Modulating alfalfa growth: impacts of plant growth regulators on physiology, architecture, and seed yield

Alfalfa seed yield is severely restricted by its indeterminate growth habit, with actual average yield only reaching 4% of potential yield in field production. Plant growth regulators (PGRs) can modulate the balance between vegetative and reproductive growth, yet research on PGR application for alfalfa plant architecture optimization and seed yield improvement remains limited. To address this gap, this study treated “XinMu No.4” alfalfa (Medicago sativa L.) with three PGRs at specific concentration gradients: flumetralin (CAG: 0.5/1.0/1.5 g L− 1), 1,1-dimethyl-piperidinium chloride (DPC: 0.25/0.35/0.45 g L− 1), and compound sodium nitrophenolate (CSN: 0.15/0.20/0.25 g L− 1), to investigate their effects on growth, photosynthetic characteristics, carbon-nitrogen metabolism, and seed yield. Key findings showed that a single spray of 0.35 g L− 1DPC resulted in complete cell structure, significantly increased chloroplast volume (by 23.6%), enhanced starch granule number (by 41.2%), and improved photosynthetic capacity (chlorophyll content increased by 47.5% and net photosynthetic rate (Pn) increased by 32.8%). This treatment also elevated nitrate reductase activity by 37.5% and sucrose synthase activity by 29.4%, enhancing carbon-nitrogen metabolic capacities. By increasing branch number to 63 branches/plant, inflorescences per plant increased by approximately 28 Pcs/Plant, ultimately boosting seed yield to 329.76 kg·hm− 2. Comprehensive membership function evaluation indicated this treatment had the highest average membership degree (0.918). Comprehensive analysis showed that 0.35 g L− 1 DPC was the most effective in shaping plant architecture and boosting seed yield.