GENETIC STUDIES OF SYMBIOTIC NITROGEN FIXATION IN SPANISH CLOVER

In this genetic study of symbiotic nitrogen fixation in the tropical legume Spanish clover (Desmodium sandwicense E. Mey), it was established that the quantitative variance found within several estimates of nitrogen fixation had a genetic basis. The diallel analysis suggested that genetic variation was present and that a major portion of this variation was attributable to differences between the parents and their F 1 crosses. The within-cross genetic variation was due solely to general combining ability suggesting the presence of heritable genetic variation. Genotype by environment interactions were also found to be significant. The analysis of the data from a cross between two lines suggested that the four nitrogen fixation estimates measured were highly correlated and this variation could be represented by one principal axis of variation. Significant differences among population means were found for all analyses. Plants with larger cotyledons and juvenile leaves fixed significantly more nitrogen than did plants with smaller cotyledons and juvenile leaves. This difference was attributed to juvenile plant vigor which may assist the onset of nodulation. Analysis of the population means suggested that non-heritable variation accounted for the predominant portion of the genetic variance when the nitrogen fixation estimates were unadjusted for differences due to juvenile plant vigor. However, when the nitrogen fixation estimates were adjusted for the effects of juvenile plant vigor the genetic variation was totally heritable. Such heritable variation is of agricultural importance.