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

Abstract

Ammonium (NH4+) and nitrate (NO3−) are the two predominant inorganic nitrogen (N) forms available to crops in agricultural soils. However, little is known about how the NH4+:NO3− ratio affect the growth of Brassica napus. Here, we investigated the impact of five NH4+:NO3− 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 NO3− as a N source extremely improved rapeseed growth compare to NH4+. However, the best growth of the N-inefficient genotype was observed under 75:25 NH4+/NO3− ratio, while it happens for the N-efficient genotype only under the sole NO3− environment. The low-NUE genotype exhibited a more developed root system, higher photosynthetic capacity, higher nutrient accumulation, and better NH4+ uptake ability under the 75:25 NH4+/NO3− ratio, resulting in a decrease of malondialdehyde (MDA) in root. However, the high-NUE genotype performed better in the above aspects under the NO3−-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 NH4+/NO3− 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.