Physiological mechanisms for increased grain protein content under experimental warming conditions in double rice cropping system

The underlying mechanisms of warming effects on rice protein content have not been thoroughly investigated in the double rice cropping system. Here, a 2-year field experiment was conducted to clarify the physiological mechanisms related to nitrogen (N) uptake and assimilation under warming. The results showed that warming significantly increased albumin (16.7%), globulin (2.9%), and glutelin (26.1%) contents in early rice, while it increased prolamin (6.3%) and glutelin (13.4%) contents in late rice. The increased protein content under warming was associated with the elevated N concentration in the panicle, which was partly caused by the enhanced N uptake in early rice but not in late rice. A 15 N pot experiment demonstrated that warming improved total N uptake from soil in both early and late rice; however, N uptake from fertilizer was increased and decreased by warming in early and late rice, respectively, resulting in improved total N uptake in early rice but not in late rice. Additionally, our results confirmed that increased soil net N mineralization rate and root activity contributed to the increased N uptake from soil under warming for both early and late rice. Furthermore, the activities of key enzymes, including glutamine synthetase, glutamate synthase, glutamic-oxaloacetic transaminase, and glutamate-pyruvate transferase, were increased, while protein hydrolysis was suppressed by warming in both early and late rice. Our findings indicate that the increase in protein components under warming conditions is due to improved N uptake in early rice and increased protein synthesis in both early and late rice.