Physiological mechanisms of yield improvement in historical U.S. soybean germplasm.

摘要

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 ( epsiloni), converted light energy into biomass ( epsilonc), and partitioned biomass into seed ( epsilonp) were examined. Seed yield increased on average by 26.5 kg ha-1 yr-1, and the increase in seed yield was driven by improvements in all three efficiencies. Although the time to canopy closure did not change in historical soybean cultivars, extended growing seasons and decreased lodging in more modern lines drove improvements in epsiloni. Greater biomass production per unit of absorbed light resulted in improvements in epsilonc. Soybean seed biomass increased at a rate greater than total above-ground biomass, resulting in an increase in epsilonp. It is thought that there is little room for further improvements in epsilon i and epsilonp as 84 years of traditional breeding has driven these efficiencies close to theoretical maxima. epsilonc is still well below its theoretical maxima and is, therefore, a target for future yield gains.;Next, in order to investigate the potential mechanisms underlying the increase in epsilonc with cultivar year of release (YOR), photosynthetic (A) and respiratory capacity was measured within this set of historical germplasm over 3 growing seasons. Traditional soybean breeding has improved epsilonc through greater rates of A with no change in respiratory capacity. The gains in A were driven by increased rates of stomatal conductance ( gs) and water use and not improved photosynthetic capacity. Thus, greater carbon gain in modern varieties was only apparent under times of ample water supply. These results suggest that as climate change increases the water demand of crops, past strategies for increasing conversion efficiency will have reduced effectiveness.;Finally, the transcript abundance of putative "yield enhancing genes" (YEG) were determined in these historic soybean cultivars to examine potential genetic drivers of past yield advancement. YEG are single genes, that when altered, have the capacity to increase plant growth and yield. Of the fifteen YEG examined in this study, six had gene expression levels that correlated with cultivar YOR and yield in at least one growing season. Three of these genes encode Rubisco activase which is hypothesized to increase yields by increasing rates of photosynthesis. YEG also include those that encode vegetative storage proteins which are important in the storage and transfer of carbon and nitrogen to sink tissues. Future work is needed to understand the underlying mechanisms of how altered YEG transcript abundance is affecting yield and if the alteration of these genes further will lead to additional yield gains.;This dissertation research provides insight into the physiological mechanisms underlying past yield gains and identifies targets for future yield improvement. At the whole canopy level, traditional breeding efforts have increased epsiloni, epsilonc, and epsilonp, and identified that improving epsilon c has the most potential of further increasing yields. On the leaf-level, photosynthesis has been improved through greater water use, indicating that past improvements made in epsilonc may not be effective in warmer future. Finally, at the genic level, putative YEG were identified as correlating with yield in soybean, and therefore, have the potential ability to increase yields further in the future.