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

Abstract

The capacity for vernalisation varies among genotypes of spring rape, Brassica napus L., and any vernalisation response reduces their suitability for cultivation in the Mediterranean environment of south-western Western Australia. The genetic basis of this variation was studied in the F₁ generation of a diallel cross among 5 inbred genotypes derived from a cross between 2 cultivars differing in sensitivity to vernalisation. In non-vernalised plants the number of leaf nodes at flowering, time to flowering, stem height at flowering, and time to initiation of stem elongation showed substantial genetic variation, both additive and dominance effects being significant. The time to initiation of stem elongation was not vernalisation-sensitive, but for the other 3 characters the response to vernalisation showed significant genetic variation, largely additive. The genetic effects controlling the number of leaf nodes at flowering were clearly different from those controlling stem height at flowering, and hence could be manipulated independently by selection. Moreover, the genetic effects controlling these characters in the absence of vernalisation were different from those controlling the response to vernalisation. Discontinuities in the distributions of the parent means and other statistics for the number of leaf nodes at flowering suggested that major Mendelian genes may largely control this character and its response to vernalisation. Comparison of the number of leaf nodes at flowering in the presence and absence of a vernalising stimulus is therefore suggested as a selection criterion for the elimination of vernalisation response. However, in the absence of a vernalising stimulus, the time to flowering will be the character easiest to influence by selection.