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Article

Localized Phosphorus Application Promotes the Growth and Nutrient Content of Pitaya (Hylocereus polyrhizus) Seedlings by Improving Root Morphology

by
Jiamin Wu
1,2,†,
Chen Chen
2,†,
Youhui Fan
1,2,†,
Yunze Ruan
3,
Junfeng Qu
4,
Fanrong Pan
4,
Zhiliang Chen
4 and
Wei Gao
1,2,*
1
Sanya Institute of Breeding and Multiplication, Hainan University, Sanya 572025, China
2
School of Tropical Agriculture and Forestry, Hainan University, Danzhou 571737, China
3
School of Breeding and Multiplication, Hainan University, Sanya 572025, China
4
China BlueChemical Ltd., Beijing 100029, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2026, 16(2), 173; https://doi.org/10.3390/agronomy16020173
Submission received: 8 December 2025 / Revised: 24 December 2025 / Accepted: 8 January 2026 / Published: 9 January 2026
(This article belongs to the Special Issue Soil Health and Nutrient Cycling Mediated by Plant-Microbial-Soil Interactions)
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Versions Notes

Abstract

Localized phosphorus (P) application stimulates root proliferation in nutrient-rich zones, aligns root growth with soil P distribution, and enhances nutrient uptake efficiency. However, whether localized P supply benefits pitaya remains unclear. In this study, pitaya seedlings were used in a rhizobox experiment with four P treatments: no P (No-P), uniform P (Uniform-P1, 50 mg kg−1), localized P (Local-P1, 50 mg kg−1), and uniform high P (Uniform-P2, 200 mg kg−1). Phosphorus treatments significantly affected shoot growth. Localized P supply produced greater shoot biomass than both uniform and high uniform P. It also increased shoot nitrogen and potassium content but did not significantly affect P content. Localized P application strongly promoted root development. Compared with Uniform-P1, Local-P1 increased root biomass, total root length, root surface area, and root volume by 142.11%, 121.77%, 110.25%, and 100.19%, respectively. Moreover, total root length, root surface area, and root volume were positively correlated with nitrogen and potassium content as well as total biomass. These findings demonstrate that localized P supply improves root morphology, enhances nitrogen and potassium acquisition, and promotes shoot growth in pitaya.

1. Introduction

Phosphorus (P) is an essential macronutrient for plant growth and development [1]. It is a key component of nucleic acids and phospholipids, and it plays a crucial role in genetic information transfer, material transformation, and energy metabolism [2,3]. Phosphorus fertilization effectively promotes plant growth and root development, thereby improving crop yield and quality [4,5,6,7]. However, farmers often apply P in excess of crop requirements, and combined with inappropriate application methods, this leads to P surplus in farmland and low fertilizer use efficiency within a single growing season [8]. As a non-renewable resource, phosphate rock reserves are being rapidly depleted, with easily accessible reserves expected to run out within the next few decades. In China, high-quality phosphate rock accounts for only 7% of the country’s total P reserves [9]. Limited reserves and excessive or improper application not only increase production costs and waste resources but also pose serious environmental risks [10,11,12,13,14]. Rational P management and strategies to improve P use efficiency are therefore urgent priorities for sustainable agricultural production.
Phosphorus (P) in soil exhibits poor mobility, is easily fixed, and diffuses slowly [15]. After fertilizer application, soil cations such as Fe3+, Al3+, Ca2+, and Mg2+ rapidly react with soluble P to form insoluble compounds, while microbial processes further contribute to immobilization [16]. As a result, only 10–25% of applied P is available for crop uptake [17,18]. Limited P mobility is therefore a key constraint on plant P acquisition [19,20]. Localized fertilization, which concentrates nutrients in specific soil zones, has been proposed as an effective strategy to reduce P fixation by limiting contact with soil cations or Fe/Al oxides [21]. In intensive systems, applying nutrients at proper distances and depths near roots enhances crop productivity, increases fertilizer use efficiency, and reduces environmental risks [22,23,24]. Numerous studies demonstrate that localized P supply improves P utilization, recovery, and availability [25,26,27,28,29].
Roots are highly plastic during development [30]. Their architecture strongly influences plant growth and nutrient use efficiency. For phosphorus (P), which is easily fixed and poorly mobile in soil, greater root–soil contact enhances absorption. Fertilizer placement regulates root growth and distribution. It shapes root architecture and enables crops to maximize nutrient capture [31,32]. Localized P supply improves plant growth by aligning spatial root development with soil P availability [33]. Plant roots exploit nutrient-rich soil patches by altering morphology or increasing nutrient uptake rates [34,35,36]. For example, localized P increased maize first-order lateral root density and length in fertile layers as compared with uniform application [37]. In maize–faba bean intercropping, localized P increased maize root density but had no effect on faba bean [38]. Similarly, studies on Brassica chinensis showed that localized P increased root length density in nutrient-rich zones and improved P uptake compared with homogeneous P [39]. These findings indicate that localized P supply substantially alters root morphology and physiological uptake capacity, with species-specific differences in response.
Pitaya is an important economic crop in tropical regions. It is valued for its nutritional, medicinal, and ornamental properties [40,41,42]. The fruit is widely favored by consumers worldwide. In recent years, rising economic benefits have driven rapid expansion of pitaya cultivation. In China, planting now exceeds 66,700 hectares [43]. Preliminary research showed that P input in pitaya production ranges from 1000 to 1200 kg ha−1 yr−1, while soil available P exceeds the abundant threshold (>40 mg kg−1) by 6.3-fold [44,45]. However, phosphate fertilizer use efficiency remains low in pitaya systems. This highlights the need for more efficient P fertilizer management. Currently, optimization strategies for P fertilizer application in pitaya have only been reported with regard to the application rate [45]. Drip and sprinkler irrigation are the main practices in pitaya production. Among them, drip fertigation supplies nutrients directly to the root zone. This localized method optimizes root and shoot traits, enhancing water and nutrient use efficiency [46]. Despite these advantages, research on localized P supply in pitaya is limited. Its effects on root morphology, structure, and physiology remain unclear. Therefore, clarifying how pitaya seedling roots respond to different P supply methods is of great theoretical and practical significance for P management in pitaya production.
This study investigated how localized and uniform P supply affects root morphology and nutrient content in Hylocereus polyrhizus, using a rhizobox experiment. The objectives were to (i) characterize root morphology under different P supply methods, (ii) evaluate nutrient content under localized P supply, and (iii) examine correlations among shoot growth, soil nutrient content, and root development across P treatments. We hypothesized that localized P application promotes root growth, thereby improving plant biomass and nutrient content.

