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Genetic Changes in Oat Bulk Populations under Winter Survival Stress¹

Convergent crossing was used to produce 58 different winter oat (Avena by zantina K. Koch and A. sativa L.) hulk populations, and these populations were subjected to 4 years of winter survival stress in Pennsylvania. Two types of observations were subsequently made to determine the effect of natural selection on these populations. First, all populations were compared in the F₈ and _F7 generations to determine performance for juvenile growth habit, freezing resistance in a laboratory test, and winter survival in the field. Second, F₈ and _F7 individual plants were isolated from 12 populations and classified for juvenile growth habit, maturity, and plant height. Derived F₁ and F₈ lines from six of these populations were compared for freezing resistance.

Thirty-four of the 58 populations increased significantly in freezing resistance and changed similarly for winter survival in the field. The majority of the populations that changed greatly for these chadracteristics were low performers in the F₂. Initially elite populations made small, nonsignificant changes under natural selection. Nonresponsiveness to natural selection in the latter populations did not reflect low genetic variability, since a wide range in freezing .resistance was observed among derived lines, regardless of generation or population.

In every population, the F₈ growth habit ranged from intermediate to very erect. By the F₇ generation, 28 populations were significantly more decumbent in juvenile growth habit. Plants from 10 out of 12 populations were significantly more decumbent by the F₇ generation; and all populations were later in maturity. Plants with decumbent juvenile growth were latest in maturity, and these two characteristics were correlated within each generation of all populations. Five of 12 populations were taller by the F₇ generation, and the others did not change in average plant height under natural selection. Changes in maturity and plant height were independent of natural selection on winterhardiness.

If the bulk population approach is to be used for natural selection for winterhardiness, controlled selection pressure during one or more early generations may have to be applied so that the tendency to increase gone frequencies for lateness and tallness can be offset.

Key Words: Winterhardiness • Freezing reslstance • Natural selection

² Research agronomist, ARS-USDA, and professor of plant breeding. The Pennsylvania State Univ., University Park, PA 16802.

Received for publication February 18, 1975.