Abstract
BackgroundIn 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.ResultsUnder 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.ConclusionsThe 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.
Highlights
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 is a major abiotic stress factor limiting plant growth and crop productivity
Under normal light condition, when compared with the control (NH4+:NO3− = 0:100), leaf area, canopy spread, fresh weight, dry weight and chlorophyll content in plants fed with NH4+:NO3− (15:85) were significantly increased by 33.8, 30.5, 77.9, 72.9 and 33.7%, respectively (Table 1)
Application of NH4+:NO3− (10:90) under LL condition led to a significant increase in total chlorophyll content, almost reaching the levels of plants under normal light
Summary
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. Conclusions: 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. Calvin cycle contains 11 different enzymes which catalyze 13 reactions in the three phases of carboxylation, reduction and regeneration. It is initiated by the enzyme ribulose-1, 5bisphosphate carboxylase oxygenase (Rubisco) which catalyses the carboxylation of the CO2 acceptor molecule, ribulose-1, 5-bisphosphate (RuBP). Photosynthesis is limited mainly by light intensity, light use efficiency and CO2 assimilation capacity among other factors
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