2. Materials and Methods

2.1. Plant Materials and Growth Conditions

The experiment was conducted in the greenhouse of Hainan University, in Jianfeng Town, Ledong County (18°63′ N, 108°75′ E). The site has an average annual temperature of 24 °C and receives approximately 2200 h of sunshine annually. The pitaya cultivar used was Hylocereus polyrhizus cv. “Dahong”. Fresh cuttings originated from intermediate segments of cladodes with similar length were selected, each weighing 130 ± 2 g. Soil was collected from Donghe Town, Dongfang City, Hainan Province (18°54′ N, 108°88′ E). It was air-dried, sieved through a 2 mm mesh, and stored in labeled plastic bags. The soil had the following properties: pH 6.92, organic matter 2.71 g kg−1, alkali-hydrolyzable nitrogen 16.78 mg kg−1, available phosphorus 7.30 mg kg−1, and available potassium 93.20 mg kg−1.

2.2. Experimental Design

The rhizobox experiment included four treatments: (1) no P supply (No-P), (2) uniform P supply at 50 mg P kg−1 soil (Uniform-P1), (3) localized P supply at 50 mg P kg−1 soil (Local-P1), and (4) uniform high-P supply at 200 mg P kg−1 soil (Uniform-P2). The total amount of phosphorus applied in Uniform-P1 and Local-P1 treatment were the same. The Uniform-P2 treatment was included primarily to assess whether localized low-phosphorus supply could achieve similar nutrient content as a uniform high-phosphorus supply. Each treatment included four replicates arranged in a randomized block design. The planting container was a custom-made detachable rectangular rhizobox (length 22 cm, width 3 cm, and height 32 cm), with one side made of transparent plexiglass. Each rhizobox was divided into three layers: upper (0–8 cm), middle (8–16 cm), and lower (16–32 cm). In the localized P treatment, all fertilizer was applied to the middle layer (8–16 cm), whereas in the Uniform-P1 and Uniform-P2 treatments, fertilizer was evenly applied across all layers.
Each rhizobox was filled with 1.9 kg of soil mixed with basal nutrients as a solution: (NH4)2SO4 100, MgCl2·6H2O 50, CaSO4·2H2O 100, MnSO4·H2O 3.25, ZnSO4·7H2O 0.79, CuSO4·5H2O 0.50, H3BO3 0.17, and Fe-EDTA 3.25 mg kg−1 soil. Potassium dihydrogen phosphate was used as phosphate fertilizer. The potassium introduced by the phosphate fertilizer was supplemented with K2SO4. Pitaya cladodes cuttings were planted in each rhizobox. Once roots had developed, 50 mL of a 15 mmol L−1 (NH4)2SO4 solution was applied to each seedling every 15 days. Water and nutrient solution levels were maintained constant throughout the experiment. Each rhizobox was repositioned at 7-day intervals, and pitaya seedlings were harvested 102 days after planting.

