Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Ethylene (C2H4) occurs in trace amounts in gasoline and natural gas and is produced when these substances are burned. It also is present in wood and tobacco smoke. Ethylene is a plant hormone produced by plants during their growth and development. However, ethylene produced through defective heating equipment can be detrimental to greenhouse crops, because it is produced in greater quantities. Ethylene pollution influences the activities of plant hormones and growth regulators, which affect developing tissues and normal organ development, many times without causing leaf-tissue damage. Injury to broad-leaf plants occurs as a downward curling of the leaves and shoots (epinasty), followed by a stunting of growth. Other symptoms of excess ethylene exposure include the abscission of flower buds (figure below), petals or leaves; water-soaking of older leaves; chlorosis; and wilting of flowers. Crops vary in their sensitivity and response to ethylene toxicity. The degree to which a crop is affected depends on the variety, temperature, ethylene concentration, and the duration of exposure. High temperatures and high light levels will increase the severity of ethylene damage. In high tunnels that burn propane, kerosene or use motors that burn gasoline and have poor or no ventilation, even minute amounts of this pollutant can cause severe damage to tomatoes. Unvented unit heaters in greenhouses can at times also cause problems. These problems tend to increase in very tight greenhouse structures, i.e., those that have little exchange with the outside air. Symptoms of ethylene damage can be very subtle, especially if there are no plants grown in clean air available for comparison.
Proper heating system installation and maintenance are the best ways to prevent problems. A maintenance plan should include cleaning the unit heater and fuel orifice twice a year. Propane flames should have a small yellow tip when properly adjusted and natural gas flames should be a soft blue with a well-defined inner cone. To ensure proper combustion, heater units should have a clean air intake and should be vented to the outside with a stack, which keeps exhaust gas from being drawn back into the greenhouse through the ventilation system.
