Physiological changes from 58 years of genetic improvement of short-seasonsoybean cultivars in Canada

7In Canada, yield of short-season soybean [Glycine max (L.) Merr.] cultivar s has increased by approximately 0.5% per year since they were first cultiv ated in the early 1930s. Future yield gains may be dependent on an understa nding of the changes made to soybean cultivars by breeding and selection. O ur 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, ac ross 4 years. Growth analysis provided data on leaf area and dry weight. Ph otosynthetic rate per leaf area was measured at several stages of developme nt each year. Yield and harvest index were determined at maturity. The numb er 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, photosynth esis, and stomatal conductance and a decrease in leaf area index, Today's c ultivars are more efficient at producing and allocating carbon resources to seeds than were their predecessors.