2.3. Root Sampling and Morphological Analysis

At harvest, shoots and roots were separated. Roots were washed with deionized water. They were immersed in 50% alcohol to prevent deformation and stored at 4 °C for morphological analysis. Root samples were scanned with a flat scanner (Epson Expression V800, Seiko Epson Corporation, Suwa, Japan) at 600 dpi. The scanned images were analyzed with WinRHIZO Pro 2009b software (Regent Instruments Inc., Quebec, QC, Canada) to quantify total root length, root surface area, root volume, and root diameter. Roots were classified into five diameter categories: 0–0.3, 0.3–0.6, 0.6–0.9, 0.9–1.2, and >1.2 mm. For each category, root length and its proportion of the total root length were calculated. The scanned roots were oven-dried at 80 °C to a constant weight to determine root biomass. Specific root length (SRL, cm g−1), specific root area (SRA, cm2 g−1), root tissue density (RTD, g cm−3), and root-to-shoot ratio (RSR, g g−1) were calculated as follows: SRL = total root length/root dry weight; SRA = root surface area/root dry weight; RTD = root dry weight/root volume; and RSR = root dry weight/shoot dry weight.

2.4. Mineral Element Determination

Aboveground branches were cut into pieces and heated at 105 °C for 30 min. They were then dried at 80 °C to a constant weight, and dry weight was recorded. Dried shoot and root samples were finely ground and digested with H2SO4 and H2O2. Digest solutions were analyzed for nitrogen using the Nessler’s reagent colorimetric method, for phosphorus using the molybdo-vanadophosphate method, and for potassium using the flame photometer method [47]. Plant N, P, and K content were calculated as the product of nutrient concentration and the corresponding dry matter mass.

2.5. Soil Sampling and Determination of Available Nitrogen, Phosphorus, and Potassium

After plant harvest, soil samples were collected from each layer after homogenization. Samples were air-dried, sieved, and analyzed for alkali-hydrolyzable nitrogen, available phosphorus, and available potassium. For nitrogen, soil was hydrolyzed with 1.0 mol L−1 NaOH, and the released NH3 was absorbed by H3BO3 and titrated with standard acid to calculate alkali-hydrolyzable nitrogen content [47]. Available phosphorus was extracted with 1.0 mol L−1 NH4F and 0.5 mol L−1 HCl and quantified using the molybdenum blue spectrophotometric method at 700 nm [47]. Available potassium was measured by flame photometry [47].

2.6. Statistical Analysis

Experimental data were processed and visualized using Excel 2019, Origin 2021, and Adobe Photoshop 21.0.1. Statistical analyses were performed by one-way analysis of variance (ANOVA) in IBM SPSS Statistics 26, and Duncan’s multiple range test (p < 0.05) was applied to assess treatment differences for each parameter presented in the figures and tables.

3. Results

3.1. Effects of Phosphorus Application Methods on Pitaya Seedling Biomass

The P application method strongly influenced plant growth (Figure 1). It had distinct effects on shoot and root development. Among the treatments, localized P supply (Local-P1) produced the greatest shoot biomass, reaching 18.33 g plant−1 (Figure 1a). Shoot biomass under Local-P1 was significantly higher than under Uniform-P2. However, it did not differ from Uniform-P1. Local-P1 also enhanced root growth. Root biomass increased by 53.33%, 142.11%, and 142.11% compared with No-P, Uniform-P1, and Uniform-P2, respectively (Figure 1a). The root-to-shoot ratio under Local-P1 was also significantly greater than under Uniform-P1 and Uniform-P2. Compared with Uniform-P1, Local-P1 increased the root-to-shoot ratio by 108.33% (Figure 1b).

