Genetic control of the pre-anthesis development of spring rape (Brassica napus L.). I. Diallel analysis of variation in the field

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A l0 x 10 complete diallel cross of spring rape cultivars was sown at three different times in the field to obtain basic information on the genetic control of pre-anthesis development. Analyses of variance of the diallel cross data indicated that both duration of the vegetative phase and leaf number at initiation were highly heritable in all sowings although dominance effects were also significant. No significant reciprocal differences were detected. Variance v. covariance (Vr–Wr) graphs for duration of the vegetative phase indicated that neither gene interaction nor correlated gene distribution was important. The order of parents along the regression line varied greatly between sowings. Late-flowering parents were found to possess a much higher frequency of recessive genes in the April and September sowings than in the June sowing when temperatures during early development were generally lower. It is suggested that this variation in the disposition of parents in the Vr–Wr graphs may reflect differences in genes for vernalization requirement which are expressed most strongly under higher temperatures. In the case of number of leaves formed prior to initiation, the slope of the regression line deviated significantly from unity in the first and second sowings, but the magnitude of this deviation declined with successive delays in sowing. Gene interaction and/or correlated gene distributions were apparently significant under certain conditions. The order of parents along the regression line also varied with sowing date, but not in as regular a manner as with duration of the vegetative phase. Differences in duration of the vegetative phase and leaf number between the Japanese cultivar Isuzu and all other parents in the September sowing appeared to be determined by one gene, or at most a small number of major genes.