(13)CO2 FEEDING STUDIES ON THE METABOLISM OF CARBOHYDRATES, ASCORBATEAND OXALATE IN SPINACH (SPINACIA-OLERACEA L.) (REPRINTED FROM PLANT NUTRITION FOR SUSTAINABLE FOOD-PRODUCTION AND ENVIRONMENT, 1997)

Hydroponically grown forty-day-old spinach (Spinacia oleracea L.) plan ts (two cultivars, Okame and Lead) were exposed to (CO2)-C-13 for 6 ho urs in the light using a steady state (CO2)-C-13/(CO2)-C-12 feeding sy stem. The C-13 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 (CO2)-C-13 feeding, and the relative specific activity (RSA, C-13 abundance of metabolites/C-13 abundance of atmosph eric (CO2)-C-13 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% a t 6 hrs in the leaf blades of Okame and Lead, respectively. The RSAs o f oxalic acid were 2.3% and 2.3% at 3 hrs and 4.9% and 4.4% at 6 hrs i n the leaf blades of Okame and Lead, respectively. The C-13 incorporat ion into the metabolites of the petioles and roots was also investigat ed. Ascorbic acid was synthesized from currently fixed C and partly fr om 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 th e leaf blades.