looking for bubbles following the application of soapy water. The heater should have a stable flame with a blue cone center. A yellow or orange flame indicates there is not enough oxygen for complete combustion of the fuel. Incomplete combustion can result in the production of harmful gases including ethylene and carbon monoxide. An outside air supply duct may be needed to provide enough oxygen for complete combustion, and proper air exchanges help remove contaminants like ethylene gas from the greenhouse.

Indirect fired heaters must be vented outside of the greenhouse so that the byproducts of combustion are not released directly into the growing environment. A few considerations for non-power- vented heaters include the height of the exhaust stack and placement near vents that are bringing air into the greenhouse. Greenhouses should follow the 2:3:10 rule if you are using passive ventilation from a vertical chimney stack. The top of the chimney stack must be at least 2 feet above the ridge of the roof. If the chimney placement is on the edge of the roof, then the top of the stack must be at least 3 feet above the roof surface. This chimney top must be high enough above the roof of the greenhouse such that it is at least 10 feet across to the nearest roof surface. In other words, a horizontal line drawn from the top of the chimney must run at least 10 feet before it touches a roof surface. Concurrent heating and venting for relative humidity management or high wind velocities can create a negative pressure in the greenhouse that results in a downdraft that sucks the exhaust gases back into the greenhouse. Power-vented heaters use a blower to actively exhaust the flue gases, and this prevents problems with back drafts. These vents can run horizontally out the wall of the greenhouse and should be at least 3 feet from any air intakes.

Figure 5. Tomatoes are a very ethylene sensitive crop that can be used as an indicator plant to detect ethylene contamination in the greenhouse. Tomatoes exposed to ethylene show epinasty or the downward growth of the petioles.

Unvented burners have been used by growers to enrich the growing environment with CO₂, but all combustion byproducts including ethylene are also released into the greenhouse. Recently, higher capacity direct-fired heaters have become available for heating production environments. These heaters are not vented and all products of combustion go into the greenhouse for heating. If these heaters are installed according to the manufacturer’s instructions and have proper air exchange to reduce relative humidity and ensure efficient combustion, they can be run without producing ethylene. For peace of mind with any heater, vented or non-vented, grow a few tomatoes near them as indicator plants.

The best way you can determine if you have ethylene contamination in your greenhouse is to carefully monitor plants that are sensitive to ethylene for the symptoms listed above. If you suspect that you may have an ethylene problem, you can place indicator plants in the greenhouse. The best indicator plant is tomato, which will show epinasty or downward bending of the leaves when exposed to very low concentrations of ethylene. Place young tomato plants near heaters or any source you suspect may be producing ethylene (Figure 5). Tomatoes will be one of the most ethylene-sensitive plants in your greenhouse, so if you observe symptoms of epinasty you should have time to remediate the problem and prevent permanent damage to your crops. You must immediately remove or turn off the source of ethylene and ventilate the area to remove the ethylene gas. As mentioned, ethylene damage may easily be confused with other types of stress that cause similar symptoms.

If you suspect you may have an ethylene problem or you would just like more information please feel free to contact us. We can use an instrument called a gas chromatograph to determine if ethylene levels in your facility are high.

Avoid using any equipment or vehicles that use combustion engines. Use only electric carts to move plants within the greenhouse. If you must use propane-powered forklifts in loading docks, make sure these areas are well ventilated to remove any ethylene that might be generated.

Inquiries about ethylene damage have increased considerably in recent years. Losses due to ethylene contamination can be devastating to both small and large greenhouse producers. If you are aware of the potential sources are able to recognize the symptoms, crop losses can be prevented.

Michelle L. Jones is the D.C. Kiplinger chair in floriculture and an associate professor at The Ohio State University. Reach her at Jones.1968@osu.edu. Peter Ling is an associate professor at The Ohio State University. Reach him at Ling.23@osu.edu.