Abstract
Plug nursery systems are being introduced into Japanese bunching onion (Allium fistulosum L.) production in Japan, to promote the mechanization of transplanting. Transplants raised in plugs, however, are so young and small that their rooting is not always satisfactory or they cannot grow into a uniform stand under unfavorable conditions. It is considered that the use of cultivars with vigorous seedling growth can expand the application of plug nursery systems. In order to gain information about the mode of inheritance of the seedling growth rate in bunching onion, we conducted a diallel analysis using a 9 × 9 half diallel cross involving various types of inbred lines. Fresh weight at the transplanting stage (seven weeks after seeding) showed a high heritability in the broad sense (hB2) and a low heritability in the narrow sense (hN2). Average degree of dominance was 1.50, suggesting that the fresh weight at the transplanting stage is controlled by overdominant genes. The correlation between Vr + Wr and Pr showed that dominant genes induced heavier weight. Fresh weight at four weeks after transplantation also showed a high hB2, a low hN2, and a very high average degree of dominance (3.10), indicating the presence of a higher grade of overdominance. Plant height at both growth stages also showed overdominance. Dominant genes induced an increase in plant height. Since the value of hN2 was higher for plant height than for fresh weight, it is considered that the contribution of additive effects was larger for plant height than for fresh weight. Furthermore, it was suggested that frequency of the dominant alleles which induced an increase in fresh weight was high in the parental lines derived from the 'Senju' group, while the frequency of the alleles which induced an increase in plant height was high in the lines derived from the 'Kujyo' group. The present study revealed that the characters related to the initial growth rate showed an overdominant inheritance. This indicates that the F1 hybrid breeding method could be applied to bunching onion to develop cultivars with vigorous seedling growth, suitable for plug nursery systems.
