Influence of Two Nitrogen Forms on Hormone Metabolism in Potential Storage Roots and Storage Root Number of Sweetpotato

Abstract

In this study, we investigated the effects of two N forms on sweetpotato [Ipomoea batatas (L.) Lam.] root growth and development, zeatin riboside (ZR) and indole‐3‐acetic acid (IAA) concentrations in potential storage roots, and storage root number per plant. During the period 2015 to 2016, field experiments were performed using the sweetpotato cultivars ‘Shangshu 19’ and ‘Jixu 23’, which involved the following three treatments: no application of N fertilizer, application of 60 kg N hm−2 ammonium N, and application of 60 kg N hm−2 amide N. Our results indicated that during storage root formation, compared with the control treatment, the ammonium N treatment led to a significantly higher number of thick roots. However, the ammonium N treatment significantly reduced the average diameter of thick roots during early storage root formation and increased the average diameter during late storage root formation. Compared with the control treatment, ammonium N treatment inhibited the expression of Ibkn1, Ibkn2, and SRD1 genes during the early stage of storage root formation and also reduced the activities of IAA oxidase, peroxidase, and polyphenol oxidase enzymes, thereby ultimately reducing the ZR and IAA concentrations in potential storage roots. The opposite pattern was observed during the late stage of storage root formation. During the canopy closure period, compared with the control treatment, ammonium N treatment significantly increased the number of storage roots with diameters of 0.5 to 5.0 cm but decreased the number of storage roots with diameters >5.0 cm. At harvest, the ammonium N treatment had resulted in the highest storage root yield and storage root number per plant.