Interspecies interaction for nitrogen use efficiency via up-regulated glutamine and glutamate synthase under wheat-faba bean intercropping

Glutamine synthetase (GS) and the glutamate synthase (GOGAT) cycle are closely related to plant nitrogen (N) uptake, translocation, and remobilization, but little attention has been paid to their role in intercropping systems. A two-factor field trial involving two planting patterns (mono-cropped wheat [MW] and wheat intercropped with faba bean) and four N levels (N0, N1, N2, and N3 with respective application rates of 0, 90, 180, and 270 kg ha−1 for wheat) were studied. Nitrogen accumulation and allocation were determined, and the GS and GOGAT activities and their gene expression in the leaves were analyzed. The results showed that wheat total aboveground N accumulation and grain N accumulation were increased when wheat was intercropped with faba bean. Intercropped wheat (IW) presented higher N translocation efficiency (NTE) than MW (except for the N3 level during 2018–2019) during the reproductive growth stage. Thus, the grain yield (except for the N3 level), N harvest index (NHI), and apparent N recovery rate (ANR) were higher in IW than in MW. In addition, total GS, Fd-GOGAT, and NADH-GOGAT activities in the wheat leaves were increased by 27 %–42 %, 17 %–44 %, and 15 %–25 %, respectively, under the N0, N1, and N2 levels from the stem elongation to filling stages when wheat was intercropped with faba bean. On average, GS1, GS2, Fd-GOGAT, and NADH-GOGAT gene expression in the IW flag leaves was 1.4–4.3 times, 1.2–3.6 times, 1.2–2.2 times, and 1.4–6.7 times higher, respectively, than that in MW. Both GS and GOGAT activity and gene expression were tightly related to straw N accumulation and NTE rather than grain N accumulation based on simple correlation analysis. In addition, wheat total N accumulation at maturity was more sensitive to GS and GOGAT activity and gene expression in the flag leaves during the heading, flowering, and filling stages, and ANR only linked with GS and GOGAT activity at the filling stage. Hence, it could be concluded that up-regulated GS and GOGAT activity and gene expression induced by intercropping during the reproductive growth stages contributed to higher NTE and NUE in intercropping. These findings suggest that interspecies interactions modulated GS and GOGAT, which play a crucial role in the intercropping advantage and provide insight into the mechanism of enhanced NUE under intercropping.