Ethanolic extracts of buckwheat (Fagopyrum esculentum Möench) hulls were separated by Sephadex LH-20 column chromatography into eight fractions. Five of the fractions exhibited peroxyl radical-scavenging activity by inhibiting the oxidation of methyl linoleate in solution. Two of the antioxidant fractions contained proanthocyanidins (condensed tannins) from the color reaction of these fractions with HCl under heat treatment. Five antioxidant compounds were isolated by preparative HPLC and identified as quercetin, hyperin, rutin, protocatechuic acid, and 3,4-dihydroxybenzaldehyde. The contents of these active compounds in the buckwheat hulls were as follows:  protocatechuic acid (13.4 mg/100 g of dried hulls), 3,4-dihydroxybenzaldehyde (6.1 mg/100 g), hyperin (5.0 mg/100 g), rutin (4.3 mg/100 g), and quercetin (2.5 mg/100 g). Besides the isolation of these compounds, two major compounds that showed no peroxyl radical-scavenging activity in the extract were isolated and identified as vitexin and isovitexin. Keywords: Fagopyrum esculentum; antioxidant activity; flavonoids; phenolic compounds; proanthocyanidin

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(Jpn. J. Soil Sci.Plant Nutr., 77, 293–298, 2006) “Kitanokaori” is a new variety of wheat for bread use bred at the National Agricultural Research Center for Hokkaido Region. The grain protein content of wheat for bread use should be higher than 120 g kg−1. Much nitrogen application is necessary to obtain high grain protein content. Therefore, it is necessary to determine the optimum amount of nitrogen to obtain the required protein content and to prevent nitrogen from remaining in the soil. The nutrition diagnosis using leaf color was investigated to predict the need and the amount of top‐dressing. Field experiments were conducted for four years with nitrogen treatments in Andosol, which has moderate nitrogen fertility, and in Histosol, which is a fertile soil. The leaf color was measured using a chlorophyll meter SPAD502 (CM value) at the middle part of the leaf, avoiding the center rib. The colors of the 10·15 uppermost second leaves were measured in one plot and averaged. A close relation was found between leaf color at the full heading stage and grain protein content at harvest. Leaf color at the full heading stage is therefore a good index to control the protein content. Considering the effect of top‐dressing at the full heading stage in each CM value, the diagnosis criterion was decided. When the CM value is over 52 at the full heading stage, more nitrogen application is not needed. When the CM value is 50·52, 30 kg N ha−1 of top‐dressing at the full heading stage is needed, and when the CM value is 45·50, 60 kg N ha−1 of top‐dressing is needed to obtain a grain protein content of more than 120 g kg−1.

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