Nitrogen Application Promotes Drought-Stressed Sugar Beet Growth by Improving Photosynthesis, Osmoregulation, and Antioxidant Defense

Drought stress is the most common abiotic stress, limiting sugar beet production in arid regions. However, at present, how nitrogen (N) application affects sugar beet growth during drought stress is still unclear. In this field experiment, moderate drought stress was performed on sugar beet plants at the canopy growth stage (45 similar to 90 days after emergence (DAE)), and different doses of N (0 kg ha(-1) (N0), 150 kg ha(-1) (N1, normal N application rate), and 225 kg ha(-1) (N2, high N application rate)) were applied. After that, the leaf photosynthesis, antioxidant defense system, osmoregulation, and transcriptome were analyzed. The results showed that N supply led to upregulated expression of key genes related to chlorophyll metabolism (BVRB_5g114540, BVRB_5g105530, BVRB_1g021010, BVRB_2g047600, BVRB_005040) and antioxidant defense (BVRB_8g194440, BVRB_6g130380, BVRB_1g008510). This further led to the increases in chlorophyll content (Chl a + b) (65.32 similar to 78.77%), superoxide dismutase (SOD) (42.01 similar to 95.47%), and ascorbic peroxidase (APX) (42.71 similar to 76.77%) activity, and solid protein (SP) content (45.21 similar to 49.75%) (p < 0.05). Besides, N supply increased the dry matter accumulation by 48.44 similar to 84.40% (p < 0.05) by increasing electron flow used for ribulose biphosphate (RuBP) carboxylation (J(C)), effective quantum yield of CO2 fixation (CYRILLIC CAPITAL LETTER EFCO2), and net photosynthetic rate (Pn). However, except for SOD and APX activity, there were no significant differences in other indexes between the N1 and N2 groups. Therefore, under drought stress, the application of 150 kg N ha(-1) can improve sugar beet leaf photosynthesis, antioxidant defense, and osmoregulation, thus improving sugar beet tolerance to drought stress and promoting plant growth.