3.2. Effects of Phosphorus Application Methods on Nutrient Content in Pitaya Seedlings

The P application method significantly affected nitrogen content in both shoots and roots (Figure 2a). Compared with No-P, Uniform-P1, and Uniform-P2, localized P supply (Local-P1) increased shoot nitrogen content by 38.53%, 24.78%, and 27.36%, respectively. Root nitrogen content was also highest under Local-P1. It was 2.5- and 2.2-fold greater than under Uniform-P1 and Uniform-P2, respectively. Total nitrogen content reached 152.76 mg plant−1 in Local-P1, the highest among all treatments. Compared to No-P, Uniform-P1, and Uniform-P2, total nitrogen content increased by 38.65%, 26.36%, and 22.31%, respectively.
Shoot and total P content showed no significant differences among treatments (Figure 2b). However, root P content was significantly higher in Local-P1. It increased by 96.55% compared with Uniform-P1 and by 103.57% compared with Uniform-P2, but did not differ from No-P.
The P application method also influenced potassium content (Figure 2c). Shoot potassium content exceeded that of nitrogen and P. Localized P application enhanced potassium content more effectively than uniform application. In Local-P1, shoot and total potassium content were 26.68% and 26.91% higher than in Uniform-P2. Root potassium content under Local-P1 was also greater, reaching 2.2- and 2.1-fold that of Uniform-P1 and Uniform-P2, respectively.

3.3. Effects of Phosphorus Application Methods on Root Morphology in Pitaya Seedlings

Analysis of pitaya root traits showed that P treatments significantly affected root morphology, including total root length, surface area, volume, and diameter (Figure 3, Table 1). Total root length and root surface area followed similar trends across the four treatments (Table 1). Compared with No-P, Uniform-P1, and Uniform-P2, Local-P1 significantly increased total root length by 71.96%, 121.77%, and 124.97%, respectively. Local-P1 also increased root surface area by 56.10%, 110.25%, and 116.34%, respectively. Root volume under Local-P1 was significantly higher than under uniform P application, reaching 3.14 m3 (Table 1). With the same total P supply, Local-P1 increased root volume by 100.19% compared with Uniform-P1. Phosphorus supply method did not significantly affect the specific root length of pitaya seedlings (Table 1). Among the treatments, Local-P1 showed the smallest specific root area, which was 14.57% lower than that of Uniform-P1. Root tissue density under Local-P1 increased significantly by 23.14% compared with Uniform-P1 and 17.32% compared with Uniform-P2. Overall, localized P supply promoted longer, denser, and more compact roots than uniform supply.
Most pitaya roots had diameters <0.3 mm, and P supply mainly affected this root class (Figure 4). The proportion of root length in this class was significantly lower in No-P, Uniform-P1, and Uniform-P2 than in Local-P1. Local-P1 showed a 6.06% higher proportion of fine roots (<0.3 mm) than Uniform-P1. No significant differences were found among treatments for roots with diameters of 0.3–0.6 mm, 0.6–0.9 mm, 0.9–1.2 mm, or >1.2 mm. Thus, localized P mainly promoted the development of fine roots.

3.4. Effects of Phosphorus Application Methods on Soil Nutrient Availability

The P application method significantly influenced soil alkali-hydrolysable nitrogen, with the lowest level under localized P application (Figure 5a). Compared with Uniform-P1 and Uniform-P2, Local-P1 reduced soil alkali-hydrolysable nitrogen by 12.43% and 22.86%, respectively. Relative to the No-P treatment, soil available P content was significantly higher in the Uniform-P1, Local-P1, and Uniform-P2 treatments (Figure 5b). Furthermore, available soil P in Uniform-P1 and Local-P1 decreased by 40.09% and 45.07% compared with Uniform-P2, with no significant difference between Uniform-P1 and Local-P1. Available soil potassium was lowest under localized P application (Figure 5c). Local-P1 showed a significant reduction compared with Uniform-P2.

3.5. Correlation and Principal Component Analyses of Indicators Across Phosphorus Application Methods

Correlation analysis showed strong positive relationships between shoot biomass and root traits, including total root length, root surface area, and root volume (Figure 6). Shoot biomass was also highly correlated with shoot content of nitrogen, P, and potassium (p < 0.01). It was positively correlated with root content of these nutrients as well (p < 0.05). In contrast, shoot biomass was negatively correlated with soil available potassium and alkali-hydrolysable nitrogen (p < 0.05). Root biomass showed similar patterns, being positively associated with root traits and nutrient content, but negatively correlated with soil available K and alkali-hydrolysable N (p < 0.05 or p < 0.01).
Shoot nitrogen content was positively correlated with root traits, while root N content was negatively correlated with soil alkali-hydrolysable N. Root P content was positively correlated with root traits (total root length, surface area, volume), root-to-shoot ratio, and root tissue density. Potassium content showed strong positive correlations with root traits and root-to-shoot ratio (p < 0.01). It was negatively correlated with soil available potassium (p < 0.05). These results indicate that P supply influences nutrient content and biomass mainly by enhancing root growth and morphology.
Principal component analysis showed that the PC1 and PC2 explained 57.6% and 17.2% of the variance, respectively (Figure 7a). These components were used to calculate overall scores for each treatment. Local-P1 achieved the highest overall score (4.20), whereas Uniform-P2 had the lowest (−2.16). The ranking of overall scores was Local-P1 > No-P > Uniform-P1 > Uniform-P2 (Figure 7b).

