Independence of nitrogen supply and seed growth in soybean: Studies using an in vitro culture system

The effect of N supply on soybean (Glycine max L. Merrill) seed growth was investigated using an in vitro liquid culture system. Sucrose was maintained at 200 mM and N was supplied by asparagine and methionine in a 6.25:1 molar ratio, Media N concentrations from zero to 270 mM had little effect on cultured cotyledon dry matter accumulation rate for 7 or 14 d, but rates approached zero after 21 d when there was no N in the media. Only 17 mM N was required for maximum cotyledon growth rate up to 21 d. Cotyledon N accumulation and concentration increased in direct proportion to the N concentration in the media. The N concentration in cotyledons from a high protein genotype was higher than a normal genotype at all media N levels (0-270 mM). Soluble sugar and oil concentrations in the cotyledons were highest at zero media N and decreased as media N increased. These data suggest that the concept of seed N demand, which is thought to cause senescence in soybean, is incorrect. Soybean seeds can accumulate dry matter without accumulating N and apparently need only minimal supplies of N (17 mM) to maintain the metabolic enzymes necessary to sustain dry matter accumulation. Genetic differences in seed protein concentration seem to be regulated by the cotyledons not the supply of N.