Increasing seed protein in soybean with eight cycles of recurrent selection

Inverse relationships between seed protein and both seed yield and seed oil have limited progress in developing economically profitable high protein soybean [Glycine max (L.) Merr.] cultivars. This study evaluated changes in and limitations to seed protein concentration, associated changes in seed oil concentration, and variability for these traits during eight cycles of recurrent selection for seed protein. Seed of a line with 483 g kg(-1) protein was blended with seed of two F-2 Populations segregating for male sterility (ms2ms2). S-0 plants from random matings in the population were evaluated in each cycle for plant maturity and for seed protein and oil concentration. The 20% of plants with highest seed protein in Cycles 0 through 3 and the 10% of plants with highest seed protein in Cycles 4 through 7 were randomly intermated to initiate the next cycle. Plant maturity was not consistently associated with either seed protein or oil in the eight cycles of selection. Recurrent selection increased mean seed protein 5.8 g kg(-1) and decreased mean seed oil 2.3 g kg(-1) per cycle. A stronger inverse relationship between seed protein and oil was observed in later than in earlier cycles as indicated by steeper slopes of and decreased variability around regression lines in later cycles. Most of the alleles for this trait were accumulated in the high protein selections by Cycle 5. An average of 53% of the plants in Cycles 6 through 8 had 480 g kg(-1) or greater seed protein. Progenies of these plants would be excellent sources of phenotypes that combine high seed protein with good agronomic traits.