Tuesday, August 18, 2015

A short history of a tree-killing anesthetic gas

Ethylene is a colourless gas that was widely used as a general anesthetic in the 1920s through 1940s. It apparently works pretty well at knocking people out and is less toxic than the chloroform and ether that proceeded it on the surgical floor. However, its flammability and outright explosiveness when mixed in certain proportions with oxygen, coupled with the development of decidedly less flammable halothane and related compounds in the 50s and 60s, led to eventual obsolescence.

In addition to being good at convincing our brain to take a time out, ethylene is intrinsically involved in the growth and development of plants. It can influence everything from seed germination to fruit ripening, and plants tend to produce it as part of their response to an injury (e.g. being eaten by insects or damaged by a frost). In ancient times, farmers with fig trees inadvertently managed to hack this system. They intentionally cut into a few of their figs to bring about the release of ethylene and subsequent ripening of the other fruit on a tree.

Ethylene seeps from a happy avocado (Source)

The ability of ethylene to affect plants was identified in the 19th century. Following the introduction of public street lightning in cities across Europe, many urban trees became deathly ill. Their roots turned blue and their leaves turned yellow and fell off. This damage was eventually determined to be due to their close proximity to leaking gas lines, which carried various ethylene-containing mixtures collectively known as illuminating gas. Interestingly, while most tree species were severely affected by the leaking ethylene, it was reported that lindens (genus Tilia) were relatively resistant. Also, trees were less affected in the wintertime when they weren't actively growing.

Not pictured: Unhappy trees (Source)

Ethylene is a very efficient molecule. After a bunch of carnation growers lost a lot of money because the plants they shipped to Chicago decided not to bloom, researchers traced the problem back to ethylene in the gas being used to light the greenhouses in which the carnations were grown. Further study revealed that a dilution of 1:2,000,000 of ethylene in air is still enough to cause open carnation flowers to close within 12 hours of exposure. Fortunately, we now have both electric lighting and a much better understanding of how ethylene affects plants we grow as crops. We use the gas to do things like get pineapple plants to flower when we want and convince certain fruits (e.g. pears) to ripen after being picked. We've also come up with substances capable of inhibiting the effects of ethylene, helping to minimize its adverse effects on plants or plant parts we want to keep around.


References

Crocker W, Knight LI. 1908. Effect of illuminating gas and ethylene upon flowering carnations. Botanical Gazette 46(4):259-276. [Full text]

Goodman L, Gilman A. 1941. The pharmacological basis of therapeutics. Macmillan Company.

Lin Z, Zhong S, Grierson D. 2009. Recent advances in ethylene research. Journal of Experimental Botany 60(12):3311-3336. [Full text]

http://www.pnas.org/content/104/9/3023.full

http://postharvest.tfrec.wsu.edu/pages/PC2000F

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