Visual Analyses of Hot Spots and Frontiers in Zanthoxylum planispinum Research Based on CiteSpace

Abstract

Zanthoxylum planispinum is a type of plant with homologous properties in medicine and food, making it well-loved in China. To explore the development of the Z. planispinum field over the past 20 years, its research hotspots and frontier trends were analyzed. This study conducted database-based visualization analyses and knowledge graph analyses using CiteSpace software with data concerning Z. planispinum published in the Chinese National Knowledge Infrastructure and Web of Science databases between 2003 and 2023. Over the last 20 years, the number of Chinese and English publications on Z. planispinum has shown increasing trends. The foci of this research were mainly germplasm resources, cultivation management, seed oil extraction technology, flavonoid extraction technology, and correlation analyses of antioxidant activities. The main research frontiers included the unified standard naming and adaptation mechanisms of Z. germplasm resources, orientation cultivation, functional component extraction, processing technology research and development, and industrial chain construction. The results provide a scientific reference for the high-quality development of the Z. planispinum industry.

1. Introduction

Zanthoxylum planispinum is a plant of the genus Zanthoxylum in the Rutaceae family. Its members include the ‘Book of Songs’, commonly known as a pepper; ‘Erya’, called hui, meaning big pepper; and ‘Divine Farmer’s Materia Medica’, Zanthoxylum piperitum DC. Because of its special flavor and medicinal properties, Zanthoxylum planispinum is widely used as an important spice, traditional food seasoning and Chinese medicine. Consequently, it is grown for its raw material and oil in China [1,2].

Zanthoxylum is a perennial woody shrub or small tree. It requires high temperatures and sufficient light throughout its life cycle. It is an important drought-resistant, barren soil, and water conservation species [3,4]. There are approximately 250 species of Zanthoxylum plants in the world, and they are generally distributed in the tropics and subtropics of Asia, Africa, and North America, with a limited distribution in temperate zones [5]. There are approximately 45 members of the genus Zanthoxylum in China, including 13 varieties and 2 variants. It is widely distributed in areas southeast of Jiangsu, the coastal zone of Zhejiang, from the southwest to the southeast of Xizang, and from the plains to the high-altitude mountains [6,7].

At present, research on Z. planispinum in China and other countries mainly focuses on cultivation technology [8], breeding [9], drying after harvest [10], drought resistance [11], pharmacological properties [12], active ingredients [13], extraction process optimization, and the chemical composition of its volatile oil [14,15]. However, there are works on sorting and analyzing the research on Zanthoxylum. In this study, we used CiteSpace visualization software (Version: 5.7 R5) to create knowledge maps of genus Zanthoxylum-related research, including the numbers of papers, institutions, authors, keywords, and other aspects. The following are some potential limitations to using this method: (1) when acquiring data, there may be a selection bias during screening; (2) the overall content of the paper is limited to the degree of analysis of the acquired data, such as by focusing on the status quo, development, and trends; and (3) due to time constraints in writing this paper, the data may be incomplete. We attempted to overcome these shortcomings as much as we could: research hotspots and contexts were reviewed, and the research and developmental trends in Zanthoxylum were summarized and analyzed, as were differences between the research in China and abroad, including hotspots and future developmental directions. The development of Sichuan pepper fields was also analyzed to provide a scientific reference for the development of a high-quality Sichuan pepper industry.

2. Data Sources and Research Methods

In this study, CiteSpace version 5.7 R5 software was adopted as a research tool for bibliometric visualization and analysis. This scientific software was developed by Dr. Chen Chaomei’s team and can be used to create knowledge maps [16]. Compared to similar software such as VOSviewer (Leiden University, Leiden, The Netherlands) and Ucinet (Analytic Technologies, Collegeville, PA, USA), CiteSpace is less complex, is easy to master, features a powerful relational algorithm, and offers several representation options, including clusters, themes, timelines, and time zones. However, in comparison to software like VOSviewer, CiteSpace has some noticeable drawbacks, such as high memory consumption and the potential overlap of node labels.

