In Canada, yield of short‐season soybean [Glycine max (L.) Merr.] cultivars has increased by approximately 0.5% per year since they were first cultivated in the early 1930s. Future yield gains may be dependent on an understanding of the changes made to soybean cultivars by breeding and selection. Our objective was to examine physiological differences associated with seed yield increase within a group of historical cultivars. At Ottawa, Ontario, we grew 14 cultivars representing seven decades of breeding and selection (1934–1992) in a randomized complete block design with four replications, across 4 years. Growth analysis provided data on leaf area and dry weight. Photosynthetic rate per leaf area was measured at several stages of development each year. Yield and harvest index were determined at maturity. The number of days to maturity and the total plant dry weight were not affected by the year of cultivar release. Seed yield, harvest index, and photosynthetic rate were found to have increased by 0.5% per year, while leaf area index decreased by 0.4% per year. The increase in seed yield with year of release was significantly correlated with an increase in harvest index, photosynthesis, and stomatal conductance and a decrease in leaf area index. Today's cultivars are more efficient at producing and allocating carbon resources to seeds than were their predecessors.

Soybean [Glycine max (L.) Merr.] production in short‐season areas has increased greatly because of improvements in cultivars and production practices. Previous studies of genetic improvement in soybean have reported yield increases of 0.5 to 1% per year. To evaluate the genetic improvement of short‐season germplasm, 41 cultivars ranging from maturity group 000 to 0, released from 1934 to 1992, were grown for 8 station‐years in Ontario and Quebec. Yield, maturity, plant height, lodging score, 100‐seed weight, seed protein and oil levels, and yield stability were regressed on year of release to determine if improvements have been made. Yield has been improved about 0.5% per year during the period under study; however, since 1976 yield has been improved about 0.7% per year. There is evidence that the rate of genetic improvement of seed yield is accelerating. Significant lodging reduction was also observed. Seed protein levels have been reduced 4 g kg‐1 yr‐1 and seed oil levels have increased 4 g kg‐1 yr‐1. Although yield has been increased, yield stability has remained constant.

An association between early maturity and tawny pubescence has been observed in short‐season soybean [Glycine max (L.) Merr.]. The objectives of this study were to determine if a single locus controls early maturity; and if there is a new locus linked to E1 and T or, alternatively, a third allele at the E1 locus. A cross was made between ‘Harosoy’ isolines OT89‐5 (e3e3 e4e4) and OT94‐47. PI 196529 was the donor of early maturity in the backcrossing program which developed OT94‐47. A total of 229 F2 plants and the parents were grown under 20‐h photoperiods produced by incandescent lamps. OT94‐47 flowered in 43 ± 1.4 d, OT89‐5 flowered later in 57 ± 4.2 d, and the F2 population fit a 3 late: 1 early flowering ratio. Early‐flowering F2 plants produced F3 families that flowered similarly to OT94‐47. Later‐flowering F2 plants either segregated for flowering date or flowered similarly to OT89‐5. To test for allelism with E1 and linkage with T, a cross was made between OT93‐26 (E1E1 e3e3 e4e4 TT) and OT94‐47. F2 plants were classified as parental types or intermediate and equivalent to OT89‐5 in maturity. Maturity and pubescence color were recorded in 376 F3 progeny rows. The data did not fit a single locus model or a two loci dominant epistasis model (12:3:1). The E7 allele was partially, but not completely, dominant over the e7 allele. Therefore, a new locus E7 is proposed. Using the F3 segregation data, linkage between T and E1 was estimated to be 1.3 ± 0.6 centimorgan (cM). Linkage between E1 and E7 was estimated to be 6.2 cM and linkage between E7 and T was estimated to be 3.9 cM. E7 is a new flowering, maturity, and photoperiod sensitivity locus tightly linked to both E1 and T E7E7 results in later flowering and maturity, and sensitivity to long photoperiods produced by incandescent lamps when compared to e7e7

Soybean [Glycine max (L.) Merr.] genotypes have been identified that show differential sensitivity to light quality. The use of different lamp types emitting light of different quality changed flowering responses to long days. The objective of this study was to investigate the flowering response of three photoperiod‐sensitivity loci to long days of various light qualities. ‘Harosoy’ near‐isogenic lines were grown under 20‐h‐long days of different light qualities, as measured by the bichromatic ratio of red to far‐red quanta (R:FR), and under light quality gradients. Generally, the photoperiodic response was greater, i.e., later flowering, with decreased R:FR. The El allele was most sensitive to light quality and required an R:FR approximating that of natural daylight for response to long days. The E3 allele was the least sensitive, and the E4 allele showed intermediate sensitivity to light quality. The earliest‐flowering near‐isogenic line (e1 e3 e4), previously found to be insensitive to long day length, showed sensitivity to long days of low R:FR light. Long days of high R:FR light were not effective in delaying flowering for some genotypes. Sensing the R:FR ratio is the function of phytochrome in light‐grown plants. The three loci studied in this work responded differentially to changes in R:FR, suggesting either a close relationship between soybean E alleles and phytochrome or the possibility that some photoperiod‐sensitivity loci are part of the phytochrome family of genes.

