Both Biomass Accumulation and Harvest Index Drive the Yield Improvements in Soybean at High and Low Phosphorus in South-West China

Soybean breeding is typically conducted in soils with favourable soil phosphorus yet soil P levels are typically low in sub-tropical regions. This study was conducted to determine yield progress of soybean cultivars released between 1995 and 2016 in sub-tropical parts of south-west China and grown at both low and high P availability. The principal factors that changed indirectly with breeding and that contribute to soybean yield were also determined. Field experiments were conducted in three consecutive years with two different sites each year. A more detailed investigation of traits contributing to soybean yield was investigated in one year at two sites. The rate of genetic gain in seed yield was around 2% per year and this was found at both low P and high P. Thus, breeding under favourable conditions has also resulted in substantial gains in soils with low P. Yield gains were attributed to a longer duration of pod filling but there was no change in the duration from sowing to flowering. The extended pod filling duration increased both above-ground biomass and harvest index (HI) at both P levels and both were important in contributing to yield gains. The increase in HI was also a result of the changed allocation of assimilates away from stems and instead to the pods and growing seeds. The allometric analysis (ratios between log transformations of the leaf, stem and pod dry weight) showed that the stem weight decreased as pod dry weight became heavier whereas leaf weight remained constant as stems became lighter as a result of breeding. Breeding also progressively increased pod number and the total seed number but there was no change in average seed weight.