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CROP PHYSIOLOGY & METABOLISM

Regulation of Seed Protein Concentration in Soybean by Supra-Optimal Nitrogen Supply

^a USDA-ARS, Dep. of Crop Science, Raleigh, NC USA
^b USDA-ARS, Dep. of Soil Science, North Carolina State University, Raleigh, NC 27695-7620 USA

dan_israel{at}ncsu.edu

The physiological and biochemical basis for increased seed protein concentrations (SPC) observed in restriction-index, recurrent-selection breeding programs with soybean [Glycine max (L.) Merr.] are poorly understood. The hypothesis that soybean SPC is regulated by the supply of nitrogenous substrates available to the seed was evaluated. Effects of supra-optimal external N on seed storage protein accumulation, amino acid concentration and composition in leaves and seeds at R5, and levels of specific storage protein subunits were measured. Genotypes with different SPC (NC 107, normal; N87-984-16, intermediate; and NC 111, high) were grown in controlled-environment chambers and supplied with 30 mM N as NH₄NO₃ from V5 to maturity or from R5 to maturity. Control plants received 10 mM N throughout the growth cycle. Relative to control, supra-optimal N increased SPC of NC 107 and N87-984-16 by an average of 28%. Greater enhancement of protein accumulation than of dry matter accumulation in the seed resulted in SPCs of 460 to 470 g kg^-1, which are appreciably greater than concentrations observed for these cultivars grown in the field. Supra-optimal N also increased SPC of the high protein line (NC 111) by 15%, but this increase resulted entirely from a decrease 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 R5 by an average of 21 and 46%, respectively. Enhanced accumulation of the ß subunit of ß conglycinin which does not contain methionine and cysteine accounted for the increase in SPC. While enhanced N availability increased the SPC of a normal protein line into the high range, availability of sulfur amino acids in the developing seed determined which storage protein subunits were synthesized from the extra N.

Abbreviations: DAF, days after flowering • DAT, days after transplant • SPC, seed protein concentration • YFEL, youngest fully expanded leaves

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