Optimizing Paclobutrazol Application for Regulating Dwarfing in Ougan (Citrus reticulata cv. Suavissima): Comprehensive Insights from Growth, Photosynthesis, and Physiological Responses

Rapid urbanization and increasing land scarcity have made urban agriculture and efficient space utilization critical directions in modern agriculture. Ougan, a fruit tree valued for both its economic and ecological benefits, holds significant promise for dwarfing cultivation techniques. In this study, a root-irrigation method was used to apply paclobutrazol at various concentrations (200, 500, 1000, 1500, and 2000 mg/L) to Ougan seedlings, with a control group for comparison. Growth parameters include an average daily increase of plant height, stem girth, new branches, and new branch girth, as well as physiological indices such as leaf SPAD values, leaf nitrogen content, net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and transpiration rate, were measured during both spring and summer growth periods. The results demonstrate that PBZ exerts a distinct concentration-dependent regulatory effect on Ougan growth: higher concentrations significantly inhibited plant height while promoting increases in stem diameter, with several parameters exhibiting a unimodal response. Short-term (spring) PBZ application enhanced certain photosynthetic parameters, such as net photosynthetic rate and stomatal conductance; however, prolonged exposure (summer) resulted in a decline in photosynthetic efficiency and overall leaf physiological status. Through comprehensive evaluation using principal component analysis and PLS-SEM, the 500 mg/L PBZ treatment was identified as achieving the optimal balance between growth inhibition and the maintenance of photosynthetic and nutritional status, closely approximating the ideal dwarfing effect. This study elucidates the complex regulatory effects of PBZ on the growth, photosynthesis, and carbon assimilation of Ougan through natural climate, providing robust technical parameters and theoretical support for future dwarf cultivation practices. These findings facilitate the development of dwarf fruit trees into bonsai vegetation, demonstrating significant horticultural application potential.