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Article

Effects of Nitrogen Application on Soluble Sugar and Starch Accumulation During Sweet Potato Storage Root Formation

by
Hong Tham Dong
1,*,
Yujuan Li
2,
Philip Brown
1,
Delwar Akbar
1 and
Cheng-Yuan Xu
1,2
1
Central Queensland University, Rockhampton, QLD 4670, Australia
2
Charles Darwin University, Casuarina, NT 0810, Australia
*
Author to whom correspondence should be addressed.
Horticulturae 2025, 11(7), 837; https://doi.org/10.3390/horticulturae11070837
Submission received: 6 June 2025 / Revised: 10 July 2025 / Accepted: 12 July 2025 / Published: 15 July 2025
(This article belongs to the Section Plant Nutrition)
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Abstract

Nitrogen is an essential element for plant growth, and both insufficient and excessive use of nitrogen have been shown to negatively affect sweet potato production. Nitrogen supply can affect carbon metabolism in plant storage organs; however, limited studies have examined its effects on the accumulation of non-structural carbohydrates (soluble sugar and starch) during the formation of sweet potato storage roots. Two pot trials were conducted to evaluate the effects of different nitrogen application levels and timings on the accumulation of non-structural carbohydrates during the formation of sweet potato storage roots. In the first experiment, plants were supplied with 0, 50, 100, or 200 mg/L of nitrogen. In the second experiment, the optimum nitrogen rate (100 mg/L) for storage root formation from the previous experiment was applied at five different times: nil N supply and nitrogen applied at planting or 3, 7, or 14 days after planting. A significant highest starch accumulation in roots during the first 35 days after transplanting was recorded in the 100 mg/L treatment. However, sweet potato required more nitrogen after storage root formation, as indicated by higher non-structural carbohydrate accumulation in roots (1905 mg/plant) in the 200 mg/L treatment at 49 days after planting. Earlier nitrogen applications promoted soluble sugar and starch accumulation in plants during storage root formation, with up to 5697 mg of non-structural carbohydrate accumulated in a plant. The study provided agronomic indicators that moderate nitrogen should be available in soil before or on planting day.

Graphical Abstract

1. Introduction

Nitrogen (N) is an essential element in structural proteins, enzymes, chlorophyll, nucleic acids, and other organic compounds that are vital for the structural integrity and metabolic functions of plants, particularly for photosynthesis [1,2]. The importance of N for plant growth and productivity of crops has been recorded [3,4]. Nitrogen influences critical physiological processes such as photosynthesis and the activity of cambium, both of which are related to the yield and quality of storage roots (SRs) [5].
Several studies have found that N supply affected the development of vegetative and storage organs in plants [6,7,8]. These studies showed that the response of root growth to N supply depended on genotype and developmental stages. A previous study in sweet potato found a positive correlation between N fertilisation and growth of vine [9]. The study showed that components of storage root (SR) yield, such as root length and size during the first two months of growth, were affected by N application. A high level of N stimulated vine growth and inhibited root initiation of sweet potato [10,11,12]. Excessive N application inhibited the formation of sweet potato SRs [13]; for example, a high N content of 210 mg/L reduced the number of sweet potato SRs compared to lower N levels [14]. In comparison, N deficiency generally has a negative impact on storage organ development and growth. However, moderate N deficiency has been reported to reduce shoot growth and enhance root development of sweet potato [15]. Nitrogen deficiency significantly reduced the number of sweet potato SRs [16].
Non-structural carbohydrates (NSCs) which are mainly soluble sugar and starch, are used in important metabolism of plants such as respiration, anabolism, and cell development [17,18]. In sweet potato, starch is mainly stored in SRs, and the formation of SRs is related to its synthesis and accumulation [19,20,21]. A previous study showed that nitrogen supply significantly affected starch content in SRs of sweet potato [22]. Nitrogen forms (ammonium N and amide N) have been reported to affect sucrose and starch content of sweet potato SRs during the initiation stage [16]. The starch content and accumulation in sweet potato SRs were affected by nitrogen application levels [23].
In sweet potato, NSCs, including soluble sugar and starch, are essential for triggering and supporting SR development in the early stage of growth [24]. While previous studies have examined the influence of N fertilisation on NSC accumulating during storage root development or at commercial harvest stages [22,23], the effects of N application rates and N application timings on NSC dynamics of sweet potato during the SR formation have not been thoroughly investigated. Current knowledge on how nitrogen management affects soluble sugar and starch accumulation during SR initiation in sweet potato remains limited. Therefore, the aims of this study were to investigate the effects of N management on the accumulation of soluble sugar and starch during the formation of SRs. We specifically addressed the following questions: How does the N supply level affect the accumulation of soluble sugar and starch in sweet potato seedlings during the formation of SRs? How does N application timing influence soluble sugar and starch accumulation during SR formation and development of sweet potato? The Orleans cultivar was used in this study, as it is one of the most popular varieties in Australia and has consistent yields for early, middle, or late-season plantings [25]. Our findings provide insights into how N management strategies can be optimised to regulate carbohydrate allocation during SR formation. This has practical significance for improving early SR initiation and potentially enhancing the marketable yield of sweet potato.

