There are several phytotoxic gases that can be produced by heating systems used for greenhouse climate control. This especially is true when the heater or furnace is not maintained properly. Closed greenhouses (and high tunnels) have the ability to retain these gases with plant damage as the end result. This article is written to inform greenhouse vegetable growers of pollutant gases that can become problematic, how they are produced, and measures that can be taken to prevent their occurrence.
The three most problematic and potentially damaging pollutant gases in greenhouse vegetable production include ethylene, sulfur dioxide and oxides of nitrogen. All of these gases potentially are part of the fumes generated in the combustion chamber of heating systems that employ propane, natural gas, fuel oil, coal or wood as a fuel source. Of the three, ethylene is by far the most common and holds the greatest potential for plant damage and yield reduction.
Ironically, ethylene is a growth hormone naturally produced by plants in minute amounts during their growth and development. It often is referred to as the "ripening" hormone and is responsible for, among other things, regulating the ripening of fruit. For example, tomatoes destined for shipment over long distances usually are picked green (breaker stage). Upon receipt of the fruits hundreds or even thousands of miles away, they are exposed to ethylene gas. As a result, they soon turn red and soften. Please note I did not use the term "ripen", for the latter (to the author) implies the development of flavor.
Ethylene levels high enough to cause plant damage in greenhouses or heated high tunnels most often result from the incomplete combustion of fuel by defective heating equipment. Typical symptoms of ethylene exposure include epinastic growth (downward curling of the leaves and petioles) followed by general stunting of growth. Other symptoms of exposure to high levels of ethylene include the abscission of flower buds petals or leaves; water-soaked appearance of older leaves; chlorosis; and wilting of flowers.
Unfortunately, tomato, our primary greenhouse crop is one of the most sensitive plants to this pollutant. Couple this fact along with the other environmental conditions present in a closed greenhouse or high tunnel (high temperature and relatively humidity), and it is not surprising to see severe ethylene damage on tomato.
The second pollutant of concern, sulfur dioxide, is produced when a fuel such as coal is burned. Many sources of coal contain sulfur as an adulterant. Upon combustion (oxidation) sulfur dioxide is produced. Symptoms of sulfur dioxide damage include chlorotic "stippling" of the leaves at lower levels and interveinal leaf necrosis at high levels. Tomato fruit damage?
Finally, the air that we breathe is comprised primarily of nitrogen gas (about 80%). High temperature combustion (oxidation) holds the possibility that oxides of nitrogen (e.g. nitric oxide) will be produced. When this happens in a greenhouse heater with faulty heat exchange tubes, the oxides of nitrogen are accidently introduced into the greenhouse. Symptoms similar to those caused by sulfur dioxide at higher levels often result. At lower levels, the leaves may simply appear darker or have downward curving leaf margins, or both.
Prevention is the best cure for air pollution problems in the greenhouse or heated high tunnel. In order to allow for complete combustion of fossil fuels, adequate oxygen is required. As a general rule, one square inch of free area (an unobstructed opening to the outside) should be provided for each 2000 Btu of heater rating. Heating systems should be cleaned on a yearly basis and checked for possible leaks in the heat exchange tubes. Exhaust pipes should be checked to make sure bird or wasp nests that would restrict air flow are not present. Finally, become familiar with the symptoms of ethylene damage and monitor plants closely. In most cases, ethylene damage is reversible, but only if the source of the pollutant is eliminated.
REVISED: February 21, 2017