Nitrogen limitation affects carbon and nitrogen metabolism in mung bean (Vigna radiata L.)

Studying the effects of nitrogen limitation on carbon, nitrogen metabolism, and nutrient uptake of mung bean is a scientific issue. In this study, urea (CO(NH₂)₂, 125 kg hm^-2) was applied at the V2, V6, R1, R2, and R4 stages, respectively, to ensure sufficient N resources during the growth process of mung beans. This study found that nitrogen limitation inhibited mung bean photosynthesis and reduced photosynthetic efficiency, which was manifested by reducing Pn (net photosynthetic rate), Gs (stomatal conductance), Tr (transpiration rate), and Ci (intercellular carbon dioxide concentration). Second, nitrogen limitation reduced N metabolism-related enzyme activity, such as NR (nitrate reductase), GOGAT (glutamate synthase), and GDH (glutamate dehydrogenase), indicating that nitrogen limitation inhibited the process of nitrogen metabolism, reducing nitrogen assimilation. Meanwhile, topdressing N fertilizer can promote the P and K uptake, and improve the partial factor productivity of P and K, which suggests that nitrogen limitation reduced P and K use efficiency. In addition, this study found that Lvfeng5 responded more significantly to nitrogen fertilizers, and had higher nitrogen use efficiency or better adaptability compared with Lvfeng2. This study provided valuable insights into the physiological and metabolic responses of mung beans to nutrient deficiency.