2.1. Data Source

The Chinese National Knowledge Infrastructure (CNKI) database was used as a data source, and data were retrieved from August 2003 to August 2023. The CNKI database search process was as follows: select advanced search, set the theme to ‘Zanthoxylum planispinum’, set the language to Chinese, and set the search scope to academic journals. A total of 6325 relevant literature sources were retrieved. After a manual screening, 4.325 papers were excluded, such as those from repeated sources, reviews, and conferences, and 2000 relevant literature sources were exported as data samples.

In addition, the Web of Science (WOS), an English literature database, was also used as a data source. The search process was as follows: select the ‘Web of Science Core Collection’, set the topic to ‘Zanthoxylum planispinum’, set the search period from August 2003 to August 2023, and set the language to English. After a manual screening, duplicated sources, reviews, conferences, and nonstandard papers were excluded, and 227 relevant literature sources were exported as data samples.

2.2. Research Method

The CNKI and WOS documents were exported in the ‘Refwork’ and plain text formats, respectively. Both files were prefixed with ‘download’, and they were imported into the CiteSpace software. The data downloaded from the CNKI database required a format conversion. Duplicate documents were screened using the ‘Remove Duplicates’ feature, and an information visualization analysis was then performed on the data. The operation parameter settings were as follows: Time slicing was set from 2003 to 2023, time slice was set to 1 year, and the selected node type was ‘Author’, ‘Institution’, or ‘Keywords’ according to the research’s needs. The threshold (Top N) was then set to 50, and the default values of other parameters were retained.

3. Analysis of Z. planispinum’s Research Status

3.1. Publication Volume Analysis

Figure 1 shows the statistical trend in the annual publication volume of the literature sources on Zanthoxylum. It reflects the developmental trend and highly productive periods of Zanthoxylum research in China and abroad over the past 20 years. From the distribution of the CNKI Chinese literature, the number of published papers from 2003 to 2007 was relatively stable, fluctuating between 15 and 24. In 2008, the number of published papers reached 53, and from 2008 to 2012, the annual number of published papers was greater than 50, with a peak of 65 in 2011. From 2013 to 2022, the number of published papers in China showed rapid growth; the average annual number of published papers reached 150, with a peak of 215 papers in 2020. From 2015 to 2023, 1435 papers were published, accounting for 71% of the total number of papers retrieved. Thus, research on Z. planispinum has gradually become popular, forming a new research hotspot.

The distribution of the WOS literature allowed the number of English-language publications to be divided into two periods. First, from 2003 to 2013, the number of publications remained relatively stable. Then, the period from 2013 to 2023 was one of overall growth. The number of published papers decreased significantly in 2020 but peaked in 2021–2022, with 46 publications each year.

From 2003 to 2022, the numbers of domestic and foreign literature sources fluctuated but showed overall increasing trends, and the research conducted by scholars in the field of Z. planispinum has increased. During 2020, the number of domestic Sichuan pepper papers peaked, whereas the number of English-language papers declined. This indicates that Chinese and foreign research on Zanthoxylum was not synchronized and that Chinese scholars are influential in the study of Zanthoxylum. Please note that because the data in this study were collected in August 2023, the 2023 data are not representative of the full year.

3.2. Published Paper Author Analysis

A network cooperation analysis was carried out to generate a co-occurrence map of the literature authors. In this study, the network was established as follows: number of nodes in the map (N) = 693, number of connections between nodes in the map (E) = 952, and network density in the map (Density) = 0.004. As shown in Figure 2a, there were more isolated nodes in the map than connections, indicating that authors who published papers in Chinese preferred independent research and lacked collaborative communications.

