Synergistic effect of biostimulants and nitrogen fertilization on crop performance of barley (Hordeum vulgare L.) under rainfed conditions

Optimizing nitrogen management and utilizing biostimulants are essential strategies to improve cereal productivity and fertilizer use efficiency under rainfed agricultural systems. Glycine betaine and zeatin are effective biostimulants known to enhance plant metabolic activity, growth performance, and grain quality. In this study, five nitrogen application rates (0, 80, 160, 240, and 320 kg N ha − 1 ), three biostimulant treatments (control, glycine betaine, and zeatin), and six barley varieties (Ramata, Placidia, Nonius, Sultan, Kıral-97, Altıkat) were evaluated in a two-year field experiment under rainfed conditions. This study investigated both the individual and synergistic effects of nitrogen rates and biostimulant treatments on the growth, yield formation, and agronomic nitrogen use efficiency of barley. Glycine betaine and zeatin applications significantly improved overall measured agronomic traits, with the highest values generally observed under zeatin treatments. The highest grain yield was recorded in the Nonius variety (6.33 t ha − 1 ), while the lowest was observed in the Altıkat variety (5.17 t ha − 1 ), compared to the control, glycine betaine and zeatin applications increased average grain yield by 3.64% and 8.01%, respectively. The highest grain yield increase was achieved at a nitrogen application rate of 240 kg N ha − 1 (116.71% higher than the unfertilized control). An application rate of 80 kg N ha − 1 yielded optimal results for agronomic nitrogen use efficiency (AE) and partial factor productivity of nitrogen (PFP N ), whereas 240 kg N ha − 1 provided the optimal balance for overall yield contribution. Foliar biostimulant treatments, particularly zeatin and glycine betaine, effectively enhance barley productivity and agronomic nitrogen use efficiency across varying nitrogen application rates under rainfed field conditions. Integrating moderate-to-high nitrogen rates with targeted biostimulant treatments represents a viable management strategy for sustainable barley production.