Agronomic, Hormonal, and Seed Quality Responses of Soybeans to Four Spray Treatment Regimes Under Irrigated Conditions in Xinjiang: A Three-Year Field Study.

Soybean production commonly faces challenges including severe pod abscission, incomplete seed filling, and uncoordinated source-sink relationships that limit yield potential. Plant growth regulator application may influence source-sink relationships and thereby affect soybean (Glycine max (L.) Merr.) yield formation. This three-year field study (2023-2025) evaluated the effects of four spray programs on soybean yield, quality, total above-ground biomass partitioning, and endogenous hormone dynamics under Xinjiang's irrigated production conditions. The four treatments were an untreated control (CK), naphthaleneacetic acid alone (NAA; 300 g ha-1), naphthaleneacetic acid plus prohexadione-calcium (NPC; 300 + 450 g ha-1), and naphthaleneacetic acid plus prohexadione-calcium and iron chlorin e6 (NCE; 300 + 450 + 45 g ha-1). The spray programs were applied at the fourth-trifoliolate and full-pod stages. Results showed that NCE treatment consistently produced the greatest yield increases (13.1-14.4%) compared with the control. This response was associated with greater middle-node pod retention, increased 100-seed weight (3.6-6.4%), and improved reproductive organ biomass allocation (44.7-53.3% at maturity vs. 40.9-45.7% in controls). NCE significantly elevated leaf trans-zeatin content (37.1-91.9%) within 24 h after application and improved seed protein concentration by up to 11.2%, while seed residues of all applied compounds remained well below safety thresholds (<0.05 mg kg-1). Correlation analysis revealed strong positive relationships between trans-zeatin levels and both reproductive biomass allocation (r = 0.74-0.90, p < 0.01) and grain yield (r = 0.86-0.96, p < 0.01). These findings indicate that NCE was the best-performing of the four tested spray programs under Xinjiang's irrigated production conditions, although the individual contributions and possible interactions of the three compounds require further factorial evaluation.