Genetic control of root shape at different growth stages in radish (Raphanus sativus L.)

Since the changes in the root shape of radish (Raphanus sativus L.) during development cannot be easily observed, it is often difficult to determine the suitable time after sowing for the selection of genotypes with desirable root shapes. To determine the optimum time for selection of radish root in breeding, we conducted diallel analyses of the root shape at different growth stages and investigated the changes in the mode of inheritance of the root shape. A set of diallel crosses of 6 inbred lines was harvested 6 times from 40 to 75 days after sowing. The major shape characteristics, associated with principal components of elliptic Fourier descriptors (EFDs), included the ratio of length to width (AP1), the bluntness of the distal part (AP2), and the curvature of the middle part (BP1). ANOVA indicated that the growth stages, genetic effects, and their interaction were significant in all 3 characteristics. The interaction did not affect appreciably the difference in the mode of inheritance of AP1 among growth stages, suggesting that the time of harvest did not affect the selection of this characteristic. The interaction caused a certain degree of heterogeneity in the ordering of genotypes and in the mode of inheritance of AP2 and BP1 at different growth stages. In AP2, the degree of dominance increased at the later growth stages. In BP1, the genetic parameters varied, but showed no apparent trend with the growth process. The results suggest that AP1 can be efficiently selected at an early stage, i.e., at the beginning of thickening growth. For AP2 and BP1, the selection at a stage around the normal harvest time is recommended. However, the fluctuations in the time of selection within a 2-week period around the normal harvest time did not affect the selection of the characteristics, because the interaction was not significant within this period.