Price’s law was used to analyze the core author group: the minimum number of papers published by the core authors (Mp) = 0.749 × the number of papers published by the most productive author (Nmax) [17,18]. In this research, the number of core-author papers published in Chinese was ≥ 4.6. There were 99 core authors in this field who published 937 articles, which accounted for 46.8% of the publications found. This does not conform to Price’s law (>50%), indicating that a core author group in the field of Z. planispinum has begun to form but has not been established. Authors who publish papers in Chinese should strengthen their cooperation, as this would allow the sharing of resources, technology, and knowledge. This, in turn, would promote Zanthoxylum research and allow its full potential to be realized through the development of a high-quality industry.

Similarly, the highest number of papers published by authors in English was 24. According to Price’s Law, there were 29 core authors, with 178 articles accounting for 78.4% of the publications. This is in line with Price’s law (>50%). The core group of authors formed and is relatively stable. In this study, a network was established as follows: N = 268, E = 537, and Density = 0.015. Here, the Density value is higher than in the Chinese studies, indicating that the network density in the map is greater and that there were cooperative teams. There are fewer isolated nodes in the graph, indicating that communication between authors was more frequent compared with the authors who published papers in Chinese (Figure 2b).

3.3. Visual Analysis of Research Institutions

To increase the accuracy of the analysis, the top 30 institutions were selected as the locations of the 6325 prickly pepper-related Chinese literature sources in the CNKI database. As shown in Table 1, the institutions with large numbers of publications related to Sichuan pepper were Northwest A-&-F University, Southwest University, Sichuan Agricultural University, and Guizhou Normal University, with 162, 130, 116, and 94 publications, respectively. Thus, the institutions responsible for publishing relevant articles in China were universities.

Chinese research institutions that study Z. planispinum are mainly distributed in Sichuan, Shaanxi, Gansu, Guizhou, and other provinces, which is also consistent, to a certain extent, with the main planting areas of Z. planispinum. The representative institutions were Sichuan Agricultural University, Northwest Agriculture and Forestry University, Gansu Agricultural University, Guizhou Normal University, and other colleges and universities.

4. Research Hotspot and Frontier Analyses of Z. planispinum

4.1. Research Hotspot Analysis

High-frequency keywords can be combined with the centrality to explore research hotspots in related fields [19]. The frequency of each keyword, which can represent an article’s content, is positively correlated with the number of research results [20]. This also reflects the hotspots and core areas of interest in a field, to a certain extent, and this allows people to understand the field’s developmental process more clearly. In this study, the network characteristics of N = 690, E = 1800, and Density = 0.0076 indicate strong correlations among nodes. Table 2 lists the top 20 high-frequency keywords used in the literature.

A Local Linear Regression algorithm was used to conduct a cluster analysis of the Keyword co-occurrence maps. In this study, modularity index (Q) = 0.5184 and contour value (S) = 0.8487 (Q > 0.3; S > 0.7) were used. The cluster structure was significant, worthy of conviction, and had reference significance. The cluster sequence number is inversely proportional to the cluster size, and the largest cluster was labeled #0 [21]. In total, 11 clusters were formed through this cluster analysis (Figure 3).

The authors combined keyword co-occurrence and keyword cluster analysis to analyze the literature in the Chinese National Knowledge Infrastructure database related to Zanthoxylum. The research hotspots for Zanthoxylum were mainly concentrated on the germplasm resource management of Zanthoxylum, Z. planispinum cultivation and management technology, Zanthoxylum seed oil extraction, Zanthoxylum flavonoid extraction technology optimization, and antioxidant activity correlation analyses.

4.1.1. Germplasm Resource Management

In China, the study of Zanthoxylum has involved actively exploring the breeding and cultivation of good varieties. However, due to historical and geographical factors, as well as language differences among various places, the classification criteria and naming methods vary [22]. There is a lack of authoritative and unified naming rules for Zanthoxylum, and homonyms are commonly used, which results in resistance to resource utilization [23]. Thus, various provinces have different naming methods for Z.planispinum varieties, such as July Z. planispinum varieties and August Z. planispinum varieties, as well as other names that do not directly express the type of Z. planispinum. The existing naming system also results in certain reading and cognitive difficulties for readers. The establishment of unified naming rules will lead to the effective utilization and management of germplasm resources.