2. Materials and Methods

2.1. Pot Experiments

The study consisted of two experiments that were conducted in a glasshouse at Bundaberg Research Facility (24°50′54″ S 152°24′14″ E). During the nitrogen level experiment (Experiment 1), the average daily maximum and minimum temperatures inside the glasshouse were 27.8 °C and 15.7 °C, respectively. The average daily maximum and minimum relative humidity were 79.4% and 49.3%, respectively. For Experiment 2, the average daily maximum and minimum temperatures were 32.9 °C and 20.5 °C, respectively, while the average daily maximum and minimum relative humidity were 86.9% and 43.1%, respectively.
In experiment 1, there were four treatments with different rates of N supply, being 0, 50, 100, and 200 mg/L in modified Hoagland solution (hereafter N0, N50, N100, and N200, respectively). The composition of nutrients in the solutions was reported in our prior study [26]. Plants were watered with respective nutrient solutions every two days to field capacity from planting. The amount of nutrient solution for each pot varied from 80 to 150 mL depending on plant ages and weather conditions. Experiment 2 consisted of five treatments with different started N timing applications, including none, on planting day, and on 3, 7, and 14 days after transplanting (DAT) (hereafter T0, T1, T3, T7, and T14, respectively). Hoagland’s modified solution lacking N [27] was utilised for the T0 treatment (see Table S1). The other treatments were supplied with the modified Hoagland’s solution supplemented with 100 mg/L N added in the form of NH4+ and NO3− (in a 1:1 ratio), which was identified as the optimal rate for supporting SR formation in the first experiment. Plants in experiment 2 were watered with N-free nutrient solution every two days until the scheduled N application dates. All pots in this experiment received the same amount of nutrient solution at each application, varying from 120 to 180 mL pot−1 depending on plant size and weather conditions.
The Orleans sweet potato cultivar was employed for the study. This is a variety known for its orange flesh, light rose skin, and uniform, elliptical shape [25]. All cuttings used for the experiments were healthy and uniform, were at least 20 cm long, and had five fully opened leaves. One cutting was planted horizontally in each pot, which was filled with 4 L of washed river sand, with three nodes below the sand surface and two fully opened leaves above ground. The pots were arranged in a completely randomised design (CRD). Tap water was added to the pots to achieve field capacity three days before transplanting. After transplanting, plants were watered with the same amount of nutrient to field capacity every other day. Plants were sampled on 10, 21, 35, and 49 DAT. At each sampling point, three plants per treatment were chosen randomly and dug up carefully to minimise root damage and washed thoroughly in water to remove all sand.
Fresh vine and root samples for each plant were collected after harvesting and kept separately in paper bags. In sweet potato, SRs are a part of the root system, so root samples in this study included fibrous roots, lignified roots, and SRs. Samples were dried in a preheated oven at 90 °C for 90 min to stop enzymatic sugar conversion in tissues and then converted to 70 °C for an additional 48–72 h to a constant weight [28]. Samples were stored in a −80 °C freezer after drying until extraction.

2.2. Non-Structural Carbohydrate Analysis

Soluble sugar and starch contents in both vines and roots were extracted by methanol:chloroform:water (MCW) solution [29,30]. As root biomass at 10 DAT was insufficient for analysis, NSC analysis for roots was conducted at three sampling dates starting from 21 to 49 DAT when sweet potato SRs started to form based on anatomical changes [26]. Overall, each sample was extracted three times by MCW solution. Samples were centrifuged for 10 min at 3000× g to separate the supernatant (containing soluble sugar) from the pellet (containing starch). Then, the supernatant was stored in a refrigerator while the pellet was placed in a fume hood overnight to dry. The next day, perchloric acid (35%) was added to the tubes containing the dried pellet to extract starch.
The measurements of soluble sugar and starch were conducted the day after extraction. The colorimetric phenol-sulfuric acid assay was used to determine the content of sugar and starch [31]. The soluble sugar supernatant was taken from the fridge and measured for the volume of the water: methanol fraction (soluble sugars) using the graduation on the centrifuge tube (generally 6.5 mL). A six-point standard curve for glucose (ranging from 0 to 100 µg mL−1) was prepared. The determination of glucose content was done with three repetitions. The absorbance of samples was recorded at a wavelength of 490 nm using a spectrophotometer. The amount of sugar and starch in each sample was determined based on the standard curve. The contents of sugar or starch in samples were calculated accordingly.

