SULFUR AVAILABILITY, COTYLEDON NITROGEN SULFUR RATIO, AND RELATIVE ABUNDANCE OF SEED STORAGE PROTEINS OF SOYBEAN/

The nutritional value of soybean [Glycine max (L.) Merr,] seed protein could be enhanced by increasing its concentration of the S-containing amino acids, methionine and cysteine. Two greenhouse pot studies and one field study were conducted with soybean grown under varying levels of S availability to observe the relationship between S availability, seed S content, and relative abundance of poor and high quality stora ge proteins. Abundances of the beta-subunit of beta-conglycinin (poor quality) and of glycinin (high quality) seed storage proteins were det ermined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis ( SDS-PAGE). Cotyledon-S concentration more than doubled, and the N:S ra tio of the seed decreased sharply (from about 40-20 g N g S-1), as S a vailability increased from 12 to 62 mg available S per plant in the fi rst greenhouse trial. The amount of the poor-quality beta-subunit of b eta-conglycinin was linearly related to the N:S ratio of cotyledon tis sue and varied from less than 15 up to 40% of storage proteins. On the other hand, the high-quality glycinin fraction of storage protein sho wed a linear, negative relation to N:S ratio of cotyledon tissue and d ecreased from 60 to less than 30% of storage proteins as the N:S ratio increased under S stress. Even in high S environments the beta-subuni t of beta-conglycinin comprised 10% or more of total storage proteins. Since poor quality storage protein was synthesized even in high S env ironments, we hypothesize that the plant's ability to reduce sulfate a nd synthesize S-containing amino acids during seed filling may be a fa ctor limiting soybean protein quality.