Soybean genetic progress of maturity group IV cultivars under well-watered and drought stress conditions in central Argentina between 1984 and 2014

Context: Soybean genetic gain in central Argentina has traditionally been estimated under high-yielding conditions. To the present, the contribution of breeding efforts over a water gradient is not known. Objective: To evaluate soybean yield genetic gain under contrasting water availability scenarios to (i) investigate breeding contributions to soybean yield potential and drought stress tolerance, and (ii) evaluate the physiological mechanisms behind the observed yield increases. Methods: We used six cultivars belonging to maturity group IV, which were released into the market between 1984 and 2014. These cultivars were evaluated under well-watered and drought-stressed conditions. Nitrogen (N) capture, partitioning, and concentration were evaluated to understand the physiological attributes that explain seed yield changes over time. Results: On average, the water-stressed condition yielded 61 % less compared to the well-watered. Absolute yield genetic progress was higher in the well-watered environment (31.6 and 12.8 kg ha-1 year-1 for the well-watered and stressed conditions, respectively), but relative yield gain was similar (1 % year-1). Absolute yield changes over time were correlated to higher total N capture (2.1 and 0.5 kg N ha-1 year-1 for the well-watered and stressed condition, respectively). Biological N fixation from emergence to maturity was identified as the driver of this differential N uptake (1.4 and 0.5 kg N ha-1 year-1 in the well-watered and water stressed environment, respectively). More than proportional increases in absolute biological N fixation genetic progress during vegetative stages explained the higher biological N fixation in newer genotypes grown under well-watered conditions (0.5 and 0.03 kg N ha-1 year-1, for the well-watered and stressed condition, respectively). Conclusions: Within the limited set of tested varieties, breeding efforts delivered higher yield potential with some degree of drought stress tolerance. Biological N-fixation modifications are responsible for the absolute yield genetic gain differences in well-watered vs. drought environments. Higher BNF in vegetative stages explained the higher yield potential of the latest cultivars released to the market in this study. Implications: This study complements genetic gain estimations in central Argentina by evaluating the yield trajectories under contrasting water scenarios. Future investigations should focus on the genetic determinants of the processes related to total N uptake and the biological N fixation.