Regulation of seed protein concentration in soybean by supra-optimal nitrogen supply

The physiological and biochemical basis for increased seed protein concentr ations (SPC) observed in restriction-index, recurrent-selection breeding pr ograms with soybean [[Glycine max (L.) Merr,] are poorly understood. The hy pothesis that soybean SPC is regulated by the supply of nitrogenous substra tes available to the seed was evaluated. Effects of supra-optimal external N on seed storage protein accumulation, amino acid concentration and compos ition in leaves and seeds at R5, and levels of specific storage protein sub units were measured. Genotypes with different SPC (NC 107, normal; N87-984- 16, intermediate; and NC 111, high) were grown in controlled-environment ch ambers and supplied with 30 mM N as NH4NO3 from V5 to maturity or from R5 t o maturity. Control plants received 10 mM N throughout the growth cycle. Re lative to control, supra-optimal N increased SPC of NC 107 and N87-984-16 b y an average of 28%. Greater enhancement of protein accumulation than of dr y matter accumulation in the seed resulted in SPCs of 460 to 470 g kg(-1) w hich are appreciably greater than concentrations observed for these cultiva rs grown in the field, Supra-optimal N also increased SPC of the high prote in line (NC 111) by 15%, but this increase resulted entirely from a decreas e in yield. Supra-optimal N supplied to NC 107 and N87-984-16 from V5 until R5 increased total free amino acid concentrations in seeds and leaves at R 5 by an average of 21 and 46%, respectively. Enhanced accumulation of the b eta submit of beta conglycinin which does not contain methionine and cystei ne accounted for the increase in SPC, While enhanced N availability increas ed the SPC of a normal protein line into the high range, availability of su lfur amino acids in the developing seed determined which storage protein su bunits were synthesized from the extra N.