Several isoflavones, such as genistein, daidzein, and their glucosyl and malonyl forms were detected by HPLC–UV in cotyledons, together with glycitein forms in the hypocotyl of two 1-maturity class contrasting soybean varieties (Ales with high isoflavone and low protein concentrations vs. Nikir, showing opposite behaviour). Cultivation involved drip irrigation and rainfed conditions, factorially combined with two levels of nitrogen supply (0 and 100 kg ha−1 at R1–R3 stages) at Legnaro (NE Italy) for two years (2006–2007). Yield components and hypocotyl and cotyledon isoflavone concentrations were measured in stems and branches separately. Ales was confirmed as having a higher total cotyledon isoflavone concentration (TCIC) than Nikir, with higher fractions of total daidzein in cotyledons and total glycitein in hypocotyl. In 2007, a 40% decline in total hypocotyl isoflavone concentration (THIC) was observed compared with 2006, due to a higher daily range temperature regime during seed filling. Irrigation increased seed yield and protein, but its effect on TCIC was negligible. Irrigation reduced THIC (∼8%) in the drier year (2007), perhaps due to greater hypocotyl weight (dilution effect) and lower fungal disease attack. TCIC and THIC were generally increased by the unusual nitrogen fertilisation practice (TCIC in year 2: 1.85 vs. 1.58 mg g−1), especially in Nikir. The most important finding was a 24.4% cotyledon (1.54 vs. 1.96 mg g−1) and 4.2% hypocotyl (14.9 vs. 14.3 mg g−1) greater isoflavone concentration in seeds of branches compared with those of the main stem, and this may have a practical application in sowing density. It is currently concluded that nitrogen fertilisation improves the nutraceutical properties of soybean, although its effect depends on the activity of the isoflavone biosynthetic pathway and the concurrent extent of seed bulging.
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