2.3. Statistical Data Analysis

All data recorded from each sampling event were analysed using one-way ANOVA using the IBM® SPSS® software statistical package (version 25; IBM, New York, NY, USA) to test the specific effects of N treatments at specific times. Because different plants were sampled in each harvest, two-way ANOVA (rather than repeated measure ANOVA) was used to analyse the interactive effects of N treatments and sampling times on soluble sugar and starch accumulation. This analysis allowed for testing the global effects of N applications, time, and the interactive effect of N by time over the study period. The contents of soluble sugar and starch were arcsine-transformed, while the accumulations were square root transformed for analysis in SPSS. All the post hoc tests were conducted with Tukey HSD. A p-value of less than 0.05 was regarded as statistically significant. Graphs were produced using the SigmaPlot® software package (version 14; SYSTAT Software, Inc., San Jose, CA, USA).

3. Results

3.1. Effect of N Rates on the Content of Soluble Sugar and Starch During the SR Initiation

The content of soluble sugar in both vines and roots was the highest in the N0 treatment (Figure 1), and higher rates of N applications led to lower content of soluble sugar. During SR formation, the soluble sugar content in vines fluctuated, whereas a consistent increase was observed in roots (Figure 1A,B). Starch content in roots grew over time in all treatments (Figure 1D). In the vine, starch content increased clearly over time in treatment N0, while it remained stable in the N100 and N200 treatments (Figure 1C). Low N availability inhibited this process. The content of starch in vines in the N0 treatment increased over time and was higher than those of any N supply levels.
The NSC content in vines and roots followed similar patterns to starch, which accounts for about 80% of NSC. The N0 treatment showed the highest content of NSC in both vines and roots, compared to other treatments (Figure 1E,F), suggesting the suppression of photosynthate translocation to SRs due to N deficiency. Both starch and NSC content in the N50 treatment were significantly lower than those in N0 but still higher than the N100 and N200 treatments. There was no statistically significant difference found in the NSC content between the N100 and N200 treatments.

3.2. Effect of N Application Timing on the Content of Soluble Sugar and Starch During SR Initiation

All four N fertilisation timing treatments reduced the soluble sugar content in both vines and roots compared to the T0 treatment (Figure 2A,B). The T1 treatment had the lowest content of soluble sugar at most sampling times. There was not much change in the soluble sugar content in vines (Figure 2A). However, the figure for roots increased noticeably in all treatments (Figure 2B). All four N fertilisation timing treatments had similar contents of soluble sugar in roots during the study period, which was significantly lower than the T0 treatment.
The effects of N application timings on the starch content were significant among treatments at all sampling times (Figure 2C,D). In vines, the starch contents were highest in the T0 treatment at all sampling dates except 10 DAT. At 10 DAT, T1 and T3 treatments showed the highest contents of starch at 180 and 191 mg g−1 dwt, respectively, which were significantly higher than that of other treatments. At 21 DAT, vine soluble sugar varied widely among treatments, and the earlier N was added, the lower the rate of soluble sugar, suggesting availability of N photosynthate use by stimulation of growth. This variation among treatments faded over time. In roots, the highest starch contents were also observed in the T0 treatment at various harvesting points. Other treatments with different N fertilisation timings had lower starch content. At 21 DAT, root starch showed similar patterns as that in vine, with lower content being observed in earlier N application timing treatments. After that, it appears that N application time did not affect NSC contents. In this experiment, total NSC (the sum of soluble sugar and starch) contents in both vine and roots followed similar patterns to starch content (Figure 2E,F).

