Enhanced glutamine synthetase activity improves grain nitrogen assimilation and low-nitrogen tolerance in sweet corn

While reducing nitrogen (N) input is essential for sustainable agriculture, maintaining yield under low-N conditions remains a major challenge, particularly for N-sensitive crops such as sweet corn. However, the physiological mechanisms underlying N uptake, allocation, and assimilation in low-N-tolerant sweet corn genotypes remain poorly understood. A two-year field experiment (2022-2023) was conducted using two sweet corn cultivars (MT6855 and TYH6) under four N application rates (0, 90, 180, and 270 kg N ha^-1). Low N input significantly reduced the low-N tolerance index, agronomic traits, N accumulation, and N assimilation in both cultivars. Across two years, MT6855 exhibited a higher low-N tolerance index than TYH6 by 164.2 %, 30.8 %, -2.5 %, and 0 % under 0, 90, 180, and 270 kg N ha^-1, respectively. MT6855 also showed significantly greater N concentration and accumulation in the ear, together with higher N harvest index and partial factor productivity, despite its comparatively lower plant and ear heights. Notably, MT6855 consistently maintained greater grain free amino acid concentration, as well as higher nitrate reductase (NR) and glutamine synthetase (GS) activities across all N levels and both years. Random forest analysis identified GS (26.6 %) and NR (24.3 %) activities as the primary contributors to low-N tolerance. These findings demonstrate that MT6855 achieves improved low-N tolerance through enhanced N assimilation and preferential allocation to the ear, providing valuable insights for breeding and cultivation strategies to improve low-N tolerance in sweet corn.