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Selection for Seedling Cold Tolerance in Safflower with Modified Controlled Environment Chambers¹

The unpredictable occurrence of cold stress in the southwestern U. S. makes selection in the field for seedling cold tolerance in safflower (Carthamus sp.) a sporadic effort. Use of a conventional cold chamber has not been practical for us because temperatures within it differed by as much as 3 C among positions only 30 cm apart, and they changed among subsequent tests. Installation of a turntable made it possible to subject seedlings growing in pots placed within the same radius (circle) a similar cold stress. We were then able to differentiate known genotypes for degree of seedling cold tolerance with as few as 10 plants of each. This degree of precision, along with controlled hardening temperatures for as little as I week before the cold stress, gave considerable flexibility in selection for seedling cold tolerance. In a fourway cross in which one parent was a cold tolerant selection out of the wild species, Carthamus flavescens Spreng, it was possible to program a cold stress (_16 C for hours) in which none of the most cold tolerant domestic parent, about 50% of the wild parent, and 0.5% of the seedlings in the segregating population survived. With controlled hardening temperatures and programmed cold stresses, time involved in breeding and genetic studies of seedling cold tolerance in safflower need be limited only to the time required for growing the filial generations.

Key Words: Carthamus tinctorius L. • C. flavescens Spreng • C. lanatus L.

² Research agronomist, ARS, USDA, and professor, Dep. of Plant Sciences, Univ. of Arizona, and research technician ARS, USDA, Tucson, AZ 85721.

Received for publication December 11, 1976.