When plants encounter ethylene, a gas they also produce naturally as a hormone, the result is softening and ripening in the case of fruit, and wilting and fading in the case of flowers.



To delay these effects, growers spray plants with products, such as EthylBloc for flowers and SmartFresh for fruits and vegetables, that contain a compound that blocks ethylene’s action on plants.



But how this compound, 1-methylcyclopropane or 1-MCP, works at the molecular level remains uncertain, despite several theories chemists have proposed in the scientific literature.

In a research paper published in the April issue of Chemistry & Biology, a team led by Michael Pirrung, a University of California, Riverside, chemistry professor, offers a novel theory for how “anti-aging” products block ethylene in plants, prolonging their shelf life.

The authors propose that a chemical reaction occurs between 1-MCP and naturally-occurring copper in plant cells. This knowledge could guide researchers in their attempts to discover new ethylene-blocking chemicals for preserving the freshness of fruits, vegetables and flowers for longer than currently is possible.

Until now, researchers believed that a complex—a chemical structure consisting of molecules that are weakly connected to one another—formed between 1-MCP and copper.

“A complex is loose and can break apart easily—something we don’t see happening in the case of 1-MCP in plants,” Pirrung says. “A chemical reaction, which is far stronger than a complex, accounts for why 1-MCP is so effective.”



Plant cells possess copper-containing ethylene binding sites called ethylene receptors. When ethylene comes into contact with the receptor, it binds chemically with the copper, which inactivates the receptor. The inactivation results in the cell breaking down, which, in turn, initiates aging and the death of plant tissues.

1-MCP works by beating ethylene to the receptors. By binding with a sufficient number of receptors chemically and permanently, it forever makes them insensitive to ethylene. The plants do not perceive ethylene thereafter, preventing ripening and wilting.

Unlike ethylene, 1-MCP does not inactivate the receptors. As a result, the cells do not break down, which prolongs the freshness of flowers and fruit after harvest and extends their shelf life.

Because 1-MCP is an unstable gas, growers face a challenge in delivering it to fruits and flowers.

When the powder is dissolved in water, 1-MCP is released as a vapor that travels through the air, making its way eventually to plants’ ethylene receptors.



In their paper, the researchers show that 1-MCP reacts with copper to give a highly reactive chemical intermediate. Called a carbene, the intermediate will react with essentially any other kind of chemical group in the area.

Pirrung says knowledge of a chemical reaction between 1-MCP and copper in ethylene receptors could help researchers design new compounds, such as non-gaseous compounds, that are capable of working as well as 1-MCP but are not as difficult to handle.

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