Prohexadione calcium boosts pepper stem rigidity: coordinated regulation of hormonal crosstalk and lignin deposition drives xylem reinforcement

Robust seedlings are critical for mechanized transplanting efficiency, with plant growth regulators (PGRs) playing a key role in seedling cultivation within plant factory systems. However, as a new type of PGR, limited research has focused on prohexadione calcium (ProCa), and its effects on pepper (Capsicum annuum L.) seedling development. This study aimed to identify the optimal foliar ProCa concentration and evaluate its stem-strengthening mechanisms in pod pepper seedlings. Results demonstrated that 200 mg/L ProCa significantly enhanced seedling vigor. Key findings included: 1) a 15.31-21.51% reduction in plant height coupled with increased stem diameter and dry matter accumulation; 2) improved root vitality (8.74-9.55%) and seedling index (12.31-29.32%); 3) enhanced stem mechanical properties, including compressive strength (+ 13.33%), bending force (+ 32.08%), and tensile resistance (+15.70%). Physiological analyses revealed that ProCa application regulated endogenous hormone levels and increased lignin accumulation, thereby promoting vascular bundle formation and xylem development. In summary, these results suggest that the foliar application of ProCa benefits the cultivation of robust seedlings by promoting pod pepper growth and enhancing the mechanical properties of stems. Overall, these findings lay a foundation for utilizing ProCa in mechanized transplanting processes for pod pepper seedlings.