Effect of various plant factors on the expression of creeping-rootedness in lucerne (Medicago sativa L. complex)

Although agronomically interesting, selection of creeping-rootedness in lucerne, that is, the ability to form adventitious shoots on horizontal, spreading roots, remains difficult because of the complex and still unclear genetic control of this character, coupled with the possible occurrence of external factors that affect unpredictably its expression. Two experiments were undertaken on progenies of creeping-rooted plants, to verify previous findings and inferences, and test novel hypotheses, concerning the effect of various plant factors on the expression of creeping-rootedness. The ultimate aim was to provide information on the most appropriate germplasm, procedures, or indirect selection criteria, that would improve the efficiency of breeding for this character. The factors here examined included genotype, age, kind of progeny, vigour, spreading ability, and underground morphology. Some genotypes appeared to be good donors of the character, whereas others did not. The proportion of creeping-rooted plants increased in all cases with the plant age. Selfing consistently depressed the expression of the character compared with clonal propagation or crossing. It is suggested that the effect of age and selfing on the penetrance of creeping-rootedness is mediated through their effect on plant vigour. Both aerial and root vigour showed indeed an effect per se on the presence of the character. Beside the vigour, possible indirect selection criteria were identified in the frequencies of swollen zones and latent stem primordia on horizontal roots. Proportion of creeping-rooted plants in progenies after one year of growth appeared as a simple and useful criterion to predict future trends. Consistent with previous conclusions, the results provided evidence that a combination of family and individual selection may enhance the character expression.