Photoperiod response is one factor responsible for the regional adaptation of soybean [Glycine max (L.) Merr.] cultivars. Few photoperiod response studies have been carried out with lines containing alleles for late maturity at only one or a few loci. An understanding of the photoperiod response of early‐maturing soybean lines would facilitate cultivar development in short‐season areas. The objectives of this study were to investigate the photoperiod response of early‐maturing ‘Harosoy’ near‐isogenic lines with indeterminate and determinate growth habit and to examine the genetic model for sensitivity to natural day length extended to 20 h with incandescent lamps [incandescent long day length (ILD)]. Harosoy near‐isogenic lines were grown in the field under natural day length and ILD. The same lines were also grown in growth cabinets under 12‐ and 20‐h photoperiods with cool white fluorescent plus incandescent lamps. Under natural day length, E3 and E4 alleles each delayed flowering 5 d and maturity 15 d while the E1 allele delayed both flowering and maturity ≈ 16 d compared with the alternative early‐maturing alleles. The E3 and E4 alleles each delayed flowering 30 d under ILD compared with natural day length. The E1 allele did not delay flowering or maturity under ILD compared with natural day length. Under 12‐h days in a growth cabinet, there were no differences among near‐isogenic lines for flowering or maturity, but the loci responded differently in the 20‐h photoperiods. The E3 allele exhibited the largest photoperiod response, delaying flowering 24 d and maturity 84 d, compared with 12‐h photoperiods. These photoperiod‐sensitivity loci produced differential photoperiod responses that may he useful for short‐season cultivar development.

breeding in short-season regions resulted in a yield improvement of 0.5% per year with an associated decrease Yield progress of short-season soybean [Glycine max (L.) Merr.] in protein concentration and some improvement in lodgcultivars in Canada has been approximately 0.5% per year since the early 1930s. Our objective was to identify changes in agronomic traits ing tolerance. In a related experiment, we grew 14 cultiassociated with yield increase within a selection of historical cultivars.vars selected from the original 41 to examine physiologi-Where applicable, we measured phenotypic stability of these traits.cal changes from 58 yr of genetic improvement (Mor-At Ottawa, ON, we grew 14 cultivars, representing seven decades of rison et al., 1999). We found that increased seed yield breeding and selection , in a randomized complete block was significantly correlated with decreased leaf area and design with four replicates, across 6 yr. Data were collected on seed increased photosynthetic and stomatal conductance yield, seed weight, plant height, plant population, lodging susceptibilrates per unit leaf area. ity, and foliar disease symptoms. Seed number per plant was calculatedOur objective in the current study was to examine from yield, seed weight, and plant population. Seed protein and oil the changes in agronomic traits associated with 58 yr of concentration were measured. The increase in seed yield with year soybean breeding in short-season cultivars. Specifically, of release was associated with a significant increase in the number of seeds produced per plant. There was no relationship between seed we examined plant height, lodging score, foliar damage yield and seed weight. A significant decrease in seed protein concen-score, plant stand, seed weight, seeds per plant, seed tration with year of release was offset by a significant increase in seed yield, and protein and oil concentration. We determined oil concentration. Newer cultivars were more phenotypically stable the phenotypic stability of these agronomic traits.for plant height than older cultivars. Modern cultivars were more efficient at establishing, supporting, and filling seeds on a per-plant MATERIALS AND METHODS basis than older cultivars.Abbreviations: CV, coefficient of variability; MG, maturity group.

The genetic model for maturity in soybean [Glycine max (L.) Merr.] is a series of near‐isogenic lines, but they do not span the natural variation for early maturity. The objectives of this study were to determine if a single gene in OT98‐17 controls early maturity and if this is a new locus. A cross was made between ‘Maple Presto’ and OT98‐17, an early‐maturing Maple Presto–derived backcross line. A total of 201 F3 progeny rows from this population and Maple Presto were grown at Ottawa, ON, in 1999. In 2000, F4 progeny rows were grown and 150 late‐maturing and 51 early‐maturing families were observed to fit a 3:1 ratio (n = 201, X2 = 0.01, P = 0.90). The early‐maturing allele was transferred to a ‘Harosoy’ background, and isolines were grown from 2002 to 2006 at Ottawa, ON. The isolines were 9 and 6 d earlier maturing in Maple Presto and Harosoy backgrounds, respectively. To determine the independence of this locus, simple sequence repeat molecular markers were used to identify three candidate regions. The gene E8 specifically mapped to linkage group C1 between Sat_404 and Satt136. No other maturity gene has been mapped to this region. The two other candidate regions were both related to maturity quantitative trait loci on molecular linkage group L and may be inadvertently selected along with early maturity. The gene symbol E8e8 has been assigned by the Soybean Genetics Committee. E8E8 results in later maturity and e8e8 results in early maturity. The earliest Harosoy maturity isoline is now rated as maturity group 000.

Developing high‐yielding, high‐protein soybean [Glycine max (L.) Merr.] lines is difficult because of the inverse relationship between seed yield and seed protein content. The objective of this study was to evaluate single cross and rapid back cross breeding methods to achieve both high seed yield and high protein content. ‘AC Proteus’ was used as the high‐protein source and ‘Maple Glen’ was used as the adapted high‐yielding parent. Six single plants were randomly selected from each of 149 F3 progeny rows to develop 886 single cross derived lines. Reciprocal back cross populations were made using 60 F1 plants from the single cross and the high‐yielding parent, Maple Glen. Ten single plant selections were randomly taken from each of 80 F1‐derived families to develop 800 back cross derived lines. In 1994, all lines were tested in the field. About 20% of lines were retained for testing through this breeding project from 1994 to 1996 although selection intensity differed across populations and years. In 1997, six single cross and nine back cross lines were tested at six locations in Eastern Canada. The seed yield and protein content of the single cross lines were not significantly different from the back cross derived lines. All the selected lines had higher seed protein content than did Maple Glen. Both breeding strategies produced lines with significantly higher seed protein content than Maple Glen. However, none of these lines had significantly higher seed yield than Maple Glen or a recently released high protein cultivar, AC Proteina. These populations exhibited a very low or no association between seed yield and protein (r = −0.06 to −0.21). Therefore, the parents may be useful sources of alleles which do not exhibit the usual pleiotrophic effect of low seed yield and high seed protein. The use of a back cross to the adapted parent following the single cross was not beneficial in the development of high yielding lines.