Top-soil root architecture characteristics of obsolete and modern soybean cultivars and shoot and seed nutrient contents
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI--COLUMBIA AT REQUEST OF AUTHOR.] Soybean (Glycine max Merr.) is one of the most important cash crops worldwide. It is widely used to produce oil and protein for food and animal feed. Soybean grain yield has increased considerably over the last century thanks to management and genetic improvement. Critical work over several decades has contributed to a steady increase in soybean yield. Breeding for greater yields has altered shoot characteristics resulting in, for example, increased light interception and enhance partitioning of photosynthates to seeds; however, the effects of breeding on the soybean root system remain unclear. Although root system architecture traits play a key role in water and nutrient acquisition, the relationship between genetic gain in soybean yield and root system architecture has not yet been characterized thoroughly. Therefore, the objective of my dissertation research was to examine top-soil root system characteristics of maturity group IV soybean varieties released between 1930 and 2005, and their relationship to yield and mineral nutrient uptake. To this end, 24 soybean varieties, including two or three per decade of release, were grown at two locations in 2016 and 2017. A broad range of root architecture traits were assessed at the beginning seed fill stage using visual an imaging approaches. Additionally, shoot dry biomass, plant height, and yield were determined. The experiments revealed significant positive relationships with the year of cultivar release (YOR) for root architecture traits that are related to greater root length in the top soil. The score of overall root complexity, which was considered to be an integrated measure of the top-soil root architecture, increased linearly with cultivar YOR. Yield measurements were consistent with previously reported increases with YOR and averaged between 16.48 kg ha-1 yr-1 and 20.56 kg ha-1 yr-1 in the four environments. A significant positive correlation between overall root complexity and yield was found in three of the four environments, indicating that changes in top-soil root architecture contributed to the observed increases in yield with YOR. Examination of the relationships between root system characteristics and shoot and seed nutrient contents revealed positive correlations for most root phenotypes. Additionally, consistent significant correlations between shoot K, S, Mn, Zn, and Cu concentrations and their concentration in the seeds were observed across the four environments. Compared to seed nutrient concentrations, shoot nutrient concentrations were more variable across environments This study revealed that increases in genetic yield gain of soybean were associated with an increase in root system complexity in the top soil, and presents evidence for the role of root system architecture in improving soybean performance. Also, these findings provide interesting insights into the relationships between root architecture characteristics and nutrient uptake as well as the relationship between shoot tissue and seed nutrient concentrations and contents across different environments and cultivars released over 75 years. The findings emphasize the promise of selection for desirable root architecture traits in breeding programs to compensate for the reduction in the nutritive value of soybean seeds in new cultivars.
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