3.3. Effect of N Rates and N Timing Application on the Soluble Sugar and Starch Accumulation in Plants During SR Initiation

In general, the total soluble sugar and starch, as well as NSC accumulation, increased over time in all treatments except the N0 treatment (Figure 3). The accumulation of these NSCs was positively related to N application levels in most observations. In the first three weeks after transplanting, the total soluble sugar accumulation in vines was similar in all treatments (Day 10: p = 0.51; Day 21: p = 0.15) (Figure 3A,B). After that, the effects of N application levels on vine soluble sugar were significant (Figure 3C,D). In contrast, vine starch accumulation showed significant differences among treatments over this study period. At the final sampling, the N200 treatment had significantly higher NSC in vines and roots, while the N0 treatment had the lowest NSC with only 71 mg plant−1 (Figure 3D). In roots, soluble sugar and starch accumulation followed a similar trend of N supply treatment to vines.
The two-way ANOVA results showed that the effects of N levels, harvesting times, and the interactive effect of N and time on the total NSC in vines, roots, and the ratio of vine and roots were statistically significant (Table 1). The differences among N treatments significantly increased over time, and the vine-to-root NSC ratio decreased over time (Table 2), and more significantly when N was supplied.
The effects of N application timings on total NSC in both vines and roots followed a similar trend to that observed with different N application levels (Figure 4). As plants were watered with the same N concentration in solution, earlier N treatments received more N cumulative input than late applications, resulting in more NSC accumulation. In this experiment, the no N supply (T0 treatment) had a significantly lower amount of soluble sugar and starch accumulated in vines and roots than in other treatments. During the first three weeks after transplanting, the accumulation of NSC was mainly in vines. However, from 35 DAT onwards, more NSCs were accumulated in roots compared to vines (Figure 1C,D). Results in this study confirmed that NSC accumulation in sweet potato is affected by N application.
The effect of N fertilisation timing on total soluble sugar in vines and roots was statistically significant at all sampling times except 10 DAT (Figure 4). At 10 DAT, there was no treatment effect on the soluble sugar accumulated in vines (Figure 4A). After that the lowest and highest soluble sugar accumulated in vines and roots was recorded in T0 and T1 treatments, respectively (Figure 4B–D). At 49 DAT, there was no statistical difference found in the soluble sugar accumulation among the T1, T3, and T7 treatments (Figure 4D).
Starch accumulation in both vines and roots increased over the study period. Applications of N from planting to 14 DAT increased the total starch accumulated in plants compared to no N application (T0). In general, the earlier N was applied, the more starch accumulated in the plant by 49 DAT.
Two-way ANOVA results showed that all effects of N application timing treatments, sampling times, and interaction between them had significant differences in soluble sugar, starch, and total NSC accumulation in vines and roots (Table 3). Earlier N application resulted in a lower vine-to-root NSC ratio (Table 4). This result confirms that timely N availability improves the translocation of NSC from vines to storage in SRs.

3.4. Relationship Between NSC Accumulation and SRs

N application levels and timing influenced SR number and SR weight at final harvesting (49 DAT) (see Tables S2 and S3). The relationships between NSC accumulation in roots and SR components (SR number and SR yield) as affected by both N application level and N application timing at the end of SR formation (49 DAT) based on the anatomical features [32] were positive (Figure 5). However, higher correlations were recorded between NSC and SR weight (R2 = 0.93) compared to that of between NSC and SR number (R2 = 0.74).

4. Discussions

4.1. Effect of N Levels on the Content of Soluble Sugar and Starch During the SR Initiation

The results of this study align with previous studies that N application affected soluble sugar and starch content in both vines and roots. In a previous study, a negative correlation between N supply and the content of soluble sugar and starch was observed in the MD810 and Jewel cultivars [12]. In a recent study, Si et al. (2018) found that N application reduced sucrose content during the early growth of the cultivars Shangshu 19 and Juxu 23 [16]. Similar results were recorded in other crops [33,34]. Previous studies have suggested that N deficiency inhibited the development of plant structure, including initiation and expansion of new leaves, and slower growth consumed less NSC, leading to accumulation of soluble sugar and starch in the [12,33]. Although our data did not directly measure growth rate or photosynthesis rate, the observed trends are consistent with these findings.
When N is sufficient, roots grow over time and function as a sink, actively receiving photo assimilates from the vine and accumulating starch [35]. Low N availability inhibits root sink strength and limits starch translocation. The treatment with the lowest N level (1 mM) had the highest content of starch in vines and roots [20]. Results in this study are similar to results observed in other crops such as corn, potato, and wheat [34,36,37]. Although high rates of N application promoted overall plant biomass production, they resulted in lower NSC content in tissues [38]. Normally, plants temporarily store NSCs in leaves before re-translocation to other parts [35].

