Genotypic Variation in Eco-Physiological Processes Affecting Soybean Yield: Role of Biomass Partitioning and Nitrogen Uptake.

Soybean [Glycine max (Merr.)] seed number per unit area explains most of the variation observed in yield across genotypes. The determination of seed number per area is a complex processes that can be modeled as a function of: (a) crop growth rate during the critical period, (b) the proportion of growth partitioned to reproductive structures and (c) the minimum assimilate requirement to set a seed. Additionally, crop growth rate during the critical period can be analyzed in terms of: (d) total nitrogen uptake and (e) nitrogen use efficiency during that period. We hypothesize that genetic improvement in yield (and hence seed number) can be originated from different eco-physiological strategies. For example, increased seed numbers can be determined by increased crop growth rate or, alternatively, by better partitioning of growth to reproductive structures. Additionally, improvement in crop growth rate could be attained by increased total nitrogen uptake or by a more efficient nitrogen use during the critical period.  To test the hypothesis we will assess the above described variables (a-e) in a set of 42 “current” commercial and 22 “old” commercial cultivars (>10 years old). Both “new” and “old” cultivars were chosen for closeness in maturity groups (2.0 to 3.0). The 64 cultivars will be evaluated in a randomized complete block experiment at two locations in Iowa.  Results from this screening will expose different eco-physiological strategies to attain higher yields associated with past breeding and will likely identify opportunities for further genetic improvement. In addition, results from this experiment may have important practical implications for crop management since cultivars having increased yield by different strategies may possible respond differentially to growing environment.