Chiang Mai Journal of Science

Paclobutrazol (PBZ), a gibberellin biosynthesis inhibitor, was investigated for its effects on vegetative growth, flowering, and leaf anatomy in Chrysanthemum (Dendranthema grandiflora cv. Chompoo-Phan). This study evaluated PBZ application at 0, 100, 200, 300, and 400 ppm via soil drenching twice over a 13-week experimental period. Results demonstrated that PBZ significantly decreased plant height, canopy width, leaf area, and stem diameter in a concentration-dependent manner, while promoting lateral branching and enhancing compactness traits. Anatomical assessments revealed distinct leaf modifications, including increased palisade and spongy mesophyll thickness (characterized by increased cell number (hyperplasia) and cell size (hypertrophy)), total leaf thickness, and stomatal density, particularly at higher concentrations. Regarding reproductive traits, flower number increased significantly at 200 ppm, although flower diameter decreased, and flowering time was delayed with increasing PBZ levels. Biomass analysis indicated a physiological trade-off: shoot and leaf fresh and dry weights declined, whereas root biomass increased significantly at moderate concentrations. Furthermore, photosynthetic pigment contents (chlorophyll a, b, total chlorophyll, and carotenoids) were enhanced, peaking at 300–400 ppm. Overall, PBZ significantly reduced vegetative growth and delayed flowering while increasing flower number, inducing anatomical reinforcement, and enhancing pigment accumulation. These findings suggest that 200 ppm is the optimal concentration to improve plant compactness and floriferousness, demonstrating high potential for converting this cultivar into a marketable potted plant.