4.2. Effect of N Application Timing on the Content of Soluble Sugar and Starch During SR Initiation

As plants were supplied with the same N solution, plants in the delayed N application treatment did not receive N before the treated days. Therefore, early N treatments received more cumulative N input compared to delayed N application treatments. Results from this study suggested that soluble sugar and starch content in both vines and roots were higher under N deficiency conditions (Nil N supply and delayed N treatments) during 21 and 35 DAT. This finding is aligned with previous studies in sweet potato and other crops. The soluble sugar content in vines was negatively associated with N levels [12]. In another study, Kim et al. (2002) reported that higher content of starch in vines and roots was found in the lower N treatments, because N limited plant growth and consumption of photosynthates [20]. Similarly, the highest NSC content was obtained under N deficiency in many crops, such as pepper, olive, and microalgae [39,40,41]. Additionally, delayed N application led to growth limitation and caused soluble sugar and starch accumulation in vines, as indicated by significantly higher content of soluble sugar and starch in the late N fertilisation timing treatment during 21–35 DAT. These results reinforce the information of N in regulating sweet potato growth and NSC accumulation in SRs. Numerous studies have found a negative relationship between N application rates and NSC contents [20,42].
The total NSC content in both vines and roots followed a similar pattern to starch content. One possible explanation is that NSCs were temporarily stored in leaves before relocation to roots after SR formation [7], while most storage NSCs were starch (>25% in roots at 49DAT).

4.3. Effect of N Rates and N Timing Application on the Soluble Sugar and Starch Accumulation in Plants During SR Initiation

The accumulation of NSC was positively related to N application levels. This indicates that the accumulation of soluble sugar in vines and roots increased with N availability [36], suggesting a general promotive effect of N on sweet potato growth and photosynthate accumulation. Nitrogen application could promote photosynthetic activities in plants, which could increase NSC accumulation [43]. Our results are consistent with previous findings in sweet potato that sucrose and starch accumulation in sweet potato storage roots increased over time [44] and in response to N application rates [23]. Similar patterns have also been observed in potato [45] and wheat [37].
In roots, the soluble sugar and starch accumulation followed patterns similar to those in vines under different N supply treatments. This observation supports the idea that N applications promoted overall plant growth and enabled the process of photosynthesis [46], contributing to greater NSC accumulated in plants [43]. In a previous study, total soluble sugar accumulated in potato leaves was associated with rates of N application [34], and suitable N application significantly increased NSC accumulation in wheat and maize [36,37,47].
Low N availability inhibited plant growth [46], which may reduce assimilate translocation and lead to higher accumulation of NSCs in leaves [47]. A lack of N supply was reported to inhibit the movement of NSCs from vine to roots [12]. Similarly, N application improved the translocation of NSC from stem to the storage organ, such as grain [43]. In our study, the earlier N application corresponded with greater NSC accumulated in vines and subsequently in roots. These suggests that N promoted sweet potato plant growth and led to photosynthate accumulation. This findings are in line with recent results that the total NSCs accumulated in vines increased significantly after 10 DAT, while that of roots grew after 21 DAT [23]. A possible reason for that would be temporary storage of NSCs in leaves before relocation to roots after SR formation [7]. The influence of N applications on NSC accumulation has also been studied in different plant species. Studies in wheat and rice suggested that suitable N application significantly increases the accumulation of NSC in stem and spikelet [37,43].

4.4. Relationships Between NSC Accumulation and SR Number

The positive correlations between NSC accumulation in roots and the number of SR or fresh SR weight at 49 DAT suggest that, although NSC was analysed for the whole root system, including the storage root and auxiliary roots, it remains a reliable indicator of SR development. Such positive correlation between NSC and storage organ yield was consistent with a previous study in rice and wheat [47,48], in which NSC were stored in grains.

