Optimum Nitrogen Application Promotes Sweetpotato Storage Root Initiation

Storage root formation of sweetpotato (Ipomoea batatas (L.) Lam) is a complex developmental process relating to the activity of cambium. Little information is available on the relationship between nitrogen (N) application levels and the initiation and development of sweetpotato storage roots (SRs). This study aims to examine how N application rates promoted/inhibited the formation and development of storage roots (SRs) for sweetpotato cultivar ‘Orleans’ during the first 8 weeks after planting. Cuttings were grown in coarse river sand culture supplied with modified Hoagland nutrient solution at four different rates (0 (N0), 50 (N50), 100 (N100) or 200 (N200) mg L−1) of N. The results showed that N100 treatment promoted the formation of primary and secondary cambium, resulting in a significant higher rate of SR formation between 21 and 56 days after transplanting (DAT). Due to the higher N demand after formation of SRs, N200 treatment displayed faster growth, higher N acquisition and the highest efficiency of N use after 35 DAT, but the SR formation rate and SR number per plant remained insignificantly lower than N100 when differentiation of adventitious root was mostly completed (49 DAT), suggesting irreversible an effect of N rate during SR initiation, which eventually affects SR number. The results suggested that the optimal substrate N level for sweetpotato SR initiation is lower than that for following SR growth, which should be considered in the fertilisation scheme.