Partial replacement by ammonium nutrition enhances Brassica napus growth by promoting root development, photosynthesis, and nitrogen metabolism

Abstract

Nitrogen (N) is crucial for plant growth, available primarily as nitrate (NO₃⁻) and ammonium (NH₄⁺). However, its presence in soil is often limited, necessitating strategies to augment N availability. This study delves into the enigmatic interplay between NO₃⁻ and NH₄⁺ in fostering the growth of Brassica napus, an important oil crop worldwide. Here, we examined the growth responses of 49 B. napus varieties to five NH₄⁺:NO₃⁻ ratios (12:0, 9:3, 3:9, 1:11, 0:12). In general, the biomass of 49 rapeseed varieties increased with the decrease of NH₄⁺ to NO₃⁻ ratios in the growth environment. However, different varieties may respond diversely to the mixed N sources, or sole NO₃⁻ or NH₄⁺ condition. For some cultivars, the mixed N supply significantly enhanced the plant growth compared with the sole NO₃⁻ conditions. Thus, we further investigate the morphological, physiological and molecular response of rapeseed to the mixed N source condition using sole NO₃⁻ as a control. The results show that partial replacement by ammonium nutrition in the environment can promote rapeseed root development, net photosynthetic rate and NO₃⁻ reduction compared to NO₃⁻-only conditions. Using transcriptome analysis, we found a total of 399 and 465 genes which were differentially expressed in root and shoot under A1N11 compared to A0N12 treatments, respectively. Genes involved in photosynthesis, N uptake and assimilation were upregulated by mixed N supplies. These findings highlight that the mixed N supply primarily stimulates B. napus growth by enhancing root development, photosynthesis and N metabolism in the shoot. Such insights are crucial for optimizing N form selection in B. napus to enhance plant performance and N use efficiency.