Comparison of diurnal variation of nitrate and sugars in lettuce and predictions of a model based on metabolism
2011
Gent, M.P.N.
The diurnal variation of nitrate and sugars may vary with plant size and the ability to take up, synthesize, and use metabolites. Three sequential plantings of Bibb lettuce germinated at 2-week intervals were grown in hydroponics to provide plants differing in size. These plants were sampled at 3-hour intervals from 600 to 2100 hours on 14 August 2007. Samples were frozen and freeze dried, and dried tissue was extracted and analyzed for soluble sugars and nitrate using LC chromatography. There was a greater increase during the day in dry/fresh weight ratio for small compared to large plants. On a fresh weight basis, tissue nitrate of small plants was only half that for larger plants. It did not vary with time. Soluble sugars were similar for all plant sizes early in the day, but increased two-fold more in the light for small compared to large plants. To model this behavior, plants were simulated as a ‘big-cell’ with simplified metabolic reactions and transport processes sensitive to internal metabolite concentrations. Photosynthesis and nitrate reductase were affected by irradiance and leaf area index. Amino acid, protein, and structure synthesis depended on concentrations of ammonium and sugars. Use of nitrate in leaves was affected both by nitrate reductase and availability of sugars. Sugars were produced by photosynthesis and consumed by respiration and processes related to growth. The strengths and weaknesses of this model are discussed.
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