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Cotton yields linked to heat tolerance.

Cotton breeders have discovered a shortcut to breeding superior Pima cotton plants that could reduce the time and expense needed to develop better commercial varieties.

"During the past 40 years, plant breeders have succeeded in making Pima cotton more heat tolerant and have increased fiber yields threefold. While researchers and growers appreciated the improvement, they didn't know until recently exactly how the change came about," says John W. Radin, an Agricultural Research Service plant physiologist formerly located in the Cotton Physiology/Genetics and Host Plant Research Unit in Phoenix, Arizona.

"Our studies in growth chambers, greenhouses, and fields show that water transpiration from plant leaves has increased. Modern cotton plants give off more water when temperatures are high, as in the desert areas of Arizona and California where Pima thrives. Just as human perspiration cools the body, evaporation from leaves cools off cotton plants. Avoiding heat stress during critical growth stages is the key to high cotton yields," says Radin who conducted this research at ARS' Western Cotton Research Laboratory in Phoenix.

The results prove that cotton breeders have been unknowingly selecting for increased evaporative cooling while selecting for increased heat tolerance and yield. This finding is a big step forward: Now it may be possible to look directly at leaf temperatures and identify the genetic lines possessing heat tolerance. Crop improvement will be faster and breeders may not have to rely on repeated field-testing to confirm a new variety's heat tolerance.

"Finding a linked relationship between an agronomic trait (higher yield) with a physiological trait (increased transpiration) makes selection easier because superior plants are easily spotted in the field. Without such a link, locating plants with increased heat tolerance and higher yields requires complicated and expensive tests," says Richard G. Percy, ARS plant geneticist at the Maricopa Agricultural Center, Maricopa, Arizona, where field studies were conducted.

Modern varieties of Pima cotton, Gossypium barbadense, are better suited for desert areas than older varieties, partly because they transpire more water. Despite greater transpiration rates, modern cultivars possess a higher water use efficiency and produce more pounds of cotton per unit of water used than older, more heat-sensitive cotton varieties. This is because successive stages in the breeding program have resulted in plants with smaller leaves and greater stomatal density. Leaves from most advanced cotton lines are now about 45 percent smaller than those from a 40-year-old cultivar.

Early Pima cotton varieties yielded best when air temperatures were between 85 [degrees] F and 90 [degrees] F, but prevailing temperatures in the Southwest during the growing season often exceed 100 [degrees] F. ARS biologist Paul J. Pinter at the U.S. Water Conservation Laboratory, Phoenix, has used modern remote sensing techniques to measure leaf temperatures. He finds that leaf temperatures of modern varieties can be as much as 10 [degrees] F to 15 [degrees] F cooler than the surrounding air. That's about double the temperature difference of older varieties.

This was a joint project of ARS, University of California-Los Angeles, and the Supima Association. UCLA scientists included plant physiologists Eduardo Zeiger and Zhenmin Lu.
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Author:Senft, Dennis
Publication:Agricultural Research
Date:Jul 1, 1993
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