Accumulation of nitrogen and dry matter by soybean seeds with genetic differences in protein concentration

Soybean [Glycine mar (L.) Merrill] yields often decline as seed protein levels increase, but the processes responsible are not clearly understood. We compared dry matter and N accumulation by individual seeds of three high protein genotypes (K 1431, KS 4402sp, NE 3396) and three commercial cultivars (Pennyrile, Flyer, Hutcheson) in the field near Lexington, KY (38 degrees 01 degrees N lat; 84 degrees 35' W long) for 3 yr to determine if increasing protein concentration decreased the seed growth rate. Plants were grown in 76.2-cm rows using conventional production practices and overhead irrigation to minimize soil moisture deficits. The rate of dry matter accumulation by individual seeds (SGR(DM)) varied from 4.2 to 6.1 mg seed(-1) d(-1) and this variation was closely associated with mature seed size, but it was not affected by seed N concentration. The rate of N accumulation (SGR(N), mg N seed(-1) d(-1)) was closely associated with seed N concentration at maturity across all genotypes and years (r(2)=0.76). Thus, SGRN and SGRDM seemed to vary independently as the N concentration in mature seeds increased. The duration of seed filling (estimated by the effective filling period) was not related to mature seed N concentration. Since higher seed N concentration had no effect on the rate or duration of seed dry matter accumulation by individual seeds, the purported negative effect of seed protein levels on yield may be more closely associated with whole plant phenomena than those operating at the individual seed level.