Retrospective study in US commercial sorghum breeding: II. Physiological changes associated to yield gain

Understanding physiological changes in response to long-term selection for yield can inform breeding decisions and hasten genetic gain. The objective of this study was to characterize changes over time in yield-relevant physiological traits for hybrids with different years of release for grain sorghum (Sorghum bicolor L. Moench). Field trials were conducted during the 2018 and 2019 seasons in 8 site-years across the states of Kansas and Texas including 20 commercially available grain sorghum hybrids released by Pioneer between 1963 and 2017. Detailed yield-related physiological traits were determined in 2 site-years including grain yield and its components, grain filling, plant biomass, panicle length, and water-soluble carbohydrates (WSC) during the reproductive period. Consistent with estimates using historical yield data, sorghum yield improvement was 27 kg ha(-1) yr(-1). For the 2 site-years with detailed yield-related physiological traits, no changes in final grain weight, grain-filling duration, and rate over time were documented. In contrast, grain number increased at a rate of 100 grains m(-2) yr(-1). Modern hybrids had larger panicle size and showed greater accumulation of WSC during vegetative period (as measured at the start of flowering) and greater remobilization of WSC during the reproductive period (after flowering) to grain, thus, maintaining grain size on the increased grain number per unit area and harvest index. These findings suggest that WSC dynamics play a critical role on past genetic yield gain in sorghum and its potential for future improvements should be considered.