Root Growth Plasticity and Nitrogen Metabolism Underpin Prolonged Cold Stress Tolerance at Tillering Stage in Japonica Rice

Cold stress impairs crop productivity through cascading inhibition of root growth, nitrogen (N) metabolism, and photosynthesis, yet the systematic linkages among these physiological disruptions remain poorly understood. It is crucial to elucidate the mechanisms by which cold-tolerant varieties maintain root growth and N-metabolizing enzyme homeostasis. This two-year field study investigated how cold duration at the tillering stage impacted root traits, N metabolism, photosynthesis, and their relationships with the yield of two japonica rice varieties differing in cold tolerance. A cold-tolerant (Dongnong 428) and a cold-sensitive variety (Songjing 10) were grown in a paddy field for two consecutive growing seasons in 2021 and 2022. Cold water (15 °C) was irrigated for 0 (denoted as D0), 5 (D5), 10 (D10), and 15 days (D15) during the tillering stage. Compared to D0, cold-water treatments significantly reduced root traits and total dry weight of both varieties. Cold stress significantly impaired N metabolism and photosynthesis, leading to significant reductions in N efficiency. The magnitude of these changes turned to greater with cold-water treatment duration. Dongnong 428 showed stronger cold tolerance, attributed to its maintenance of superior root traits and photosynthetic performance, as well as higher activities of enzymes in the roots, which sustained N assimilation and utilization. These factors primarily contributed to Dongnong 428 achieving 11.6–20.9% higher yields compared to Songjing 10. Cold stress during the tillering stage disrupts root growth and photosynthesis, impairs plant N acquisition ability, resulting in substantial yield loss. Cold-tolerant varieties maintain superior root morphology/functionality and photosynthetic performance.