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United States Department of Agriculture

Agricultural Research Service

Chilly Nights Confuse Some Plants’ Inner Workings / November 20, 1996 / News from the USDA Agricultural Research Service

Plant physiologist inserting the leaf of a tomato plant exposed to cool night temperatures into the airtight sample chamber of a device designed and constructed in his laboratory.

Chilly Nights Confuse Some Plants’ Inner Workings

By Dawn Lyons Johnson
November 20, 1996

URBANA, Ill., Nov. 20--Chilly nights can mean internal turmoil for warm-weather plants like tomatoes, soybeans and corn, resulting in reduced yields, U.S. Department of Agriculture scientists say.

Scientists with USDA’s Agricultural Research Service have discovered that when overnight temperatures dip below 50 degrees Fahrenheit, crucial biochemical processes that typically occur inside the plant at night may shut down--only to revive in the next day’s warmer temperatures and clash with daytime biochemical activities. The result: less efficient photosynthesis, slashed yields and a possible explanation for the geographic limits imposed on plants by their temperature sensitivity.

Plants have an internal timekeeping mechanism called circadian rhythm that regulates the internal processes over a 24-hour period, explains ARS plant physiologist Don Ort at the agency’s Photosynthesis Research Unit here.

“There are specific reactions that are timed to occur at a given period of day or night,” Ort says. “If they occur at the same time, they compete and stall photosynthesis, just as competing foot movement would paralyze a runner.”

For example, temperatures might drop below 50 degrees at 10 p.m., stalling nighttime biochemical processes in a tomato plant. When the weather warms up again at 8 a.m., the plant may resume its interrupted nighttime processes. But at the same time, the plant’s internal clock kicks off the regularly scheduled daytime biochemical activity, resulting in competition with the lingering night processes.

ARS scientists have shown that the activity of two critical enzymes, sucrose phosphate synthase and nitrate reductase, are regulated by a circadian rhythm that is disrupted by cool temperatures in chill-sensitive plants like tomatoes.

The scientists hope to use this information to override low-temperature sensitivity in several economically important crops. “If we can improve the plant’s ability to tolerate lower temperatures, even by one or two degrees, we would significantly improve the biological efficiency and productivity of these crops,” Ort says.

A report on Ort’s work is featured in the October issue of Agricultural Research, the monthly magazine of the Agricultural Research Service. The magazine is accessible on the World Wide Web in PDF file format at:

Scientific contact: Donald Ort, Photosynthesis Research Unit, Agricultural Research Service, USDA, 190 ERML, 1201 W. Gregory Drive, Urbana, Ill. 61801; phone: (217) 333-2093,

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