Early Nitrogen Applications Promote the Initiation of Sweetpotato Storage Roots

A number of published studies have focused on the effects of N application timing on sweetpotato yield. However, none of them examined the influence of the timing of N fertilisation on the initiation of storage roots (SRs), which largely determines the number of tubers per plant and substantially affect the yield. A pot experiment was conducted in sand culture to evaluate the influence of N application timing (N applied on the planting day, 3, 7 and 14 days after transplanting (DAT), denoted T1, T3, T7 and T14, respectively, and a no N applied control T0) on the plant acquisition of N and the initiation of SRs of ‘Orleans’ cultivar. A previously-determined optimal N rate for the SR formation (Hoagland solution with 100mg/L N) in the experimental system was used. Rapid SR formation was found to require N during the first week after transplanting. No or late N application delayed the initiation of SRs and reduced the number of SRs. No or delayed N application inhibited the formation of regular vascular cambium (RVC) and anomalous cambium (AC) and led to an increase in lignification of stele cells leading to significantly higher proportion of lignified roots. By contrast, plants supplied with N within 7 DAT had a higher rate of SRs and lower rate of lignified roots. Also, earlier N application promoted total plant and root growth measured as higher biomass and storage root weight, as well as higher N accumulation in plants and higher nitrogen recovery efficiency (NRE). Results suggested that the SR formation required N during the first week after transplanting. Therefore, moderate N should be available in soil before or on planting day.