4. Discussion

4.1. Localized Phosphorus Application Altered Biomass Allocation Between Shoot and Root

Plants show plasticity in both shoot and root growth, enabling adaptation to heterogeneous and changing environments [34]. Phosphorus application methods can shift growth patterns and influence P use efficiency and yield [48]. Localized P application has been shown to increase seedling dry matter and promote root proliferation [22,26,49,50,51]. In this study, localized P application improved pitaya growth compared with uniform application, consistent with findings in other crops such as rice, wheat, maize, and faba bean [52,53,54]. With the same total P applied, pitaya seedlings performed better under localized application (Local-P1) than under uniform application (Uniform-P1), with a 142.11% increase in root biomass (Figure 1a). This is because, compared with the broadcast fertilization method, the localized fertilization method applies fertilizer to only limited regions of the soil. By doing so, it minimizes the contact area between soil and fertilizer, thereby reducing soil P fixation and facilitating enhanced P uptake and utilization by plants [21]. Ultimately, this translates to a significant improvement in the growth performance of pitaya under localized P supply. Localized P application (Local-P1) also significantly promoted both shoot and root growth compared with uniform high-P application (Uniform-P2) (Figure 1). This indicated that localized P application is an effective strategy to improve P use efficiency while reducing fertilizer input.
Shoot and root development are tightly connected, forming an interdependent system. Changes in the root-to-shoot ratio across growth stages indicate shifts in the center of plant growth [54]. Under P deficiency, assimilates are preferentially allocated to the root system to enhance soil exploration, resulting in an earlier slowdown of shoot growth and an increased root-to-shoot ratio [55,56]. Previous studies show that in heterogeneous nutrient environments, this ratio often increases or remains stable [57]. In our study, localized P supply significantly increased the root-to-shoot ratio compared with uniform supply (Figure 1b). This effect was mainly driven by root proliferation. Phosphorus content in pitaya roots correlated strongly with the root-to-shoot ratio (Figure 6). Lower root-to-shoot ratios in Uniform-P1 and Uniform-P2 suggest nutrient limitation on plant growth.

4.2. Localized Phosphorus Application Improved Root Morphology

The plasticity of root morphology, structure, and spatial distribution in response to localized nutrient supply has been well documented across various plant species [35,58]. Localized P supply alters root traits and plant growth. Some plants adapt by enhancing root length, surface area, and volume [54,59,60,61,62]. They also form more lateral roots and reduce root diameter. These adjustments are compensatory response to spatial uneven P availability. Specifically, extensive root proliferation in nutrient-rich zones compensates for the uneven distribution of soil nutrients [63,64]. Furthermore, the localized application of P fertilizers containing NH4+-N acidifies the root zone membrane, thereby softening the cell walls of the roots and accelerating cell division, which in turn promotes root growth [65]. In our rhizobox experiment, P fertilizer was concentrated in the middle soil layer (8–16 cm) to facilitate root access. Results indicated that, compared with uniform P application (Uniform-P1 and Uniform-P2), localized P supply (Local-P1) significantly increased total root length, surface area, and volume. Compared with uniform P application (Uniform-P1 and Uniform-P2), localized P supply (Local-P1) greatly increased root traits. Total root length, surface area, and volume in Local-P1 were 121.77%, 110.25%, and 100.19% higher than in Uniform-P1, respectively (Table 1). The results show strong root proliferation and downward growth into nutrient-rich patches (Figure 3), which is consistent with previous findings for rice, common bean, and sorghum [53,66,67]. Under localized P supply conditions, plants showed the lowest specific root area together with the highest root tissue density (Table 1). This inverse relationship generally indicates that root dry matter allocation shifts toward denser structural components, such as thickened cortical tissues or enhanced vascular development, rather than toward expanding root surface area [68]. These morphological adjustment patterns indicate high plasticity of the pitaya root system under localized P supply.
Root diameter has a strong effect on total length, surface area, and volume [69,70]. Fine roots are essential for plant growth, as they are primarily responsible for absorbing water and nutrients. Localized P supply often stimulates fine root proliferation [31,51,53]. In this study, the proportion of roots with diameters <0.3 mm was significantly higher under localized P supply (Local-P1) than under other treatments, showing a 6.06% increase compared with uniform P supply (Uniform-P1) (Figure 4). This greater fine root abundance likely explains the increase in total root length under localized P. In summary, these root morphological changes under localized P supply can be interpreted as an efficient plant foraging strategy aimed at optimizing P acquisition [71].
Root morphology and structure largely determine nutrient acquisition. Root number and activity are closely associated with the production, transport, and allocation of assimilates, as well as crop yield formation. Correlation analysis showed that total root length, root surface area, and root volume were all significantly positively correlated with biomass (Figure 6). Previous studies reported that localized and moderate P supply effectively improved the growth in Rosa multiflora and maize by enhancing root development, particularly through changes in root morphology and physiology [33,37,51]. Localized P supply facilitates better root morphology during initial crop growth, leading to expanded soil exploration and subsequently increased plant nutrient content [26]. Therefore, localized P application likely promotes pitaya growth by strengthening the positive coupling between root morphology and nutrient absorption. Furthermore, the nutrients in soil patches also affect root physiological processes such as proton exudation and acid phosphatase activity in the rhizosphere [31,72]. It is necessary to study the potential mechanisms of root morphological changes in order to elucidate the relationship between root and nutrient content.

