The physiological basis of genotypic differences in nitrogen use efficiency in oilseed rape (Brassica napus L.)

Four field experiments were performed in the UK in harvest seasons 2007 and 2008. Each experiment consisted of 10 winter oilseed rape varieties grown at a low level of available nitrogen (N) and at a high level of available N intended to replicate commercial practice. A combined analysis of three of the experiments with significant yield differences between the N treatments showed a significant interaction between N availability and variety for yield. Across these three experiments the proportion of yield lost when crops were grown at low N compared with high N ranged from 0.23 to 0.35 among varieties. The proportion of yield lost at low N was negatively associated with crop N uptake. There was also an interaction between N supply and variety for N use efficiency (kg of seed dry matter/kg available N) within these three experiments. Varietal differences in yield at low N correlated most closely, and positively, with crop N uptake, final crop dry matter and seeds/m2, but not N utilisation (kg seed/kg N uptake). Every additional kilogram of N taken up by the crop increased yield at low N by 0.020 t/ha. The amount of N taken up after flowering was the most important phase of N uptake for determining yield differences between the varieties, with every additional kilogram of N taken up after flowering associated with a yield increase of 0.016 t/ha. Each additional 1000 seeds/m2 was associated with an additional 1.4 kg N/ha taken up after flowering. There was no correlation between yield at low N or late N uptake and individual seed size.