Nitric Oxide Is Involved in the Regulation of the Ascorbate-Glutathione Cycle Induced by the Appropriate Ammonium: Nitrate to Mitigate Low Light Stress in Brassica pekinensis

Low light intensity is common in northern China due to fog or haze, and causes stress for crop plants. To solve the problem of low light intensity stress on the growth and development of vegetable crops in China, new cropping strategies must be developed. We previously showed that an appropriate ratio of ammonium and nitrate (NH4+:NO3-) can alleviate the effect of low light stress on plants, although it is not clear what mechanism is involved in this alleviation. We propose the hypothesis that an appropriate ammonium/nitrate ratio (10:90) can induce NO synthesis to regulate the AsA-GSH cycle in mini Chinese cabbage seedlings under low light intensity. To test the hypothesis, we conducted a series of hydroponic experiments. The results indicated that, under low light intensity conditions, appropriate NH4+:NO3- (N, NH4+:NO3- = 10:90) decreased the contents of malondialdehyde (MDA), hydrogen peroxide (H2O2), and superoxide anion (O-2(-)) in leaves compared with nitrate treatment. Exogenous nitric oxide (SNP) had the same effects on MDA, H2O2, and O-2(-). However, with the addition of a NO scavenger (hemoglobin, Hb) and NO inhibitors (N-nitro-l-arginine methyl ester, L-NAME), NaN3 (NR inhibitor) significantly increased the contents of MDA, H2O2, and O-2(-). The application of N solution enhanced the AsA-GSH cycle by increasing the activities of ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and ascorbate oxidase (AAO), compared with control (NH4+:NO3- = 0:100). Meanwhile, exogenous SNP significantly increased the above indicators. All these effects of N on AsA-GSH cycle were inhibited by the addition of Hb, L-NAME and NaN3 in N solution. The results also revealed that the N and SNP treatments upregulated the relative expression level of GR, MDHAR1, APXT, DHAR2, and AAO gene in mini Chinese cabbage leaves under low light stress. These results demonstrated that the appropriate NH4+:NO3- (10:90) induced NO synthesis which regulates the AsA-GSH cycle in mini Chinese cabbage seedlings under low light stress.