4.3. Local Phosphorus Application Increased Nitrogen and Potassium Content

Plant P content is generally thought to increase with higher P inputs [73]. Many studies have tested the effects of localized P application on crop P content. It has been shown to enhance P content in plants such as rice, chili, Rosa multiflora, and Pinus massoniana [26,33,53,60]. In contrast, other studies found no significant difference in maize P content between broadcast and localized P application [31]. In this study, P supply method and rate had no effect on shoot P content in pitaya seedlings. This suggests that pitaya may have high P use efficiency and can maintain growth across a wide range of P levels. By contrast, root P content was significantly higher under localized than under uniform P supply (Figure 2b). The improved root growth of pitaya under localized P supply provides a potential explanation. Plants often upregulate high-affinity P transporters to optimize uptake in the rhizosphere [74], suggesting that localized P supply may enhance root P content in pitaya seedlings.
Nitrogen content is a key driver of plant growth and yield. It strongly influences biomass formation [75]. Graciano et al. [76] found that the positive effects of localized P application result from both increased P availability and sustained nitrogen assimilation. Our results confirm that both the amount and method of P application influence nutrient content in plants. Localized P (Local-P1) increased shoot nitrogen content by 24.78% compared to uniform P (Uniform-P1), and root potassium content under Local-P1 was 2.2-fold that under Uniform-P1 (Figure 2a,c). Shoot biomass was strongly and positively correlated with nitrogen and potassium content under all P treatments. Plant nitrogen and potassium content were also highly correlated with total root length, root surface area, and root volume (Figure 6). These findings suggest that enhanced nitrogen and potassium content play a major role in pitaya seedling growth. Improved root growth expands the absorption zone, increasing nitrogen and potassium uptake and promoting shoot growth. The results support our hypothesis that localized P application promotes root growth, thereby improving plant biomass and nutrient content. The lack of significant difference in growth and P content between plants without and with P addition can be attributed to the plants’ initial adaptive responses to low P stress [77,78,79], their efficient use of soil inherent P, and the likely rapid fixation of the applied fertilizer.

5. Conclusions

Localized P supply produced greater shoot biomass than both uniform P and uniform high-P treatments. It also increased shoot nitrogen and potassium content but had no significant effect on P content. This effect was associated with improvements in root morphology induced by localized nutrient supply. Under localized P supply, root biomass, total root length, root surface area, and root volume increased significantly, with roots < 0.3 mm in diameter accounting for the largest proportion of total root length. We conclude that localized P application enhances root morphology, improves nitrogen and potassium content, and promotes shoot growth in pitaya. Under field conditions, local P application can be achieved through methods including hole application, strip application, concurrent seeding and fertilizer application, and drip irrigation fertilization. It is an efficient fertilization strategy that optimizes root development, reduces fertilizer use, and increases nutrient use efficiency.

