Diallel analysis of vernalisation responses in spring rape (Brassica napusL.): a basis for adaptation to a Mediterranean environment

The capacity for vernalisation varies among genotypes of spring rape, Brass ica napus L., and any vernalisation response reduces their suitability for cultivation in the Mediterranean environment of south-western Western Austr alia. The genetic basis of this variation was studied in the F-1 generation of a diallel cross among 5 inbred genotypes derived from a cross between 2 cultivars differing in sensitivity to vernalisation. In non-vernalised pla nts the number of leaf nodes at flowering, time to flowering, stem height a t flowering, and time to initiation of stem elongation showed substantial g enetic variation, both additive and dominance effects being significant. Th e time to initiation of stem elongation was not vernalisation-sensitive, bu t for the other 3 characters the response to vernalisation showed significa nt genetic variation, largely additive. The genetic effects controlling the number of leaf nodes at flowering were clearly different from those contro lling stem height at flowering, and hence could be manipulated independentl y by selection. Moreover, the genetic effects controlling these characters in the absence of vernalisation were different from those controlling the r esponse to vernalisation. Discontinuities in the distributions of the paren t means and other statistics for the number of leaf nodes at flowering sugg ested that major Mendelian genes may largely control this character and its response to vernalisation. Comparison of the number of leaf nodes at flowe ring in the presence and absence of a vernalising stimulus is therefore sug gested as a selection criterion for the elimination of vernalisation respon se. However, in the absence of a vernalising stimulus, the time to flowerin g will be the character easiest to influence by selection.