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Natural antifreeze for underage trees.

Freezes over the past several years--which have slashed production in northern Florida as much as 98 percent--have caused the citrus industry to shift farther south in the state.

Cold-hardiness is an important characteristic in most crops grown in temperate zones. Therefore, supercooling--a natural protection from freeze damage--is as important in subtropical evergreen citrus as it is in deciduous temperate tree crops.

"We're taking a holistic, or whole plant, approach to preventing internal freezing of citrus tissues," says ARS plant physiologist George Yelenosky. "Instead of just looking at leaves, stems, buds, or flowers, we're using computerized sampling sensors and thermal regulators for whole trees in environmentally controlled rooms."

Although unable as yet to satisfactorily predict or manipulate supercooling to a set temperature, Yelenosky and colleagues have found that they can reinforce the process by spraying citrus leaves with abscisic acid and other compounds.

At the ARS Horticultural Research Laboratory in Orlando, Florida, Yelenosky, Heinz K. Wutscher, and Joseph C. Vu, now at Gainesville, have been using what they call "supercooling-stabilizers."

"These are simply blends of compounds that allow young, unhardened citrus trees to supercool more deeply and survive at around 22 [degrees]F," Yelenosky explains.

Young trees treated with these compounds have survived for up to 2 hours what normally would be a lethal freeze without any injury or evidence of ice forming in the tissues.

Plant physiologist Michael Bausher is investigating the role of a newly found glycoprotein, GP-24. GP-24 was found in Poncirus, the most cold-hardy type of citrus, which is also deciduous. And in collaboration with the University of Florida, Bausher is studying unique proteins associated with citrus cold-hardening, called cold acclimation proteins.

"We're also working on research to infuse antifreeze proteins (AFP2) found in arctic fish into citrus," says Yelenosky.

Orlando plant geneticist Randall P. Niedz says AFP2 could increase supercooling, reduce the amount of freezable water at a given temperature, and decrease the rate of ice spread in the tissues.

Scientists from the Plant Biotechnology Institute in Saskatoon, Saskatchewan, and Louisiana State University in Baton Rouge are collaborators in this research.

A type of inedible citrus has been shown to supercool at temperatures as cold as 50 [degrees]F. Even its flowers can supercool and survive at 21 [degrees]F; its callus, at 12 [degrees]F.

In subtropical regions like Florida, this is important since temperatures in relatively short freezes seldom drop below 18 [degrees]F, and they often remain above 21 [degrees]F. Under these conditions, according to Yelenosky, it's possible that supercooling could be developed into a significant freeze-protection mechanism.

Like deciduous tree crops, citrus requires many hours of cool temperatures to harden. Ideally, those temperatures should be between 32 [degrees]F and 50 [degrees]F. About 500 continuous hours within this temperature range are needed before a severe freeze to prepare citrus for survival. In Florida, this ideal conditioning period is never assured and rarely achieved.

"Undoubtedly," says Yelenosky, "if we could adequately manipulate supercooling for citrus trees, we could not only save millions of dollars for Florida growers, but could also revolutionize freeze-management strategies for world agriculture."--By Doris Stanley, ARS.

George Yelenosky and other scientists mentioned are at the USDA-ARS U.S. Horticultural Research Laboratory, 2120 Camden Road, Orlando, FL 32803. Phone (407) 897-7300, fax number (407) 897-7309.
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Author:Stanley, Doris
Publication:Agricultural Research
Date:Oct 1, 1992
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