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Making peach trees more cold hardy.

Their simplicity is beautiful. But a lot of nature's complexities go into making those fragile, delicately scented blossoms that decorate peach trees in the early spring.

Had the trees not earlier experienced a cold, dormant period, the blossoms would never have appeared. Nor could they exist if temperatures had been too cold during that dormant period.

As the seasons change, fruit trees acclimate themselves to changing temperatures, building their cold tolerance as winter approaches. "Deciduous trees require a certain amount of chilling to produce flowers and then fruit," explains ARS plant physiologist Michael Wisniewski.

But although a fruit tree prepares itself for seasonal temperature changes, a harsh winter can damage buds, bark, and wood in the trunk and limbs. Trees can even be killed by severe cold spells. And in the spring, blossoms are particularly vulnerable to late frosts. Any of these can spell economic disaster for fruit growers.

For several years, Wisniewski and horticulturist Ralph Scorza have been studying cold tolerance at the ARS Appalachian Fruit Research Station in Kearneysville, West Virginia.

Their perseverance paid off recently with a new theory of how plants keep themselves from freezing.

Working with scientists in the United States and Mexico, Wisniewski and Scorza identified a single gene that appears to control dormancy. And they've found a new protein that accumulates in tree bark as the tree slips into dormancy.

"We knew that living cells in woody portions of a tree avoid freezing by deep supercooling," Wisniewski says. "What this means is that the water inside each cell cools to below the normal freezing point without forming ice crystals."

In order to accomplish supercooling, the cells must maintain their fluid contents free of ice-nucleating agents and prevent water or ice crystals from entering them from spaces between the cells.

"We discovered that the pit membrane--a portion of the cell wall through which water and nutrients flow--is rich in pectins that form a barrier to keep ice from entering into the cell when temperatures drop," he says.

According to Wisniewski's research, the type, amount, and degree of cross-linking, or interconnecting, of these pectins may determine the size of the pores of the pit membrane. And the pore size determines the cell's permeability to water.

Plant cell walls are made of cellulose and hemicellulose organized into long, rodlike units called microfibrils that are the building blocks of the cell wall. Pores are spaces between the microfibrils that allow substances to move into and out of the cell.

Pectins, forming layers along the outside of the microfibrils, act as a glue. This makes the pores even smaller--almost nonexistent--thereby substantially decreasing cell wall permeability.

ARS scientists have been able to change the way plant tissue responds to cold temperatures by modifying the structure of the pectins in the cell wall.

"With this information, we may be able to breed trees for increased cold hardiness at critical times of the year," Wisniewski says.

Previous research indicated that cell wall porosity and permeability were key factors in supercooling, but until now there was no direct evidence that could support this theory.

"Our knowledge of the genetic regulation of cold-hardiness in woody plants is limited," Wisniewski says. "The time when plants become dormant happens to be the time when they are also most cold-hardy. The simultaneous occurrence of these two physiological events makes it more difficult to figure out what's going on genetically."

However, this may be easier now that Scorza and his coworkers have discovered a single gene that appears to control dormancy in peach trees.

The gene, named "EVG" for evergreen, was first found in peach trees in Mexico that never go dormant. Trees recessive for EVG require little or no cold-induced dormancy to blossom and produce fruit, "unlike the peach trees we're familiar with," Scorza says.

"We crossed the evergreen peach with a deciduous peach. Then we grew second-generation (F2) hybrid trees in Mexico, Florida, Georgia, and West Virginia to test their dormancy requirement and cold-hardiness."

Scorza's coworkers in the evergreen peach research included W.R. Okie with ARS in Byron, Georgia, J. Rodriguez-A. with the Colegio de Post-graduados in Chapingo, Mexico, and W.B. Sherman with the University of Florida at Gainesville.

The close cooperation between U.S. and Mexican scientists and among researchers at the four locations made it possible to identify this important gene. This type of cooperative scientific exchange is necessary when working with new, unique fruit germ-plasm, Scorza says.

As research progresses from the physiological to the molecular level, other genes may be identified. If so, Scorza foresees plant breeders genetically modifying plants to produce more cold-hardy varieties.

"The advantage of gene transfer is that once a gene is isolated, it can be manipulated and possibly inserted into any variety," he says.

At this time, regeneration and transformation rates of fruit crops are low compared with other species, but once a transgenic fruit or nut variety has been produced and tested, it can be multiplied indefinitely through budding, grafting, or cuttings.

For these reasons, Scorza feels that although perennial fruit and nut crops have been relatively difficult subjects for gene isolation and transfer, these crops over time could reap the greatest benefits from such technologies.

This study also produced some new, interesting results on proteins involved in cold acclimation and dormancy.

"Dormancy is generally believed to be a prerequisite for cold acclimation in woody perennials. But we found that although the evergreen peach doesn't go dormant, it does increase its hardiness during the fall," says Rajeev Arora, a plant physiologist working at the Kearneysville lab.

This indicates, he says, that halting growth may not be an absolute requirement for cold acclimation.

"However, maximum hardiness attained by the deciduous trees was more than twice that of the evergreens," Arora says.

The scientists found that two storage proteins, called 16 kD and 19 kD, accumulate to high levels in deciduous trees but not in evergreens.

"We're now trying to evaluate the role of these proteins in cold acclimation and dormancy," Arora says.

Accumulation of bark storage proteins is apparently regulated by day length. There's something in the tree's system that senses the change in day length. But the evergreen peach used as a parent in the study came from an area in Mexico where seasonal shifts in day length are minimal.

"This is really interesting," Arora says. "The evergreen peach may be missing that particular system in the tissues that senses the change in day length--which could account for its low levels of the storage proteins."--By Doris Stanley, ARS.

Scientists mentioned in this article are at the USDA-ARS Appalachian Fruit Research Station, 45 Wiltshire Road, Kearneysville, WV 25430. Phone (304) 725-3451, fax number (304) 728-2340.
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Title Annotation:includes related article on evergreen peach trees
Author:Stanley, Doris
Publication:Agricultural Research
Date:Oct 1, 1992
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