Screening the Optimal Concentration and Timing of Paclobutrazol for the Growth and Development of Container-Grown Blueberries

The blueberry variety ‘Liberty’ shows excessive vegetative growth and difficulty in flower bud differentiation under container cultivation. Paclobutrazol (PBZ), a widely used plant growth regulator, effectively modulates the balance between vegetative and reproductive growth in plants; however, its application in container-cultivated blueberries remains understudied. This study systematically investigated the effects of different PBZ concentrations (0–200 mg·L−1) on the growth and development, physiological characteristics, and fruit quality of container-cultivated ‘Liberty’ blueberries and further clarified the optimal application time. Results showed that low-concentration treatments (≤75 mg·L−1) significantly enhanced root development, increased new shoot diameter, and elevated the root-to-shoot ratio. Concurrently, it induced the coordinated thickening of palisade and spongy tissues in leaves and significantly increased the maximum photochemical efficiency (Fv/Fm) and chlorophyll content. The contents of endogenous hormones indole-3-acetic acid (IAA) and gibberellin (GA3) in new shoots were significantly reduced, while the cytokinin-to-gibberellin ratio (CTK/GA3) in flower buds was markedly elevated. These changes effectively promoted flower bud differentiation, increased bud number, and advanced the flowering time by approximately 2 days. Fruit quality was significantly improved. Under high concentration treatments, the content of malondialdehyde (MDA) continuously accumulated, and the activities of antioxidant enzymes (CAT, POD, SOD) significantly decreased. Furthermore, the efficacy of paclobutrazol weakened with the delay of application time. Comprehensive analysis indicated that the 50 mg·L−1 concentration effectively suppressed excessive vegetative growth and maximally improved fruit quality, with its application at the mid-stage of secondary shoot growth exhibiting favorable coordination of plant growth and development. This provides a theoretical basis for the application of paclobutrazol in blueberry production.