Effects of nitrogen forms on carbohydrate metabolism and storage-root formation of sweet potato

Abstract

In this study, we examined the effects of different forms of nitrogen (N) fertilizer on carbohydrate metabolism and storage root formation in sweet potato [Ipomoea batatas L. (Lam.) cv. Shangshu 19 and cv. Jixu 23] in 2015–2016. Two fertilizer treatments, ammonium nitrogen (AN) and amide nitrogen (XN), were applied at 60 kg ha−1 in a two-factor split-plot design. The effects of nitrogen form on the morphology of adventitious roots, carbohydrate metabolism in potential storage roots, and number of storage roots per plant in sweet potato were investigated. The results show that during the early growth phase, the AN treatment significantly increased the number of adventitious roots, root tips, root length density, and fresh weight of roots (in pot trials). This treatment also significantly decreased the sucrose concentration of potential storage roots and increased the activities of cell wall, vacuolar, and cytoplasmic invertases. However, XN-treated potential storage roots showed a relatively high starch concentration, activities of sucrose synthase and ADP-glucose pyrophosphorylase, and transcription of sporamin genes. At the canopy closure period, the AN treatment significantly increased the number of storage roots of 0.5–5.0 cm in diameter and decreased the number of those > 5 cm in diameter compared to the control. The XN treatment induced the opposite effects. In the harvesting period, the AN treatment produced the highest storage root yield and number of storage roots per plant. Thus, in field trials the AN treatment induced a greater increase in production by increasing the number of storage roots.