P. J. Sexton

EFFECTS OF NITROGEN SOURCE AND TIMING OF SULFUR DEFICIENCY ON SEED YIELD AND EXPRESSION OF 11S AND 7S SEED STORAGE PROTEINS OF SOYBEAN

Abstract The nutritional value of soybean (Glycine max [L.] Merr.) seed as a protein source for nonruminants is limited by the amount of the S-containing amino acids, methionine and cysteine, they contain. The nutritional quality of S soybean is known to be influenced by both S and N nutrition. The capacity of the plant for S assimilation during late seed filling is an additional factor that might limit protein quality even when S availability is more than adequate. This paper describes experiments conducted to examine the effects of S and reduced N availability on protein quality, and the capacity of the plant for assimilation of sulfate-S during seed filling. Availability of sulfate-S was varied in the hydroponic culture during various vegetative and reproductive stages and N was supplied as either urea (reduced form) or as nitrate (oxidized form). The ratio of 11S/7S seed storage proteins was used as an indicator of protein quality. The 11S protein is of greater nutritional value than is the 7S protein; therefore, the greater the 11S/7S ratio the better the nutritional quality of the seed protein. Provision of reduced N caused seed yield to increase across S treatments and the 11S/7S ratio to decline among plants that were S-sufficient but not among those that were S-deficient. Seed yield was very sensitive to S-deficiency occurring during vegetative growth, but not to S-deficiency occurring during reproductive growth. The 11S/7S ratio was strongly influenced by S-deficiency occurring during reproductive growth, but was relatively insensitive to S availability during vegetative growth. Provision of S near the middle of the seed filling period to previously S-deficient plants caused 11S/7S ratio to increase threefold over plants that were maintained S-deficient throughout seed filling. We conclude that the plant maintains a substantial capacity for S assimilation late into seed filling, and that mobilization of vegetative S is not a large source of S for developing seeds.

Differential accumulation of Soybean seed storage protein subunits in response to sulfur and nitrogen nutritional sources

Abstract Soybean (Glycine max [L.] Merr.) seed storage proteins consist of subunits that differ in amino acid profile, the β-subunit of 7S protein being essentially devoid of the S-containing amino acids, methionine and cysteine. Our objective was to examine the interaction of N and S nutrition on the relative abundance of these storage protein subunits in soybean seed. ‘Kenwood’ soybean was grown in hydroponic culture, and during vegetative growth (V2–R4.5) N was provided as 5 mM KNO3 to plants grown under sulfur-deficient (0.004 raM Na2SO4) or sulfur-sufficient (0.4 mM Na2S04) conditions. During seed fill (R4.5–R7) N was supplied as 5 mM KNO3 or 2.5 mM urea. Each N group was given S treatments of 1) no sulfur, 2) 0.4mM Na2SO4, 3) 0.2 mM L-cystine, or 4) 0.4 mM L-methionine. Effects on seed protein quality of S deficiency during vegetative growth were essentially overcome by supplying sulfate as late as R4.5. Total protein and seed storage protein were increased with urea as N source, but urea also increased the β-subunit. Provision of reduced S as methionine essentially suppressed β-subunit production, but cystine did not, suggesting that cystine did not influence methionine level in the seed. We also report the accumulation of two as yet unreported proteins which occur at extremes of S nutrition : (1) a putative β-subunit of 7S protein occurring in the embryonic axis under S-deficiency ; and (2) a ca. 14kD protein in cotyledon tissue under provision of L-methionine. Though S and N did interact to a limited extent to influence seed protein composition, major effects were from S or N acting individually.