Changes in photosynthetic traits and their responses to increasing fertilization rates in soybean ( Glycine max (L.) Merr.) during decades of genetic improvement

Abstract Backround Changes in photosynthetic traits (PTs) during the long‐term genetic improvement of soybean (Glycine max (L.) Merr.) yield have been studied, but detailed information on whether PT responses to environmental stress have improved, and their correlations with seed yield, are still unknown. Our objectives were to describe the changes in soybean PTs – leaf area index (LAI), leaf chlorophyll content (Chl), net photosynthetic rate (PN), stomatal conductance (gs), and transpiration rate (E) – during decades of genetic improvement, and to detect whether the responses to increasing fertilizer application rates (FRs) of the PTs of 13 different soybean cultivars released in various decades differed. Results All of the soybean PTs increased significantly along with the year in which each cultivar was released, under different FR treatments, indicating that PTs have improved during decades of genetic breeding. Medium FR (nitrogen) treatment (150 kg ha −1) increased PT values, to different extents, at all the investigated growth stages. Leaf area index, Chl, and PN of the old and middle cultivar groups at the full bloom (R2), full seed (R6), and beginning maturity (R7) stages decreased significantly under high FR treatment (300 kg ha−1) compared with the medium FR treatment. The former had no effect on any of the PTs of new cultivar group, or had promotive effects. Thus, the photosynthetic capacities of the new cultivars are more tolerant to high FR‐related stress than older cultivars. Conclusions The photosynthetic capacities, and tolerance to high FR‐related stress, of soybean cultivars that were released in different years improved after long‐term genetic breeding. © 2021 Society of Chemical Industry