Growth And Quality Of Pakchoi (Brassica Chinensis L.) As Affected By Inorganic And Organic Nitrogen Nutrition And Its Mechanisms

This study was carried out to determine the effect of inorganic nitrogen and organic nitrogen on growth, quality and physiological mechanisms of pakchoi {Brassica chinensis L.) under normal solution culture, wholly and partially sterile culture. The results were as follows:1. Effect of spraying amino acids on growth, nitrate content and quality in the shoot of pakchoi in both summer and autumnSpraying three amino acids (Glu, Gln, Gly) could significantly enhance yield of shoot and Gln was the best. Better effects on increasing yield of shoot were observed when amino acids were sprayed in summer than in autumn. The nitrate content of shoot decreased obviously when spraying amino acids, especially Glu in two seasons. Chlorophyll contents were improved by spraying three amino acids and the effects were better in summer than autumn, too. Of three amino acids, Gln could increase P concentration of shoot in summer and K concentration in autumn significantly. Glu and Gly significantly increased Ca and Fe concentration of shoot, but Glu, Gln and Gly significantly decreased Zn concentration in summer. Gly increased Fe concentration of shoot and Gly or Glu increased Zn concentration significantly, but Gln or Gly decreased Ca concentration and Gln decreased Zn concentration significantly in autumn.2. Effect of inorganic nitrogen and organic nitrogen on growth, quality and physiological characters of pakchoi under wholly sterile sand cultureExcept that NH₄⁺-N significantly reduced root fresh weight, all nitrogen sources significantly increased shoot and root fresh weight compared to no nitrogen treatment (CK). The trend of shoot and root dry weight was similar to shoot and root fresh weight. Of three nitrogen sources, effect of Gln-N was better than NH₄⁺-N and worse than NO3-N. The different nitrogen sources had different effect on root morphology.NO₃^--N treatment significantly increased the nitrate-nitrogen accumulation inwhole plant (minus seed') compared to CK, NH4+-N or Gln-N. There were no significant differences between CK, NH4+-N, and Gln-N. This indicated that Gln-N and NH/-N absorbed externally was not converted to internal nitrate-nitrogen. Compared to CK, all treatments significantly enhance total nitrogen and soluble protein content in shoot and root. Of three nitrogen sources, effect of NEU+-N or Gln-N was better than NO3"-N treatment. Compared to CK, NH4+-N or Gln-N significantly increased free amino acid content in shoot, while NOs'-N had no significant effect;NH4+-N significantly increased free amino acid content in root, while Gln-N or NCV-N treatment significantly decreased it. Compared to CK, all treatments significantly increased chlorophyll a, chlorophyll b, carotenoid and chlorophyll a+b content in leaf. Of three nitrogen sources, effect of Gln-N was best. Compared to CK, three nitrogen sources all significantly decreased soluble sugar and starch content in shoot, but three nitrogen sources had no significant effect on them in root except that NH4+-N significantly decreased soluble sugar content in root.Compared to CK, three nitrogen sources all significantly decreased GS activity in shoot and root, and increased GOT activity in shoot and root and GPT activity in root. However, NO3^-N significantly decreased shoot GPT activity, while NH4+-N and Gln-N had no significant effect on it. Compared to inorganic nitrogen (NH4+-N or N03"-N), amino acid-N significantly increased GOT and GPT activity in root, indicating that amino acid was transaminated in root at first, and then transformed into other amino acids, finally synthesized protein.3. Effect of different amino acid-N concentrations on growth, quality and physiological characters of pakchoi under partially sterile solution cultureShoot and root fresh weight increased when Gln-N concentration in the nutrient solution was 5mmol/L or less, and significantly decreased afterwards. Effects of Gln-N concentrations on shoot and root dry weight were similar to fresh weight. Different effects of different Gln-N concentrations on root morphology were observed.Total nitrogen and free amino acid content in shoot and root, shoot soluble protein content, shoot soluble sugar content and chlorophyll a, chlorophyll b,carotenoid and chlorophyll a+b content in leaf rose when Gln-N^ concentration in the nutrient solution increased. Although addition of Gln-N to the nutrient solution increased root soluble protein content and decreased shoot starch content compared to no nitrogen treatment, no significant difference in them was observed between different Gln-N concentrations. Gln-N concentrations in the nutrient solution unaffected soluble sugar content and starch content in root.No significant differences in shoot GOT activity were observed when Gln-N concentration in the nutrient solution was 5mmol/L or less, and significantly increased afterwards. However, the highest root GOT activity was observed when Gln-N concentration was 5mmol/L. Although addition of Gln-N to the nutrient solution significantly increased shoot GPT activity compared to no nitrogen treatment, no significant difference was