The rapeseed genotypes with contrasting NUE response discrepantly to varied provision of ammonium and nitrate by regulating photosynthesis, root morphology, nutritional status, and oxidative stress response.

Ammonium (NH) and nitrate (NO) are the two predominant inorganic nitrogen (N) forms available to crops in agricultural soils. However, little is known about how the NH:NO ratio affect the growth of Brassica napus. Here, we investigated the impact of five NH:NO ratios (100:0, 75:25, 50:50, 25:75, 0:100) on plant growth, photosynthesis, root morphology, ammonium uptake, nutritional status, oxidative stress response, and relative expression of genes involved in these processes in two rapeseed genotypes with contrasting N use efficiency (NUE). Application of NO as a N source extremely improved rapeseed growth compare to NH. However, the best growth of the N-inefficient genotype was observed under 75:25 NH/NO ratio, while it happens for the N-efficient genotype only under the sole NO environment. The low-NUE genotype exhibited a more developed root system, higher photosynthetic capacity, higher nutrient accumulation, and better NH uptake ability under the 75:25 NH/NO ratio, resulting in a decrease of malondialdehyde (MDA) in root. However, the high-NUE genotype performed better in the above aspects under the NO-only condition. Nitrate decrease MDA by reducing the activities of superoxide dismutase, peroxidase, and catalase in root of the N-efficient genotype. Moreover, significant differences were detected for the expression levels of genes involved in N uptake and oxidative stress response between the two genotypes under two NH/NO ratios. Taken together, our results indicate that the N-inefficient rapeseed genotype prefers mixed supply of ammonium and nitrate, whereas the genotype with high NUE prefers sole nitrate environment.