title: Short-term alteration of nitrogen supply prior to harvest affects quality in hydroponic-cultivated spinach (Spinacia oleracea).
abstract: BACKGROUND
  Quality-associated problems, such as excessive in planta accumulation of oxalate, often arise in soillessly cultivated spinach (Spinacia oleracea). Maintaining a higher level of ammonium (NH₄⁺) compared to nitrate (NO₃⁻) during the growth period can effectively decrease the oxalate content in hydroponically cultivated vegetables. However, long-term exposure to high concentrations of NH₄⁺ induces toxicity in plants, and thus decreases the biomass production. Short-term application of NH₄⁺ before harvesting in soilless cultivation may provide an alternative strategy to decrease oxalate accumulation in spinach, and minimise the yield reduction caused by NH₄⁺ toxicity.
  
  
  RESULTS
  The plants were pre-cultured in 8 mmol L⁻¹ NO₃⁻ nutrient solution. Next, 6 days before harvest, the plants were transferred to a nutrient solution containing 4 mmol L⁻¹ NO₃⁻ and 4 mmol L⁻¹ NH₄⁺. This new mix clearly reduced oxalate accumulation, increased levels of several antioxidant compounds, and enhanced antioxidant capacity in the edible parts of spinach plants, but it did not affect biomass production. However, when the 8 mmol L⁻¹ NO₃⁻ was shifted to either nitrogen-free, 4 mmol L⁻¹ NH₄⁺ or 8 mmol L⁻¹ NH₄⁺ treatments, although some of the quality indexes were improved, yields were significantly reduced.
  
  
  CONCLUSIONS
  Short-term alteration of nitrogen supply prior to harvest significantly affects quality and biomass of spinach plants, and we strongly recommend to simultaneously use NO₃⁻ and NH₄⁺ in hydroponic cultivation, which improves vegetable quality without decreasing biomass production.
authors.affiliation: MOE Key Lab of Environmental Remediation and Ecosystem Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, 310058, China.
authors.forename: Xian Yong
authors.initials: XY
authors.lastname: Lin
authors.forename: Xiao Xia
authors.initials: XX
authors.lastname: Liu
authors.forename: Ying Peng
authors.initials: YP
authors.lastname: Zhang
authors.forename: Yuan Qing
authors.initials: YQ
authors.lastname: Zhou
authors.forename: Yan
authors.initials: Y
authors.lastname: Hu
authors.forename: Qiu Hui
authors.initials: QH
authors.lastname: Chen
authors.forename: Yong Song
authors.initials: YS
authors.lastname: Zhang
authors.forename: Chong Wei
authors.initials: CW
authors.lastname: Jin
mesh_terms: D064751:Ammonium Compounds / Q000008:administration & dosage / Q000009:adverse effects / Q000378:metabolism*; D000975:Antioxidants / Q000032:analysis / Q000378:metabolism; D002681:China; D018556:Crops, Agricultural / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism; D005308:Fertilizers / Q000009:adverse effects; D063427:Food Quality; D055951:Functional Food / Q000032:analysis; D006801:Humans; D018527:Hydroponics; D009566:Nitrates / Q000008:administration & dosage / Q000009:adverse effects / Q000378:metabolism*; D058458:Nitrogen Cycle; D009753:Nutritive Value; D010070:Oxalates / Q000009:adverse effects / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism; D018515:Plant Leaves / Q000737:chemistry / Q000254:growth & development* / Q000378:metabolism; D018547:Plant Stems / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism; D012995:Solubility; D018724:Spinacia oleracea / Q000737:chemistry / Q000254:growth & development* / Q000378:metabolism; D013997:Time Factors
chemical_list: D064751:Ammonium Compounds; D000975:Antioxidants; D005308:Fertilizers; D009566:Nitrates; D010070:Oxalates
keywords: nitrate; oxalate; patterns of nitrogen supply; vitamin C