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.

Field and pot experiments were conducted to explore the response mechanism of endogenous hormones of potential storage root to phosphorus and its relationship with yield and appearance quality of sweetpotato using five different rates of phosphorus addition. Application of adequate amounts of phosphorus (P2 treatment, 112 kg of P2O5 ha–1 in field experiment or 0.04 g of P2O5 kg–1 in pot experiment) improved the yield and the appearance quality of sweetpotato when compared to the control treatment. This observation can be attributed to the fact that P2 treatment significantly increased the expression of Ibkn1 and APRT genes and the concentration of ZR from 20 to 40 days after planting, but the results were the opposite at 10 days after planting. In addition, an increase in the expression of SRD1, NIT4, IbMADS1, and OPR3 and the concentrations of IAA and JA from day 10 to day 40 after planting were observed. Furthermore, the expression of GA3oX4 and the concentration of GA3 decreased significantly from 20 to 30 days of planting and significantly increased after 40 days of planting. Moreover, a significant decrease in the expression of AAO and concentration of ABA was observed from 10 to 30 days after planting, and a significant increase was observed after 40 days of planting. The results show that P2 treatment promoted root development, particularly significantly increased the number of roots and potential storage roots. P2 treatment significantly increased the diameter, weight, and number of storage roots at 40 days after planting. Finally, proper phosphorus application (112 kg of P2O5 ha–1) increased the yield (enhanced from 18.99 to 25.93%) by increasing the number of storage roots per plant and improving the appearance quality by increasing the length/diameter ratio and uniformity of storage root weight.

The understanding of the effects of nitrogen sources on lignin synthesis in sweet potato during storage root formation is limited. In this study, we investigated the effects of different nitrogen source on sweet potato storage root formation and development, as well as lignin synthesis in potential storage roots. The sweet potato cultivars Shangshu 19 and Jixu 23 were used in field experiments in 2019 and 2020. Three treatments were tested: (a) no nitrogen fertilizer application (control); (b) 60 kg hm−2 ammonium nitrogen; and (c) 60 kg hm−2 amide nitrogen. The results indicate that during sweet potato storage root formation, ammonium nitrogen significantly enhanced root activity compared to that of the control. The ammonium nitrogen treatment promoted IbEXP1 and inhibited Ibkn1 and Ibkn2 expression during the early stages of storage root formation, then increased gibberellic acid and decreased zeatin riboside content, enhanced phenylalanine ammonia lyase and peroxidase activities, and promoted lignin synthesis in potential storage roots. The opposite effects of ammonium nitrogen treatment on gene expression, hormone contents, and enzyme activity were observed in the late stages of storage root formation. Relative to the control, the ammonium nitrogen treatment significantly increased the number of storage roots during canopy closure. The ammonium nitrogen treatment produced the highest storage root yield and number of storage roots per plant. These results indicated that the ammonium nitrogen can inhibit root lignin synthesis, then promote storage root formation and increase the yield of sweet potato.

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.

675 RESEARCHS torage roots yield differs among sweetpotato (Ipomoea batatas L.) varieties from nearly 60 t ha −1 in high-yielding varieties to <40 t ha −1 in low-yielding varieties. Both high-yielding and low-yielding varieties are easily overgrown (Duan et al., 2018), especially under excessive fertilization, which is common in crop production. When overgrowth appears, the distribution of assimilates between aboveground tissues and storage roots is not coordinated. Thus, storage root yield is notably decreased (Chen et al., 2012), mainly due to the low transportation rates of assimilates from leaves and stems to storage roots. Assimilate transportation from leaves and stems to storage roots is much faster in highyielding than in low-yielding varieties of sweetpotato, which results in a higher harvest index (Liu et al., 2015a). A further study indicated that the unloading of assimilates in storage roots is a major limitation for photosynthate transportation in sweetpotato varieties (Liu et al., 2015b). To design neoteric methods that improve storage roots yield and reduce resource waste caused by ABSTRACT To clarify the unloading pathway of assimilates in storage roots of sweetpotato (Ipomoea batatas L.), a combination of methods including electron microscopy, movement of the phloemmobile symplasmic tracer carboxyfluorescein, and assays of invertase and sucrose synthase activities were explored to investigate this pathway from storage roots formation to harvest. The sieve element-companion cell complex was symplasmically connected to surrounding parenchyma cells by plasmodesmata and isolated before storage root bulking (40-140 d after planting). Numerous plasmodesmata appeared among phloem parenchyma throughout storage root bulking. Images of carboxyfluorescein movement indicated that the dye was restricted to phloem and released into surrounding tissues before and after storage root formation, respectively. Sucrose synthase activity increased continually during storage root bulking, and it was much higher than that of insoluble acid invertase. Although this remained low and changed little, that of soluble acid invertase increased and remained high. The starch content in storage roots increased during bulking, but sucrose content decreased. Thus, the predominant unloading pathway switched from apoplasmic to symplasmic during storage root formation and bulking. This switch resulted in enhanced sink potential of storage roots, evidencing opportune sink-source relationships in sweetpotato.