observed between different Gln-N concentrations. Root GPT activity increased when Gln-N concentration in the nutrient solution increased, but no significant effect was observed between 5 and lOmmol/L Gln-N. Root GOT activity was averagely four times of shoot by the addition of three Gln-N concentrations and root GPT activity was averagely eight times, but root GOT and GPT activity was similar to shoot in no nitrogen treatment.4. Choices of optimal experimental condition of amino acids partially replacing NO3^-N in normal solution cultureThere was the biggest shoot fresh weight and dry weight with 12mmol/L NO3-N in the nutrient solution, while a further increase in NO3VN concentration reduced the shoot fresh weight and dry weight. The shoot nitrate content increased with increasing the N03"-N concentration in the nutrient solution except for 3mmol/L.Compared to 100%NO3"-N treatment (CK), the shoot fresh weight of plant was significantly increased when the 20% NO3"-N in the nutrient solution was replaced by Arg, while it was unaffected by Asn and Gin. However, fresh weight was decreased by most amino acids. Except for Cys, Gly, His and Arg, other amino acid treatments all depressed apparently nitrate content in the plant shoot relative to CK.Both Gin and Glu reduced shoot fresh weight and dry weight with decreasing NO3"-N/amino acid-N ratios. At 80:20 NO3'-N:Gln-N ratio, Gin had no significant121growth reduction effect on shoot fresh weight. Decreasing NCV-N/amino acid-N ratios reduced nitrate content in shoot, regardless of Glu or Gin.Addition of about 12mmol/L NO3"-N to the nutrient solution and the replacement of 20% NO3-N in nutrient solutions with Gln-N will significantly reduce nitrate content in shoots without significant reduction of crop yields. In addition, Glu-N used as a nitrogen source to replace the 20% NCV-N in nutrient solutions due to the fact that Glu had an important role in nitrogen metabolism.5. Effect of amino acids partially replacing NCV-N in nutrient solution on growth, nitrate content and quality of pakchoi under partially sterile solution cultureShoot dry weight significantly increased, but shoot fresh weight, root fresh and dry weight were unaffected when Gin replaced 20% NCV-N in the nutrient solution compared to 100% NCV-N (CK). However, shoot fresh weight dropped, root fresh weight significantly rose, while shoot and root dry weight were unaffected by Glu.Both Gin and Glu significantly reduced the nitrate content in shoot and root compared to CK. Both Gin and Glu significantly enhanced total nitrogen and soluble protein in shoot and root, root free amino acid content and chlorophyll SPAD values in leaf compared to CK. Only Gin significantly enhanced shoot free amino acid content, while Glu had no significant effect. Both Glu and Gin significantly enhanced soluble sugar and starch contents in shoot and root. Amino acids of shoot and root mainly were Asp, Glu, Glu, His and Ala in all treatments. Glu and Glu significantly enhanced some amino acid contents and did not decrease amino acid content except that Glu reduced Phe content compared to CK.6. The study of mechanism of the decrease in nitrate content in pakchoi by amino acids partially replacing the NCV-N in the nutrient solutionWhen nitrate concentration was 2mmol/L or lOmmol/L in the sterile uptake solution, addition of 2.5mmol/L or 25mmol/L amino acids (Gin or Glu) all significantly inhibited the nitrate uptake rate of root and reduced the NR activity in shoot and root of pakchoi compared to the uptake solution without amino acids (Gin or Glu).There was no significant difference in nitrate content in shoot and root between80% NCV-N and 100% NCV-N under partially sterile solution culture, indicating that the decrease in nitrate content in pakchoi by amino acids replacing 20% NCV-N was not due to the decrease in NCV-N concentration in the nutrient solution, but the role of amino acid.When Gin and Glu replaced 20% NCV-N in the nutrient solution, NR activity in shoot and root significantly decreased under partially sterile solution culture compared to 100% NCV-N, indicating that the decrease in nitrate content in pakchoi by amino acids replacing 20% NCV-N was not due to the increase in nitrate reduction in vivo.7. Effect of amino acids partially replacing NCV-N in nutrient solution on iron bioavailability in shoot of pakchoi in normal solution cultureTotal iron content in shoot significantly increased when 20% NCV-N in the nutrient solution was replaced by Arg, His or Gin, and significantly decreased by Asp or Glu, while NH4+-N or Asn treatment had no significant effect compared to 100% NCV-N (CK). Glu, Arg, His and Gin treatments all significantly increased the soluble iron content in shoot, while NH/-N, Asp, or Asn had no significant effect compared to CK. Arg, Gin, His or Asn enhanced the Caco-2 cell ferritin content in shoot, while Asp, NH4+-N or Glu decreased it compared to CK. However, only Arg significantly increased ferritin content in shoot, Glu significantly decreased it, while other treatments had no significant effect compared to CK. In all treatments, iron in pakchoi had bioavailability to some extent, indicating that pakchoi may provide a part of iron needed by human beings.