UNIFORMLY N-15-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 N-15-la beling of soybean plants has been developed. Daily net influx of nitra te, potassium, calcium, chloride and phosphate, as well as total solut ion uptake, was measured for a crop of 60 plants grown in a 3 m(2) are a. A protocol is presented which (i) maintains adequate nutrient conce ntrations for the crop, (ii) adds nutrients and water as required with no exchange of solution, and (iii) minimizes the addition and use of N-15. With this technique, 6.5 mol of N-15 were used to generate 1 kg (DW) of soybean seeds (350 g protein kg(-1); 98 atm% N-15) and 3.1 kg (DW) total biomass; recovery of nitrogen in the total biomass was 95%. N-15-labeled soy protein isolate or harvested plant parts (eg leaves or stems) can be used to investigate nutrient absorption and metabolis m in human or animal studies.