5. Conclusions

Our study demonstrated that both N application rates and timings significantly affected the soluble sugar and starch content and accumulation in sweet potato during the SR formation. Nitrogen deficiency increased the contents of soluble sugar and starch in both vines and roots during SR initiation. Moderate N supply (100 mg/L) promoted early starch accumulation, while a higher rate of N applications (200 mg/L) enhanced total NSC accumulation at a later stage (49 DAT), indicating a shift in N requirements for SR development.
Early nitrogen application, particularly at or before planting, promoted soluble sugar and starch accumulation in sweet potato vines and roots, and facilitated greater translocation of NSCs from vines to roots contributing to a higher number of SRs and fresh SR yield per plant. In contrast, delayed or insufficient N supply reduced NSC accumulation in roots and increased accumulation in vines, indicating impaired translocation of NSCs between source and sink.
These findings underscore the importance of N management, including application rates and application timings, in optimising NSC allocation during SR formation. By adopting N application strategies, growers can improve sweet potato production and SR quality, offering practical benefits for sweet potato yield and marketability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/horticulturae11070837/s1, Table S1: Hoagland nutrient solution recipes; Table S2: Effect of N application levels on SR of Orleans at 56 DAT; Table S3: Effect of N application timing on SR of Orleans at 49 DAT.

Author Contributions

Conceptualization, H.T.D., Y.L., P.B. and C.-Y.X.; methodology, H.T.D. and C.-Y.X.; validation, H.T.D. and C.-Y.X.; formal analysis, H.T.D.; investigation, H.T.D.; resources, Y.L.; data curation, H.T.D.; writing—original draft preparation, H.T.D.; writing—review and editing, Y.L., P.B., D.A. and C.-Y.X.; visualization, D.A. and C.-Y.X.; supervision, D.A. and C.-Y.X.; project administration, H.T.D.; funding acquisition, H.T.D. and C.-Y.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Central Queensland University and Australian Research Training Program (RTP) scholarship.

Data Availability Statement

Data will be made available on request.

Acknowledgments

The research was funded through CQUniversity’s Research Higher Degree Candidature Budget and Australian Government RTP scholarship. We gratefully acknowledge the support from Queensland Department of Agriculture and Fisheries, Bundaberg Research Facility, and its staff for their assistance during the experiment. Thanks are expressed to CQUniversity Bundaberg technician lab staff for their assistant in the lab work.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
DATDays After Transplanting
FSRWFresh Storage Root Weight
MCWMethanol:Chloroform:Water
NNitrogen
NLNitrogen Level
NSCNon-structural Carbohydrate
NSCRNon-structural Carbohydrates in Root
NSCVNon-structural Carbohydrates in Vines
NTNitrogen application Timing
SRStorage Root
SRtStarch in Roots
SSSoluble Sugar
SSRSoluble Sugar in Roots
SSVSoluble Sugar in Vine
SVStarch in Vine
TTime

