^<13>CO_2 feeding studies on the metabolism of carbohydrates, ascorbate and oxalate in spinach (Spinacia oleracea L.)

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Hydroponically grown forty-day-old spinach (Spinacia oleracea L.) plants (two cultivars, Okame and Lead) were exposed to ^<13>CO_2 for 6 hours in the light using a steady state ^<13>CO_2/^<12>CO_2 feeding system. The ^<13>C incorporation into sugars, organic acids, amino acids, ascorbic acid, and oxalic acid in leaf blades, petioles and roots was measured at 3 and 6 hrs ^<13>CO_2 feeding, and the relative specific activity (RSA, ^<13>C abundance of metabolites/^<13>C abundance of atmospheric ^<13>CO_2 in the chamber) was calculated. The RSAs of the sugars were 40.7% and 35.1% at 3 hrs and 47.7% and 46.3% at 6 hrs and those of the ascorbic acid were 7.0% and 6.7% at 3 hrs and 14.0% and 12.4% at 6 hrs in the leaf blades of Okame and Lead, respectively. The RSAs of oxalic acid were 2.3% and 2.3% at 3 hrs and 4.9% and 4.4% at 6 hrs in the leaf blades of Okame and Lead, respectively. The ^<13>C incorporation into the metabolites of the petioles and roots was also investigated. Ascorbic acid was synthesized from currently fixed C and partly from the reserved C. Oxalic acid synthesis was active in the leaf blades, but less active in the petioles and roots. Nitrate reduction, oxalic acid synthesis and amino acid synthesis occurred simultaneously in the leaf blades.
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