The pot, field mini-plot, and field split-plot experiments were carried out to investigate sweetpotato roots morphology(RM), number of tuberous roots per plant(NTR), leaf area index(LAI), shoots biomass, roots biomass and ratio of top to root(T/R-value) at 30, 60, 90, and 120 days after planting(DAP). At the whole growth period (0–120 d), nitrogen application promoted shoots growth, but had different effects on roots growth. At the early growing stage (0–30 d), low nitrogen rate (NR, N21) promoted roots growth and differentiation, but proper NR(N11, N22, N31) and excessive NR(N12, N23, N32) apparently inhibited roots development, causing NTR to decrease by 29.3%–36.6%(or 1.3–2.0 per plant) (P<0.05). The NTR at different nitrogen rates tended to be same in the split-plot experiment after 60 DAP. At the middle growth stage (30–60 d), proper or excessive nitrogen rates were in favor of expansion of sweetpotato source-sink relationship(SSR), enhancing shoots biomass by 35.5%–48.9% and 73.4%–79.8%, respectively(P<0.05), LAI rapidly ran up to four, but had no influence on roots biomass (P>0.05). At the late growth stage (60–120 d), excessive NR sustained shoots biomass growth, LAI and T/R at harvest reached 4.9–5.6 and 1.50–1.51, respectively, which were apparently higher than those (3.3–3.5 and 0.58–0.64) under the proper NR (P<0.05). When no nitrogen application, LAI and T/R at harvest were only 1.6–1.9 and 0.36–0.42, of which LAI was apparently lower than that under the proper NR (P<0.05). At the same time, there was a difference in fresh weight of individual tuberous root(FWITR) under different nitrogen rates. FWITR(218–345 g) under the proper NR was higher than those under no NR(181–262 g) and excessive NR(160–285 g, P<0.05), therefore, the final yield per plant under the proper NR(1.02–1.20 kg) was higher than thatunder no NR(0.88–0.91 kg) and excessive NR(0.79–0.87 kg, P<0.05). It is showed that nitrogen as base fertilizer, its less or no application is favorable for the establishment of SSR, its proper or excessive application delays SSR at early growth stage, its right or excessive application promotes the expandation of SSR at middle growth stage, and only right application would keep SSR balance at late growth stage.
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