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Horticulturae 11 00837 g001
Figure 1. Content of NSCs as affected by N rates. (A) Soluble sugar content in vines; (B) Soluble sugar content in roots; (C) Starch content in vines; (D) Starch content in roots; (E) NSC content in vines; and (F) NSC content in roots. Values are indicated as mean ± SE (n = 3). ANOVA results are based on square root transformed data. Two-way ANOVA results, including effects of N level, harvesting date, and N level by time interaction on SS, starch, and NSC contents, are shown. Different letters indicate significant differences among treatments on the same harvesting dates using one-way ANOVA (Tukey’s HSD, p < 0.05). Abbreviations: SS = Soluble sugar; NSC = Non-structural carbohydrates; NL = N level; T = Time.
Figure 1. Content of NSCs as affected by N rates. (A) Soluble sugar content in vines; (B) Soluble sugar content in roots; (C) Starch content in vines; (D) Starch content in roots; (E) NSC content in vines; and (F) NSC content in roots. Values are indicated as mean ± SE (n = 3). ANOVA results are based on square root transformed data. Two-way ANOVA results, including effects of N level, harvesting date, and N level by time interaction on SS, starch, and NSC contents, are shown. Different letters indicate significant differences among treatments on the same harvesting dates using one-way ANOVA (Tukey’s HSD, p < 0.05). Abbreviations: SS = Soluble sugar; NSC = Non-structural carbohydrates; NL = N level; T = Time.
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Figure 2. Content of NSCs as affected by N timing application. (A) SS content in vines; (B) SS content in roots; (C) Starch content in vines; (D) Starch content in roots; (E) NSC content in vines; and (F) NSC content in roots. Values are indicated as mean ± SE (n = 3). ANOVA results are based on square root transformed data. Two-way ANOVA results, including effects of N application timing, harvesting date, and N application timing by time interaction on SS, starch, and NSC contents, are shown. Different letters indicate significant differences among treatments on the same harvesting dates using one-way ANOVA (Tukey’s HSD, p < 0.05). Abbreviations: SS = Soluble sugar; NSC = Non-structural carbohydrates; NT = N application timing; T = Time.
Figure 2. Content of NSCs as affected by N timing application. (A) SS content in vines; (B) SS content in roots; (C) Starch content in vines; (D) Starch content in roots; (E) NSC content in vines; and (F) NSC content in roots. Values are indicated as mean ± SE (n = 3). ANOVA results are based on square root transformed data. Two-way ANOVA results, including effects of N application timing, harvesting date, and N application timing by time interaction on SS, starch, and NSC contents, are shown. Different letters indicate significant differences among treatments on the same harvesting dates using one-way ANOVA (Tukey’s HSD, p < 0.05). Abbreviations: SS = Soluble sugar; NSC = Non-structural carbohydrates; NT = N application timing; T = Time.
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Figure 3. Effect of N levels on the accumulation of soluble sugar and starch in plants at various sampling times. (A) 10 DAT; (B) 21 DAT; (C) 35 DAT; and (D) 49 DAT. The x-axis represents the soluble sugar (SS)/starch, and the y-axis represents the treatments. Numbers on the left and right are mean values of total NSC (SS + Starch) in roots and vines, respectively, followed by standard error (SE) (n = 3). ANOVA results are based on square root transformed data. Means followed by different letters are significantly different among treatments (Tukey’s HSD, p < 0.05). Abbreviations: SRt = Starch in roots; SSR = Soluble sugar in roots; SSV = Soluble sugar in vine; SV = Starch in vine.
Figure 3. Effect of N levels on the accumulation of soluble sugar and starch in plants at various sampling times. (A) 10 DAT; (B) 21 DAT; (C) 35 DAT; and (D) 49 DAT. The x-axis represents the soluble sugar (SS)/starch, and the y-axis represents the treatments. Numbers on the left and right are mean values of total NSC (SS + Starch) in roots and vines, respectively, followed by standard error (SE) (n = 3). ANOVA results are based on square root transformed data. Means followed by different letters are significantly different among treatments (Tukey’s HSD, p < 0.05). Abbreviations: SRt = Starch in roots; SSR = Soluble sugar in roots; SSV = Soluble sugar in vine; SV = Starch in vine.
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Figure 4. Effect of N application timings on the accumulation of soluble sugar and starch in plants at various sampling times. (A) 10 DAT, (B) 21 DAT, (C) 35 DAT, and (D) 49 DAT. The x-axis represents the N acquisition, and the y-axis represents the treatments. Numbers on the left and right are mean values of total NSC (SS + Starch) acquisition in roots and vines, respectively, followed by standard error (SE) (n = 3). ANOVA results are based on square root transformed data. Means followed by different letters are significantly different among treatments using one-way ANOVA (Tukey’s HSD, p < 0.05). Abbreviations: SRt = Starch in roots; SSR = Soluble sugar in roots; SSV = Soluble sugar in vine; SV = Starch in vine.
