Ethylene is a colorless, odorless gas composed of two carbon and four hydrogen atoms (C2H4). It is a naturally occurring plant hormone that acts as a chemical signal that controls several aspects in plant development and growth. In most plants, it is involved in flower senescence, fruit ripening and seed germination.
Ethylene can also occur in the greenhouse as a gaseous contaminant. Levels above 0.1 ppm are known to cause injury to plants during production, handling, shipping and storage (i.e. post-production). Symptoms of ethylene damage include foliar chlorosis or necrosis, drooping of leaves and poinsettia bracts, rapid flower aging and wilting, malformed leaves or flowers, pre-mature dropping of leaves or flowers, and stunted growth. Plants vary in response to ethylene, not only among different plant species, but even among plant cultivars of the same species. A plant’s response depends on the ethylene concentration, length of exposure, sensitivity of the plant species and stage of plant life cycle. Greenhouse temperature also plays a role; higher temperatures are known to have a greater impact on the plant.
The most common source of excess ethylene in a greenhouse is from heaters that are not functioning properly or are insufficiently vented. Other sources of ethylene may include: exhaust from combustion engines (equipment powered by propane), leaky gas lines or contaminated fuel, cigarette smoke and decaying plant material. Most often, when there is an elevated level of ethylene, there will also be elevated levels of carbon monoxide, which can be very dangerous to humans.