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

Nitrogen (N) is crucial for plant growth, available primarily as nitrate (NO) and ammonium (NH). However, its presence in soil is often limited, necessitating strategies to augment N availability. This study delves into the enigmatic interplay between NO and NH 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 NH:NO ratios (12:0, 9:3, 3:9, 1:11, 0:12). In general, the biomass of 49 rapeseed varieties increased with the decrease of NH to NO ratios in the growth environment. However, different varieties may respond diversely to the mixed N sources, or sole NO or NH condition. For some cultivars, the mixed N supply significantly enhanced the plant growth compared with the sole NO conditions. Thus, we further investigate the morphological, physiological and molecular response of rapeseed to the mixed N source condition using sole NO as a control. The results show that partial replacement by ammonium nutrition in the environment can promote rapeseed root development, net photosynthetic rate and NO reduction compared to NO-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.