Effect of Paclobutrazol on Growth and Storage Root Yield of Sweet Potato (Ipomoea batatas (L.) Lam.) ‘Kuri Kogane’ Grown in Planting Bags

Excessive vegetative growth in sweet potato often disrupts source–sink relationships, reducing assimilate allocation to storage roots in containerized systems. This study evaluated the effects of paclobutrazol (PBZ)—a gibberellin biosynthesis inhibitor—on the vegetative growth, physiological parameters, and storage root yield of sweet potato (Ipomoea batatas L.) cv. Kuri Kogane grown in planting bags. A completely randomized design (CRD) was implemented using PBZ concentrations of 0, 25, 50, 100, and 200 ppm. Applications at 100 and 200 ppm significantly suppressed leaf area and aboveground biomass at 2 months after planting (MAP). Conversely, 100 and 200 ppm PBZ enhanced photosynthetic pigments, significantly increasing SPAD chlorophyll meter readings (SCMRs), with 100 and 200 ppm yielding the highest chlorophyll a and total chlorophyll contents at 1 month after planting. Consequently, 100 and 200 ppm PBZ increased fresh tuber yields to 41,933 and 42,000 kg ha−1, respectively. Although the Harvest Index numerically increased to 0.61–0.63, this non-significant trend was driven primarily by the reduction in aboveground biomass rather than an increase in tuber yield. Furthermore, 200 ppm PBZ promoted foliar nutrient accumulation (N, P, K, Ca, and Mg) and elevated tuber protein content to a maximum of 6.32%, while sweetness (°Brix) and carotenoid content remained unchanged. In conclusion, PBZ application effectively restricts excess shoot growth and enhances storage root yield and protein content in container-grown sweet potato.