Author Contributions

Conceptualization, W.G.; Methodology, C.C.; Investigation, C.C. and J.W.; Data curation, J.W., C.C., Y.F. and W.G.; Formal Analysis, J.W.; Resources, F.P. and Z.C.; Writing—Original Draft Preparation, J.W. and C.C.; Writing—Review and Editing, J.W., Y.F., Y.R., J.Q. and W.G.; Supervision, Y.R.; Project Administration, W.G.; Funding Acquisition, W.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (32202586), the Hainan Provincial Sanya Yazhou Bay Science and Technology Innovation Joint Project (No: ZDYF2025GXJS131), and the Scientific Fertilization Technology System for Characteristic Crops in Hainan (RH2300006505).

Data Availability Statement

Data are available upon request from the corresponding author.

Conflicts of Interest

Authors Junfeng Qu, Fanrong Pan and Zhiliang Chen were employed by the company China BlueChemical Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Biomass (a) and root-to-shoot ratio (b) of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Error bars indicate standard errors of the means (n = 4). Different lowercase letters above the bars denote significant differences among treatments (p < 0.05).
Figure 1. Biomass (a) and root-to-shoot ratio (b) of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Error bars indicate standard errors of the means (n = 4). Different lowercase letters above the bars denote significant differences among treatments (p < 0.05).
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Figure 2. Nitrogen content (a), phosphorus content (b), and potassium content (c) of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Error bars indicate standard errors of the means (n = 4). Different lowercase letters above the bars denote significant differences among treatments (p < 0.05).
Figure 2. Nitrogen content (a), phosphorus content (b), and potassium content (c) of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Error bars indicate standard errors of the means (n = 4). Different lowercase letters above the bars denote significant differences among treatments (p < 0.05).
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Figure 3. Root morphology of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply.
Figure 3. Root morphology of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply.
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Figure 4. Proportion of different root diameters of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Different lowercase letters denote significant differences in root diameter proportions among treatments (p < 0.05).
Figure 4. Proportion of different root diameters of pitaya seedlings under different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Different lowercase letters denote significant differences in root diameter proportions among treatments (p < 0.05).
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Figure 5. Effect of P treatments on soil alkali-hydrolysable nitrogen (a), available phosphorus (b), and available potassium (c). Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Error bars indicate standard errors of the means (n = 4). Different lowercase letters above the bars denote significant differences among treatments (p < 0.05).
Figure 5. Effect of P treatments on soil alkali-hydrolysable nitrogen (a), available phosphorus (b), and available potassium (c). Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Error bars indicate standard errors of the means (n = 4). Different lowercase letters above the bars denote significant differences among treatments (p < 0.05).
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Figure 6. Correlation analysis of biomass, root morphology, nutrient content, and soil nutrient availability. Abbreviations: SB—shoot biomass; RB—root biomass; RSR—root-to-shoot ratio; TRL—total root length; RSA—root surface area; RV—root volume; SRL—specific root length; SRA—specific root area; RTD—root tissue density; SNC—shoot nitrogen content; RNC—root nitrogen content; TNC—total nitrogen content; SPC—shoot phosphorus content; RPC—root phosphorus content; TPC—total phosphorus content; SKC—shoot potassium content; RKC—root potassium content; TKC—total potassium content; SAN—soil alkali-hydrolysable nitrogen; SAP—soil available phosphorus; SAK, soil available potassium. **: Significant at p < 0.01 level, *: Significant at p < 0.05 level. The size of the central circles represents the magnitude of the correlation coefficient, with larger circles indicating that the coefficient is closer to either 1 or −1.
