Abstract: Crop productivity depends on photosynthetic source capacity. Appropriate nitrogen (N) fertilizer management is beneficial for improving growth, photosynthetic capacity and thereby increasing crops yield. A two-year pot experiment was conducted with four N treatments, i.e., conventional basal application 100 kg N·hm^-2 as control (FP), a total of 80 kg N·hm^-2 applied either 100% at basal application (JS), 100% at tuber initiation stage (35 d after transplant, KS), 50% at basal application and 50% at tuber initiation stage (35 d after transplant, FS), to examine the effects of reduced nitrogen fertilizer combined with application methods on the photosynthesis and chlorophyll fluorescence characteristics of sweetpotato (Ipomoea batatas) during summer 2016 and 2017. The results showed that the conventional basal application of a reduced N rate decreased photosynthesis of sweetpotato during the final growth phases compared to conventional application, dressing application relatively delayed late-season leaf senescence as indicated by the increased net photosynthetic rate (P_n), stomatal conductance (g_s), intercellular CO₂ concentration (C_i) and chlorophyll (Chl a+b) content during tuber expansion period. Split application of N fertilizer had noticeably higher P_n, g_s, C_i and Chl a+b than other treatments. Furthermore, split application of N fertilizer had a significantly higher photochemical efficiency of photosystem Ⅱ (F_v/F_m), quantum yield of electron transport (Φ_PSⅡ), and photochemical quenching co-efficient (q_P), but lower initial fluorescence (F_o) and non-photochemical quenching coefficient (NPQ) during tuber expansion period. The improved photosynthesis by split N was due to both increased F_v/F_m with higher electron transfer rate and reduced thermal dissipation of light energy in the tuber expansion period. Results were consistent between two sweetpotato cultivars across years. The results indicated that one-time fertilization at basal or tuber initiation stage were not conducive to sweetpotato leaf photosynthesis. The split N application was more beneficial in terms of delaying late-season leaf senescence, extending leaf function period, enhancing photosynthesis and biomass production under reduced N application rate, which would be beneficial for sweetpotato yield.

Key words: nitrogen application management under reduced rate, photosynthetic characteristics, chlorophyll fluorescence parameter, sweetpotato (Ipomoea batatas)