Recent results from a genetic gain study have illustrated the contribution of plant breeding to the improvement in seed yield of soybean (Glycine max (L.) Merr.). The objective of this research was to characterize the changes in several physiological parameters that have occurred in the released cultivars with the improvement of seed yield. Sixty maturity group III and 54 maturity group IV cultivars, released from the 1920's through 2010, were evaluated in dryland and irrigated environments at Manhattan, KS in 2010 and 2011. Genotypes were planted in four-row plots, 3.4 m long, spaced 76 cm apart, arranged in a randomized complete block design with four replications. Genotypes were evaluated for canopy temperature, leaf chlorophyll content, pollen germination, and leaf fluorescence. Canopy temperature measurements were captured at solar noon using an infrared camera multiple times after the plants reached reproductive stages of growth. Leaf chlorophyll content was measured using a SPAD meter several times from R1 continuing through R6.  In vitro pollen germination was measured using incubation temperatures of 28 and 34° C, beginning at late R1 through the end of flowering.  Leaf fluorescence was measured beginning at R1 through R6. Genotypes differed significantly for all traits measured. Seed yield increased with year of release. Canopy temperature was negatively correlated with seed yield in both the maturity group III and IV genotypes. Leaf chlorophyll content, in vitro pollen germination, and leaf fluorescence tended to be positively correlated with seed yield. Evaluation of these parameters may serve as a basis to select for seed yield, or to assess the abiotic stress tolerance of a genotype.