THE EFFECT OF GERMPLASM SOURCE ON ALFALFA ROOT CHARACTERISTICS

Cultivar intracrosses and intercrosses were made within and between six alfalfa (Medicago sativa L.) cultivars differing in relative winterhardiness. The progeny were grown in the field for 2 seedling years and in gravel culture tanks in the greenhouse. The objectives were to study genetic variability for root and shoot characters in crosses between diverse sources of alfalfa germplasm under field and greenhouse environments and to determine if a relationship between winterhardiness of the germplasm source and root characters could be detected.;Two plants were selected for high or low root fresh weight from each of six cultivars and crossed in a 12 x 12 diallel to determine the effect of root weight selection on genetic variability for alfalfa root traits. The progeny were grown in gravel culture tanks in the greenhouse.;GCA was the most important source of genetic variation for all traits. A maternal effect was observed for root diameter, root dry weight, and shoot dry weight. When plants selected for high root weights were used as the female parents, their progeny had greater root diameters, root dry weights, and shoot dry weights than the progeny of the same plants used as the male parents. The inverse was observed for plants selected for low root weight. Reciprocal crosses should be kept separate when selecting plants for root weight to utilize the maternal effect.;The expected relationship of hardy germplasm having more root branching than nonhardy germplasm was not consistently observed. Alfalfa cultivars differing in winterhardiness may or may not differ in root morphology.;In the field, no significant differences were observed among the cultivar intracrosses for any trait except number of lateral roots. The cultivar intercrosses had significantly greater root dry weights, shoot dry weights, and root diameters at the crown and at 15 cm than the intracrosses. General combining ability (GCA) was highly significant for all characters. Large genotype x environment interactions were observed for the root traits indicating that procedures need to be developed that reduce or control the effects of these interactions.