Sampling greenhouses for ethylene
Ethylene is a gaseous plant hormone that has a number of desirable and undesirable responses (Table 1). In greenhouse situations, it can induce leaf yellowing, leaf curling (epinasty), and the abscission of leaves, flower buds, and open flowers. Ethylene is active at extremely low levels in the air. Plants begin to respond to ethylene at a concentration of approximately 0.05 to 0.1 parts-per-million (ppm) in the air. Background concentrations are usually in the range of 0.002 to 0.005 ppm. A concentration of as little as 1 to 5 ppm saturates the response to this gas.
A history lesson
There are many sources of ethylene gas in the environment including ripening fruit or decaying plant material, incomplete combustion of organic fuels, and as a contaminant in natural gas. One of the early uses of ethylene dates back to before 1910 to de-green citrus fruit, although the identity of ethylene as the active component was not known at the time. In this early example, kerosene stoves were included in railcar shipments of fruit from the southern United States to northern markets in the winter, and it was noticed that the heaters enhanced the de-greening (and thereby improved orange or yellow coloration). This process became known as “sweating.” Interestingly, when the kerosene stoves were replaced with alternate types of heaters, the benefits of “sweating” disappeared. It was soon learned that it was the incomplete combustion of kerosene that caused ethylene to form and thereby de-green the fruit.
This brief history lesson provides a good example of how ethylene responses can be inadvertently derived from heating systems. Incomplete oxidation of propane and natural gas can form ethylene and other hydrocarbons. In the winter, when gas exchange for the greenhouse is typically at its lowest and heat demand the greatest, the chances of encountering a problem with ethylene are also at their greatest.
When to sample for ethylene
It is a good idea to sample the air in the greenhouse occasionally and have the gas sample analyzed for ethylene content. It might be advisable to have gas sample analyzed on startup of new heaters or heaters that have not been run for a while or if symptoms of ethylene action are noticeable in a currently heated house. It should be noted that poor growing conditions can also lead to chlorophyll loss and shedding of plant parts; a late winter/early spring growing season and high humidity can lead to conditions of a waterlogged root zone, which can induce ethylene formation by the plant.
Propane and natural gas typically have very low ethylene levels and leaks of these two combustion gases would likely not go unnoticed. Typically, the volatile material included in these gases as indicators of leaks would indicate a leak before contaminating ethylene could cause a problem with the plants in the greenhouse.
How to sample for ethylene
Sampling for ethylene in the greenhouse is simply done. If sampling is part of the startup process and the opportunity is available, collect a sample from anywhere in the range to be heated prior to igniting the heaters. Following startup and establishment of the desired temperature, one or two samples can be taken from air near the center of the range being evaluated. If only a couple heaters are present, it would be preferable to take samples in the heated air stream as it leaves the heater’s heat exchanger, before a significant amount of the heated air has mixed with the colder greenhouse air. If ethylene levels in the greenhouse exceed 0.05 ppm or an ethylene-related problem is determined to exist, it is important to identify the malfunctioning heater. In this case, samples should be pulled from the air entering and leaving the heat exchanger of each heater in the range.
An ethylene analysis requires approximately 1 milliliter of gas. Often, more than one sample is removed from the syringe to verify the gas concentration, so a 3- to 5-milliliter sample would work well. A sample is collected by filling a 5- to 10-milliliter glass syringe 3/4 full of air from the desired location. Only glass syringes will work. Plastic syringes can actually absorb some gaseous compounds. The open end of the syringe (no needle used) is sealed with a rubber septum. At the MSU Postharvest Biology and Technology Lab, we use red rubber serum vial septa. The septa allow the analysis personnel to pull a sample from the filled syringe using a second 1-milliliter syringe fitted with a needle.
It is a good idea to test the air tightness of the syringe prior to use. This is done by sealing the open end of the syringe with a septum and pressing lightly on the syringe plunger. If the depressed plunger springs slowly back three or four times, returning to essentially the same spot, the seal should be adequate. It is common for the plunger tip to shrink with age and make a poor seal. For more information on how to sample for ethylene, call your local Extension educator or testing laboratory.
What to submit for testing
The syringes should be labeled using permanent ink on the barrel of the syringe. A number or letter should suffice. It is a good idea to indicate these identifying numbers on a letter accompanying the samples and contact information needs to be included. It is not necessary to indicate the locations/sources of the samples of the gases in the syringes. It is usually best if the analysis lab samples the syringes without knowledge of their contents. The analysis lab should return a tabulated analysis sheet and a copy of the chromatograms, if available. Such documentation is critical in proceedings where proof is expected to be demonstrated.
To prevent movement of the plunger or loss of the septum during transit, it’s a good idea to secure them with adhesive tape. Syringes should be shipped in a relatively stout box and packed so they do not contact one another. Plastic bubble wrap works well to separate the syringes and prevent motion during shipping. Shipping time should not exceed two or three days.
If a problem with ethylene exists, the heater needs to be repaired immediately. If repairs cannot be quickly fixed, ventilation and ethylene scrubbing are options that can be explored. Firms that service heating and cooling of produce distribution warehouses can source ethylene scrubbers based on permanganate, carbon or catalytic filters.
Table 1. Plant responses to ethylene
|Promotes color development in fruit||Accelerates senescence|
|Stimulates ripening of climacteric fruit
||Enhances excessive softening of fruits|
|Promotes de-greening in citrus||Stimulates chlorophyll loss|
|Stimulates dehiscence in nuts||Stimulates leaf epinasty (twisting)|
|Alters sex expression in cucurbits||Stimulates sprouting of potato|
|Promotes flowering in bromeliads||Promotes discoloration (e.g., browning)|
|Reduces lodging of cereals||Promotes abscission of leaves, fruits, and flowers|
|Inhibits geotropic responses for roots and shoots|
For more information:
Randolph M. Beaudry, Professor
Department of Horticulture
A22 Plant and Soil Science Bldg.
Michigan State University
East Lansing, MI 48824-1325
Ph: (517) 355-5191 ext 303