Appropriate NH₄⁺: NO₃^- ratio improves low light tolerance of mini Chinese cabbage seedlings

In northwest of China, mini Chinese cabbage (Brassica pekinensis) is highly valued by consumers, and is widely cultivated during winter in solar-greenhouses where low light (LL) fluence (between 85 and 150 μmol m^-2 s^-1 in day) is a major abiotic stress factor limiting plant growth and crop productivity. The mechanisms with which various NH₄⁺: NO₃^- ratios affected growth and photosynthesis of mini Chinese cabbage under normal (200 μmol m^-2 s^-1) and low (100 μmol m^-2 s^-1) light conditions was investigated. The four solutions with different ratios of NH₄⁺: NO₃^- applied were 0:100, 10:90, 15:85 and 25:75 with the set up in a glasshouse in hydroponic culture. The most appropriate NH₄⁺: NO₃^- ratio that improved the tolerance of mini Chinese cabbage seedlings to LL was found in our current study.

Under low light, the application of NH₄⁺: NO₃^- (10:90) significantly stimulated growth compared to only NO₃^- by increasing leaf area, canopy spread, biomass accumulation, and net photosynthetic rate. The increase in net photosynthetic rate was associated with an increase in: 1) maximum and effective quantum yield of PSII; 2) activities of Calvin cycle enzymes; and 3) levels of mRNA relative expression of several genes involved in Calvin cycle. In addition, glucose, fructose, sucrose, starch and total carbohydrate, which are the products of CO₂ assimilation, accumulated most in the cabbage leaves that were supplied with NH₄⁺: NO₃^- (10:90) under LL condition. Low light reduced the carbohydrate: nitrogen (C: N) ratio while the application of NH₄⁺: NO₃^- (10:90) alleviated the negative effect of LL on C: N ratio mainly by increasing total carbohydrate contents.

The application of NH₄⁺:NO₃^- (10:90) increased rbcL, rbcS, FBA, FBPase and TK expression and/or activities, enhanced photosynthesis, carbohydrate accumulation and improved the tolerance of mini Chinese cabbage seedlings to LL. The results of this study would provide theoretical basis and technical guidance for mini Chinese cabbage production. In practical production, the ratio of NH₄⁺:NO₃^- should be adjusted with respect to light fluence for successful growing of mini Chinese cabbage.