LAUSR

Abstract In soybeans, exposure to non-inductive photoperiods (i.e., long or extended days) after flowering triggers a number of responses, including slower development of reproductive structures, enhanced vegetative growth and increased production of flowers. The larger number of flowers under extended photoperiods results in an increase of the number of fruits, which eventually may translate into greater grain yield. The main goal of this work was to assess whether C and N assimilation rates increase in soybeans subjected to long days after flowering. Soybean cultivars NS4619 and NS5019 were planted in the field during the normal planting season in 2016−17 and 2018−19. At flowering, two treatments were imposed: natural (Nat) or extended photoperiods (Ext) in which daylength was prolonged by 4 h with low intensity illumination from light-emitting diode lamps delivering less than 10 μmol m−2 s−1 of photosynthetically active radiation at the top of the canopy. Extended photoperiods prolonged reproductive growth, thereby delaying crop maturity. Growth of the main stem and branches was promoted by Ext, with plants showing greater number of nodes and main stem + branches dry weight at maturity. The number of pods and seeds increased by 69–87 % and 31–76 %, respectively, in Ext, and pod and seed dry weight increased significantly in 2016−17. Averaged over the whole reproductive period, crop growth rate increased under Ext in 2016−17 and 2018−2019. Ext promoted post-flowering vegetative growth more than growth of seeds, therefore harvest index was lower under Ext. The growth increase under Ext was larger if the biomass of abscised leaf blades and petioles was taken into account. Consistently with increased biomass under Ext, nitrogen accumulation by the crop was higher under extended photoperiods, with no dilution effect of N due to increased dry mass. About half of the N in plant tissues derived from biological nitrogen fixation, irrespective of photoperiodic condition, which implied that both, nitrogen fixation and uptake of soil nitrogen increased under long photoperiods. In brief, both the amounts of C and N assimilated by the crop were higher under Ext. Understanding the molecular, biochemical and physiological basis of post-flowering photoperiodic effects might offer clues to increase the yield potential of soybeans.