Canola genotypes differ in growth and yield at various ammonium:nitrate ratios and alternating ammonium and nitrate supplies

Canola (Brassica napus L.) is an important oil crop that requires high nitrogen rates, but is sensitive to ammonium (NH4+-N) toxicity. To determine whether genotypes exhibit variation in NH4+-N toxicity, and whether NH4+-N toxicity may be ameliorated by mixing nitrate (NO3−-N) and NH4+-N in various ratios, or by providing NH4+-N and NO3−-N in various sequences. Using glasshouse experiments, we tested the growth of two NH4+-N toxicity-sensitive genotypes [SC03-1 (codenamed G30) and Zhongshuang4B (G3)] and two NH4+-N toxicity-resistant genotypes [Tarcoola-22 (G26) and ZY001 (G16)] under five NH4+:NO3− ratios at the 1.5 vegetative stage and three NH4+:NO3− ratios at seed maturity. We then selected the most NH4+-N-resistant (ZY001) and -sensitive (SC03-1) genotypes to grow at alternating nitrogen form supplies until the 1.5 vegetative stage. In general, ZY001 had the highest growth and yield, especially at 50:50 and 25:75 NH4+:NO3− ratios. NH4+-N toxicity was mitigated when NH4+-N supply was combined with relatively high concentrations of NO3−-N. When alternating NH4+-N and NO3−-N supply, canola genotypes consistently performed poorly with NH4+-N supply and improved with NO3−-N. Higher concentrations of nitrate relative to NH4+-N, and the use of nitrate alone (either before or after NH4+-N fertilisation), were associated with improved canola growth and physiological parameters under glasshouse conditions. The genotype ZY001 exhibited superior performance among the four genotypes studied. Using nitrate instead of NH4+-N as a nitrogen source may improve canola growth under glasshouse conditions. The ZY001 genotype showed tolerance to NH4+-N toxicity and may be useful for future studies under field conditions.