PHYSIOLOGICAL MECHANISMS OF YIELD IMPROVEMENT IN HISTORICAL U.S. SOYBEAN GERMPLASM BY

Soybean (Glycine max Merr.) is the world’s most widely grown leguminous crop and an important source of protein and oil for food and feed. Soybean yields have increased substantially throughout the past century with yield gains widely attributed to genetic advances and improved cultivars, as well as advances in farming technology and practice. Although soybean yields do not appear to be stagnating, the current rate of gain is insufficient to meet the United Nations target of doubling crop yields by 2050. While soybean yields have been increased through traditional breeding efforts, the physiological mechanisms underlying past yield gains in the U.S. are largely unknown. Therefore, the aims of this thesis research are to gain a better understanding of the physiological basis of past improvements in soybean yield in order to help identify strategies for increasing future production. First, in a two year experiment, twenty-four soybean cultivars released between 1923 and 2007 were grown in field trials. Physiological improvements in the efficiencies by which soybean canopies intercepted light