Plant Growth Regulators Improve Soybean Yield in Northwest China Through Nutritional and Hormonal Regulation

In Xinjiang, soybean yield potential is constrained by varietal limitations and abiotic stresses. Although plant growth regulators (PGRs) can enhance crop yield, their specific impacts on soybean production, endogenous hormone regulation, and nutrient dynamics in this region have not been well characterized. This study evaluated the effects of different PGR treatments on yield formation, hormone levels, and nutrient contents through a field experiment conducted in Ili, Xinjiang, from 2023 to 2025. Foliar applications of naphthaleneacetic acid (NAA, 300 g ha−1), prohexadione-calcium (Pro-Ca, 450 g ha−1), and iron chlorine e6 (ICE6, 45 g ha−1) were applied twice—at the fourth trifoliolate and full-pod stages—with an untreated control (CK) for comparison. Compared with CK, PGR treatments increased biomass allocation to reproductive organs by 6.2%, nutrient accumulation of N, P, and K by 12.3%, 25.5%, and 6.5%, respectively, pod number by 6.92 pods per plant, seed number by 4.86 seeds per plant, and 100-seed weight by 0.47 g, resulting in 6.6–12.0% higher grain yield. Seed PGR residues were 0.009 mg kg−1. PGR application enhanced reproductive organ conversion capacity, nutrient uptake efficiency, and regulated endogenous hormone levels, clarifying internal yield-formation mechanisms and offering valuable reference for soybean research, particularly in similar latitudes.