Figure 6. Correlation analysis of biomass, root morphology, nutrient content, and soil nutrient availability. Abbreviations: SB—shoot biomass; RB—root biomass; RSR—root-to-shoot ratio; TRL—total root length; RSA—root surface area; RV—root volume; SRL—specific root length; SRA—specific root area; RTD—root tissue density; SNC—shoot nitrogen content; RNC—root nitrogen content; TNC—total nitrogen content; SPC—shoot phosphorus content; RPC—root phosphorus content; TPC—total phosphorus content; SKC—shoot potassium content; RKC—root potassium content; TKC—total potassium content; SAN—soil alkali-hydrolysable nitrogen; SAP—soil available phosphorus; SAK, soil available potassium. **: Significant at p < 0.01 level, *: Significant at p < 0.05 level. The size of the central circles represents the magnitude of the correlation coefficient, with larger circles indicating that the coefficient is closer to either 1 or −1.
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Figure 7. (a) Principal component analysis of biomass, root traits, nutrient content, and soil nutrient availability; (b) Overall scores of different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Abbreviations: SB—shoot biomass; RB—root biomass; RSR—root-to-shoot ratio; TRL—total root length; RSA—root surface area; RV—root volume; SRL—specific root length; SRA—specific root area; RTD—root tissue density; SNC—shoot nitrogen content; RNC—root nitrogen content; TNC—total nitrogen content; SPC—shoot phosphorus content; RPC—root phosphorus content; TPC—total phosphorus content; SKC—shoot potassium content; RKC—root potassium content; TKC—total potassium content; SAN—soil alkali-hydrolysable nitrogen; SAP—soil available phosphorus; SAK—soil available potassium.
Figure 7. (a) Principal component analysis of biomass, root traits, nutrient content, and soil nutrient availability; (b) Overall scores of different P treatments. Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Abbreviations: SB—shoot biomass; RB—root biomass; RSR—root-to-shoot ratio; TRL—total root length; RSA—root surface area; RV—root volume; SRL—specific root length; SRA—specific root area; RTD—root tissue density; SNC—shoot nitrogen content; RNC—root nitrogen content; TNC—total nitrogen content; SPC—shoot phosphorus content; RPC—root phosphorus content; TPC—total phosphorus content; SKC—shoot potassium content; RKC—root potassium content; TKC—total potassium content; SAN—soil alkali-hydrolysable nitrogen; SAP—soil available phosphorus; SAK—soil available potassium.
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Table 1. Total root length, root surface area, root volume, specific root length, specific root area, and root tissue density of pitaya seedlings treated with different P application methods.
Table 1. Total root length, root surface area, root volume, specific root length, specific root area, and root tissue density of pitaya seedlings treated with different P application methods.
TreatmentTotal Root Length (cm)Root Surface Area (cm2)Root Volume (m3)Specific Root Length (cm g−1)Specific Root Area (cm2 g−1)Root Tissue Density (g m−3)
No-P516.0 ± 48.0 b119.6 ± 12.4 b2.10 ± 0.18 ab1761.8 ± 406.5 a406.5 ± 27.6 b0.141 ± 0.007 ab
Uniform-P1400.1 ± 106.5 b88.8 ± 24.1 b1.57 ± 0.44 b2159.3 ± 475.1 a475.1 ± 24.9 a0.121 ± 0.005 c
Local-P1887.3 ± 111.5 a186.7 ± 25.5 a3.14 ± 0.49 a1940.8 ± 405.9 a405.9 ± 12.4 b0.149 ± 0.009 a
Uniform-P2394.4 ± 78.8 b86.3 ± 17.6 b1.51 ± 0.33 b2066.5 ± 451.6 a451.6 ± 14.1 ab0.127 ± 0.003 bc
Note: Treatments: No-P—no P supply; Uniform-P1—uniform P supply; Local-P1—localized P supply; Uniform-P2—uniform high-P supply. Values are presented as mean ± standard error (n = 4). Different lowercase letters within a column indicate significant differences among treatments (p < 0.05).
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Wu, J.; Chen, C.; Fan, Y.; Ruan, Y.; Qu, J.; Pan, F.; Chen, Z.; Gao, W. Localized Phosphorus Application Promotes the Growth and Nutrient Content of Pitaya (Hylocereus polyrhizus) Seedlings by Improving Root Morphology. Agronomy 2026, 16, 173. https://doi.org/10.3390/agronomy16020173

AMA Style

Wu J, Chen C, Fan Y, Ruan Y, Qu J, Pan F, Chen Z, Gao W. Localized Phosphorus Application Promotes the Growth and Nutrient Content of Pitaya (Hylocereus polyrhizus) Seedlings by Improving Root Morphology. Agronomy. 2026; 16(2):173. https://doi.org/10.3390/agronomy16020173

Chicago/Turabian Style

Wu, Jiamin, Chen Chen, Youhui Fan, Yunze Ruan, Junfeng Qu, Fanrong Pan, Zhiliang Chen, and Wei Gao. 2026. "Localized Phosphorus Application Promotes the Growth and Nutrient Content of Pitaya (Hylocereus polyrhizus) Seedlings by Improving Root Morphology" Agronomy 16, no. 2: 173. https://doi.org/10.3390/agronomy16020173

APA Style

Wu, J., Chen, C., Fan, Y., Ruan, Y., Qu, J., Pan, F., Chen, Z., & Gao, W. (2026). Localized Phosphorus Application Promotes the Growth and Nutrient Content of Pitaya (Hylocereus polyrhizus) Seedlings by Improving Root Morphology. Agronomy, 16(2), 173. https://doi.org/10.3390/agronomy16020173

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