Nuclear genetic control of variation in simazine tolerance in oilseed brassicas. II : Selection for simazine tolerance in a Brassica campestris population

Substantial variation in two measures of simazine tolerance-simazine tolerance index (STI) and percentage seedling survival at 15 days after sowing-was observed in a controlled environment among half-sib families obtained from a highly variable cross-composite population of Brassica campestris. Analyses of this variation yielded narrow-sense heritability estimates of 0.57 and 0.53 for STI and seedling survival respectively, indicating that there would be some scope for improving simazine tolerance through selection within this population. Three generations of selection in a hydroponic screening system was accompanied by a three-fold increase in mean STI and a substantial increase in the proportion of plants without simazine toxicity symptoms. A field study showed that the tolerant population obtained from this selection program had a much higher yield than a B. campestris population carrying cytoplasmic gene(s) for resistance in handweeded plots and plots treated with 1000 ml/ha simazine. Higher B. campestris yields should be obtained by increasing simazine concentration for more effective weed control and selecting further for a higher level of simazine tolerance. Crosses of selected tolerant B. campestris plants with the commercial B. napus cultivar Wesbrook showed that introgression of genes from B. campestris can significantly improve the simazine tolerance of the B. napus recipient parent. This is an important result as B. napus has a much higher yield potential than B. campestris in southern Australian growing areas where simazine tolerant cultivars are urgently needed.