Figure 4. Effect of N application timings on the accumulation of soluble sugar and starch in plants at various sampling times. (A) 10 DAT, (B) 21 DAT, (C) 35 DAT, and (D) 49 DAT. The x-axis represents the N acquisition, and the y-axis represents the treatments. Numbers on the left and right are mean values of total NSC (SS + Starch) acquisition in roots and vines, respectively, followed by standard error (SE) (n = 3). ANOVA results are based on square root transformed data. Means followed by different letters are significantly different among treatments using one-way ANOVA (Tukey’s HSD, p < 0.05). Abbreviations: SRt = Starch in roots; SSR = Soluble sugar in roots; SSV = Soluble sugar in vine; SV = Starch in vine.
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Figure 5. Relationship of NSC in roots and SR yield and SR number at 49 DAT as affected by N application levels and timings. Abbreviations: SR = storage root; NSC = non-structural carbohydrates.
Figure 5. Relationship of NSC in roots and SR yield and SR number at 49 DAT as affected by N application levels and timings. Abbreviations: SR = storage root; NSC = non-structural carbohydrates.
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Table 1. The main and interactive effects of N treatments and time on the accumulations of soluble sugar, starch, and NSC in sweet potato plants.
Table 1. The main and interactive effects of N treatments and time on the accumulations of soluble sugar, starch, and NSC in sweet potato plants.
FactorSSVStarch in Vine NSCVSSRStarch in Roots NSCR
N level<0.001<0.001<0.001<0.001<0.001<0.001
Time<0.001<0.001<0.001<0.001<0.001<0.001
N level × time<0.001<0.001<0.001<0.001<0.001<0.001
Two-way ANOVA results are based on square root transformed data. Abbreviations: SSV = Soluble sugar in vines; SSR = Soluble sugar in roots; NSCV = Non-structural carbohydrates in vines; NSCR = Non-structural carbohydrates in roots.
Table 2. The effect of N rates on the vine-to-root NSC ratio on different sampling dates.
Table 2. The effect of N rates on the vine-to-root NSC ratio on different sampling dates.
Treatments21 DAT35 DAT49 DATANOVA
N03.9 ± 0.5 b2.7 ± 0.22.7 ± 0.1 aNL: p = 0.02
N505.5 ± 0.3 ab2.4 ± 0.11.6 ± 0.1 bT: p < 0.001
N1007.2 ± 0.2 a2.6 ± 0.11.3 ± 0.1 bcNL × T: p < 0.001
N2006.5 ± 0.6 a3.3 ± 0.41.2 ± 0.1 c
p value0.0030.13<0.001
The table presents the mean values followed by standard errors (SE) (n = 3). ANOVA results are based on square root transformed data. Means followed by different letters are significantly different (Tukey’s HSD, p < 0.05) within columns (one-way ANOVA). The two-way ANOVA results, including N level, time, and the interactive effect of N level by time, are shown in the last column. Abbreviations: NL = N level; T = Time.
Table 3. The main and interaction effects of N treatments and time on the accumulation of soluble sugar and starch in vines and roots of sweet potato.
Table 3. The main and interaction effects of N treatments and time on the accumulation of soluble sugar and starch in vines and roots of sweet potato.
FactorVinesRoots
SSStarchTotalSSStarchTotal
NT<0.001<0.001<0.001<0.001<0.001<0.001
T<0.001<0.001<0.001<0.001<0.001<0.001
NT × T<0.001<0.001<0.001<0.001<0.001<0.01
Two-way ANOVA results are based on square root transformed data. The asterisk denotes the interaction between the two factors Abbreviation: NT = Nitrogen application timing, T = Time, SS = Soluble sugar.
Table 4. The effect of N application timing on the vine-to-root NSC ratio on different sampling dates.
Table 4. The effect of N application timing on the vine-to-root NSC ratio on different sampling dates.
Treatments21 DAT35 DAT49 DATANOVA
T03.59 ± 0.14 ab1.24 ± 0.09 a0.51 ± 0.01 aNT: p < 0.001
T12.75 ± 0.14 c0.47 ± 0.02 c0.36 ± 0.01 bT: p < 0.001
T32.46 ± 0.17 c0.41 ± 0.01 c0.37 ± 0.03 bNT × T: p < 0.001
T72.95 ± 0.08 bc0.55 ± 0.01 c0.42 ± 0.01 b
T143.8 ± 0.29 a0.91 ± 0.05 b0.43 ± 0.02 ab
p value0.001<0.001<0.001
The table presents the mean values followed by standard errors (SE) (n = 3). ANOVA results are based on square root transformed data. Means followed by different letters are significantly different (Tukey’s HSD, p < 0.05) within columns (one-way ANOVA). The two-way ANOVA results, including N level, time, and the interactive effect of N level by time, are shown in the last column. Abbreviations: NT = N application timing; T = Time.
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Dong, H.T.; Li, Y.; Brown, P.; Akbar, D.; Xu, C.-Y. Effects of Nitrogen Application on Soluble Sugar and Starch Accumulation During Sweet Potato Storage Root Formation. Horticulturae 2025, 11, 837. https://doi.org/10.3390/horticulturae11070837

AMA Style

Dong HT, Li Y, Brown P, Akbar D, Xu C-Y. Effects of Nitrogen Application on Soluble Sugar and Starch Accumulation During Sweet Potato Storage Root Formation. Horticulturae. 2025; 11(7):837. https://doi.org/10.3390/horticulturae11070837

Chicago/Turabian Style

Dong, Hong Tham, Yujuan Li, Philip Brown, Delwar Akbar, and Cheng-Yuan Xu. 2025. "Effects of Nitrogen Application on Soluble Sugar and Starch Accumulation During Sweet Potato Storage Root Formation" Horticulturae 11, no. 7: 837. https://doi.org/10.3390/horticulturae11070837

APA Style

Dong, H. T., Li, Y., Brown, P., Akbar, D., & Xu, C.-Y. (2025). Effects of Nitrogen Application on Soluble Sugar and Starch Accumulation During Sweet Potato Storage Root Formation. Horticulturae, 11(7), 837. https://doi.org/10.3390/horticulturae11070837

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