Retrospective study in US commercial sorghum breeding: III. Nitrogen internal efficiency

Retrospective studies are critical to investigate changes in physiological traits related to nitrogen (N) uptake and yield formation. N is a key yield-limiting nutrient in sorghum (Sorghum bicolor L. Moench), as in most crops, so improving its N internal efficiency (NIE, yield to N uptake ratio) could increase productivity and sustainability. Understanding changes in N efficiency across time linked to carbon (C) economy can provide insights for future breeding targets. This study aims to characterize N and C dynamics (via studying changes in water-soluble carbohydrates, WSC) of 20 commercial U.S. sorghum hybrids released from 1963 to 2017. Field trials were conducted during the 2018 and 2019 growing seasons in Riley County, KS. Seasonal changes in biomass accumulation, N uptake, and WSC were characterized during both vegetative and reproductive periods. Modern hybrids had greater NIE compared with those from the 1960s. Overall, beyond the yield gain documented for modern sorghum hybrids, the NIE increased at the expense of a reduction in grain N and, in minor proportion, due to increase in the N harvest index (NHI) at maturity for newer relative to older hybrids. Newer hybrids evidenced greater N remobilization from the stover to the grains during the reproductive period. This study demonstrates the physiological mechanisms for an increase in N and C utilization behind yield genetic gain for sorghum hybrids. Future yield gain in sorghum could be pursued by enhancing N uptake to sustain further genetic progress.