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Heritability of Cold Hardness in Flax (Linum usitatissinum L.)¹

Two hundred F₂ families from each of two crosses, ‘DeOro’ x ‘Dillman’ and ‘Linore’ x ‘B-5128,’ were studied in the F₃ and F₄ generations for reaction to low temperatures at two locations. Each generation was grown in a different year. During the first season, the plants were subjected to severe low temperatures, as low as —23C (—9F) at Chillicothe, Texas, but snow cover protected the plants so that cold injury was mild to moderate. The second season, temperatures dropped as low as —17C (2F) at Denton, Texas, without snow cover, and cold injury was moderate to severe. A cold hardiness index (CHI) was calculated from the degree of injury and the percentage of plants injured in each row. Dominance deviations could not be detected in the F₃ or F₄ generations. Due to the seasonal differences, genetic variances of families in the F₄ were often higher than variances in the F₃. Heritability estimates of CHI calculated from components of variance were .35 in the F₃ and .59 in the F₄ of the second cross. Heritability was estimated also by the regression of F₄ on F₃ which was .75 ± .15 for the first cross, and .56 ± .04 for the second cross. The regression estimates were unbiased by genotype- year and genotype-location interactions. The expected genetic advance in F₄, by selecting the top 5% in F₃, was 12 CHI units in both crosses. The observed gain in F₄ was the same as predicted in the first cross, but it was much less than expected in the second cross. Estimates of genetic variance in the F₄ increased as the season progressed indicating that selection near the end of the season would be much more efficient than selection after the first period of freezing. Growth habit appeared to be closely associated with cold hardiness. Cold hardy lines all formed a rosette of branches during the winter contrasted to the upright growth of the cold sensitive lines

Key Words: cold tolerance • winter hardiness • winter survival

² Formerly graduate student, Texas A&M University (now Research Agronomist, Department of Agriculture, Giza, United Arab Republic); Research Agronomist, Crops Research Division, ARS, USDA, and Professor, Texas Agr. Exp. Sta., College Station; and Assistant Professor, Texas Agr. Exp. Sta. College Station 77843

Received for publication April 25, 1968.