Seed yield of canola (Brassica napus L.) is determined primarily by biomass in a high-yielding environment

The better performance of hybrid canola compared with open-pollinated triazine-tolerant canola can be associated with greater biomass and harvest index. We compared several hybrid and open-pollinated canola cultivars in field conditions to (i) quantitatively analyse yield formation and identify the key drivers of yield formation process; (ii) investigate biomass accumulation and partitioning and evaluate the relative importance of biomass, harvest index and yield components. Six elite varieties, two from each of the three types (triazine-tolerant (TT), hybrid TT, and hybrid imidazolinone-tolerant (IT) or conventional (CV) (hybrid IT/CV)) of canola, were grown under the optimum crop management in the 3 years from 2009 to 2011 in the high-rainfall zone of south-western Australia. Leaf area, specific leaf area, light interception, biomass, seed yield and yield components were measured at key growth stages to determine biomass accumulation, crop growth rate (CGR), radiation-use efficiency and to investigate the relationship between yield, biomass, CGR, specific leaf area, yield components and harvest index. Hybrid IT/CV canola grew more vigorously with thicker leaves and greater leaf area, allocated more biomass into leaves, intercepted more radiation, produced higher biomass in the vegetative stage and maintained its biomass superiority throughout the whole crop cycle. It had radiation-use efficiency of 1.74 g MJ m–² photosynthetic active radiation, 28% higher (P < 0.001) than TT canola (1.41 g MJ m–² photosynthetic active radiation) and 16% higher (P < 0.001) than hybrid TT canola (1.52 g MJ m–² photosynthetic active radiation). The average CGR for hybrid IT/CV canola (12.1 g m–² day–¹) was 32% higher than that of TT canola (9.2 g m–² day–¹) from budding to the beginning of pod filling. Hybrid IT/CV canola produced 38% higher seed yield than TT canola in favourable growing conditions (2009, 2011). However, there was no yield difference between the hybrid IT/CV, hybrid TT, and TT canola in the drought year (2010). The number of pods m–² and seeds m–² was highly associated with biomass at vegetative, budding, flowering, podding and maturity and CGR from budding to podding. High yield in hybrid canola was attributed mainly to higher biomass from each phenological phase from the vegetative stage to maturity and not to improved harvest index.