Physiological response to low-nitrogen stress and comprehensive evaluation in four rice varieties

Rice (Oryza sativa L.) research has rarely focused on the response to low-nitrogen stress in different subtypes previously and lacked a low-nitrogen tolerance evaluation system. Here, we investigated the physiological characteristics under moderate and low-nitrogen stress conditions in two japonica cultivars (NG46 and NG9108) and two indica cultivars (LYP9 and 9311). Using subordinate function analysis and principal component analysis, the low-nitrogen tolerance of four rice varieties was comprehensively evaluated; stomatal conductance, total carotenoid content, and nitrate reductase NR activity were taken as the low-nitrogen tolerance evaluation system. Among the four rice cultivars, NG46 and LYP9 had significant advantages in photosynthetic gas-exchange capacity, optimizing the balance between light-harvesting capacity, the ratio of reaction center inactivation, the magnitude of decrease in heat dissipation, and nitrogen-metabolism enzyme activities. The results investigated the physiological mechanisms of rice adaptation to low-nitrogen stress and offered a reliable method for assessing low-nitrogen tolerance in rice.