Objectives A field experiment was carried out in order to determine the optimum nitrogen (N) application rate with or without applying uniconazole for dry matter accumulation (DMA), top-root ratio(T/R), N content of above and below-ground biomass, yield, N utilization efficiency and uniconazole contribution rate (UCR) of sweat potato (Jishu22).

Methods Using sweet potato cultivar of Jishu 22 as material, a field experiment was carried out by setting-up N treatments (N) and N-uniconazole treatments (N-U) with different N application rates in Pingyin, Shandong Province. The N rates in treatments were 0, 45, 90, 135, 180, 270 kg/hm², recorded as N₀, N₁, N₂, N₃, N₄, and N₅ in turn. The uniconazole application rate was 25 mg/L, using no fertilizer as control. Dry matter accumulation (DMA), N utilization efficiency and yield were recorded 85, 110 days after transplanting and during harvesting stage.

Results 1)The DMA of aboveground biomass increased with the increasing of nitrogen application rates, while the DMA in N-U treatments had no differences, except N₁-U treatment. Spraying of uniconazole significantly reduced the DMA of aboveground in N₃-U, N₄-U and N₅-U treatments. This suggested that the growth of aboveground biomass was constrained by uniconazole application. The DMA of underground biomass was significantly decreased when N application rate was higher than N₂ (90 kg/hm²) in both N and N-U treatments with the DMA higher in N-U treatments than that in N treatments. Significant correlation was noted between uniconazole and DMA of underground biomass in N₁-U and N₂-U treatments. 2) T/R ratio of N-U treatments decreased in all three growing periods, suggesting the retarded aboveground growth and accelerated underground growth by applying uniconazole. 3) Application of uniconazole increased N content of vine of Jishu22 after 85 and 110 days, while after 140 days it didn’t differ. There was a point in N content of storage root of Jishu22 between N and N-U treatments which was near the application rate of N₂ (90 kg/hm²), this indicated that 90 kg/hm² was the minimum application rate with the aim to increase the N concentration of storage root. 4) The highest yield was found in N₁ (N 45 kg/hm²) in N treatments and N₂-U (N 90 kg/hm²) in N-U treatments, respectively with no further increase in yield with excessive fertilizer application. At the same N application rate, the yield was found to increase in CK-U and N₀-U treatments by the application of uniconazole. 5) With increasing N application, fertilizer contribution rate (FCR) and agronomy efficiency (AE) changed following the pattern of “V” in N treatment with the highest value in N₁. FCR and AE decreased in all treatments except in N₂-U treatment with uniconazole. Partial factor productivity (PFP) decreased with increasing fo N application with greater PFP in N-U treatments than those in N treatments. Uniconazole contribution rate (UCR) to yield followed the order of N₂ > N₀ > CK > N₄ > N₃ > N₅ > N₁.

Conclusion Nitrogen application rate is the key factor regulating yield of sweet potato (Jishu22). The study suggested that the optimum N application rate was N₁ 45 kg/hm² in N treatment and N₂ 90 kg/hm² in N-U treatment. Higher yield and improved growth of aboveground and underground biomass were found after the application of uniconazole.