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.