Uniformly 15N-labeled soybean seeds produced for use in human and animal nutrition studies: Description of a recirculating hydroponic growth system and whole plant nutrient and environmental requirements

A recirculating hydroponic technique which enables the uniform ¹⁵N-labeling of soybean plants has been developed. Daily net influx of nitrate, potassium, calcium, chloride and phosphate, as well as total solution uptake, was measured for a crop of 60 plants grown in a 3 m² area. A protocol is presented which (i) maintains adequate nutrient concentrations for the crop, (ii) adds nutrients and water as required with no exchange of solution, and (iii) minimizes the addition and use of ¹⁵N. With this technique, 6.5 mol of ¹⁵N were used to generate 1 kg (DW) of soybean seeds (350 g protein kg^-1; 98 atm% ¹⁵N) and 3.1 kg (DW) total biomass; recovery of nitrogen in the total biomass was 95%. ¹⁵N-labeled soy protein isolate or harvested plant parts (eg leaves or stems) can be used to investigate nutrient absorption and metabolism in human or animal studies.