Nitrogen efficiency in semi-dwarf and normal hybrids of oilseed rape

The cultivation of winter oilseed rape is characterized by high rates of nitrogen fertilization. This can cause large nitrogen surpluses after seed harvest of oilseed rape. Most of the rapeseed oil is used for the production of biofuels. New regulatory standards in the European Union demand a reduction in greenhouse gas (GHG) emissions from biofuels of 60 % (by 2018) compared to fossil fuels. Energy input to produce mineral nitrogen fertilizer is responsible for a high proportion of GHG emissions in the production chain of rapeseed oil. The environmental aspects and new legislative regulations illustrate the future challenge of maintaining current seed yields while using less nitrogen fertilizer than today. The development of oilseed rape cultivars with improved nitrogen use efficiency is one contribution to cope with this challenge. The use of semi-dwarf oilseed rape types might be a step towards this direction. The aim of this study was to analyze the agronomic performance of semi-dwarf and normal oilseed rape hybrids at low and high nitrogen supply. Several nitrogen related traits were analyzed with regard to nitrogen use efficiency. The genetic mechanisms controlling these agronomic and nitrogen related traits were analyzed by QTL mapping. A population of 150 oilseed rape hybrids, segregating in 75 semi-dwarf and 75 normal hybrids was developed from a DH population. DH lines were generated from the cross 'Alesi-bzh' x 'H30'. 'Alesi-bzh' is a dwarf line carrying the bzh dwarf allel. The DH population is segregating into dwarf and normal type lines. The hybrids were tested in field experiments in Einbeck and Gottingen at low nitrogen supply (without nitrogen fertilization) in 2010/2011. In 2011/2012 and 2012/2013 108 hybrids were tested at the same locations at low and high nitrogen supply. On average, semi-dwarf hybrids produced a significantly higher seed yield (2.4 dt/ha) at low nitrogen supply compared to normal hybrids. At high nitrogen supply both growth types produced similar seed yields with a non-significant difference of 0.2 dt/ha. In general semi-dwarf hybrids formed significantly less straw in all environments than normal hybrids. The combinations of higher or similar seed yields and less straw yield resulted in a higher harvest index of semi-dwarf hybrids at both nitrogen supply levels. Semi-dwarfs had a significantly higher nitrogen utilization efficiency compared to normal hybrids at low and high nitrogen supply. That means they produced more kg seed per kg nitrogen taken up. The difference between the growth types was 2.7 and 1.1 kg seeds per kg nitrogen taken up at low and high nitrogen supply, respectively. Therefore, semi-dwarf hybrids revealed a higher nitrogen use efficiency at both nitrogen levels. The variation in nitrogen uptake efficiency contributed more to genetic variation in nitrogen use efficiency than variation in nitrogen utilization efficiency. QTL mapping identified a QTL on linkage group (LG) A06 with pleiotropic effects on seed yield, straw yield, harvest index and nitrogen use efficiency at low and high nitrogen supply. Due to the position of the QTL it is most likely the effect of the dwarf gene Bzh itself. Another QTL with pleiotropic effects on seed yield, harvest index and nitrogen use efficiency was identified on LG A07 at low nitrogen supply. Based on the results of the present study semi-dwarf hybrids produce higher seed yields and show a higher nitrogen use efficiency than normal type hybrids, especially at nitrogen deficient conditions.