4.1.2. Zanthoxylum Planispinum Cultivation and Management Techniques

Cultivation and management have always been a research hotspot in the Zanthoxylum field, and such research is necessary to support the development of the Zanthoxylum industry. In recent years, with the continuous expansion of the Zanthoxylum planting area and the gradual increase in production, the price of Zanthoxylum has shown a downward trend. Moreover, due to increases in production costs, the benefit to farmers has been reduced, and the sustainable development of the industry is now confronted with severe challenges [24]. Production costs may be reasonably reduced by improving cultivation management technology [25]. This would also decrease the impact of declining Z. planispinum prices for farmers. Consequently, this is a current research focus. For example, pepper self-regulation mechanisms, reasonable pruning, fertilization using the natural decay of weeds into nutrients, and back-feeding pepper trees reduce the use of chemical fertilizers and pesticide damage to tree species [26]. This not only saves money spent on fertilizers and pesticides but also indirectly reduces labor costs. Cultivation management that saves labor and capital costs is important for the sustainable development of the Zanthoxylum industry.

4.1.3. Zanthoxylum planispinum Seed Oil Extraction

Zanthoxylum seeds are required to produce Zanthoxylum oil, which contains polyunsaturated fatty acids necessary for the human body and has a high commercial value [27]. The solution extraction method is commonly used to extract Zanthoxylum seed oil, but because of the thick waxy layer on Zanthoxylum seed surfaces, the conventional solvent extraction method often needs a series of complicated degumming, dewaxing, and other processes to attain the edible standard, as the numbing taste of Zanthoxylum seed oil is unpopular with consumers [28]. These steps can be eliminated with supercritical fluid technology, but the cost is prohibitively high for large-scale production [29]. Zhang et al. [30] combined solvent and supercritical extraction processes and obtained a maximum extraction rate of 13.7% for Zanthoxylum seed oil after experimentally optimizing the parameters. They not only shortened the extraction process, but also achieved a higher quality of Zanthoxylum seed oil, with a clear and bright color. However, the original numbing taste of Zanthoxylum was retained. Sun et al. [31] used supercritical CO₂ equipment to extract Zanthoxylum seed oil. The critical CO₂ extraction and the relative α-linolenic acid content produced by the response surface method were optimized using a single-factor experiment. The optimal extraction process was as follows: extraction pressure, temperature, and time were set to 30 Pa, 51 °C, and 90 min. The extraction rate of Zanthoxylum seed oil was 12.52%, and the α-linolenic acid content was 4.45%. New extraction technology and the optimization of the Zanthoxylum seed oil extraction process will promote the production of high-quality Zanthoxylum seed oil.

4.1.4. Flavonoid Extraction Technique Optimization and a Z. planispinum Antioxidant Activity Correlation Analysis

Flavonoids have biological effects such as anti-oxidation, anti-mutation, anti-aging, anti-tumor, and antibacterial properties [32,33]. The optimization of the Z. planispinum flavonoid extraction process has been the focus of researchers in recent years. Wang et al. [34] optimized the ultrasound-assisted extraction of flavonoids using the ring surface method using a single-factor test. The optimized process parameters were as follows: The ethanol concentration was 85%, the solid–liquid ratio was 1:20 (g/mL), the temperature was 70 °C, and the extraction time was 35 min. The yield of flavonoids was 12.16%, and the process was stable and reliable. Flavonoid molecules contain a certain number of phenolic hydroxyls, which can quickly transfer hydrogen protons to free radicals, terminate the free radical oxidation reaction, and show strong antioxidant properties [35,36]. Mu et al. [37] experimentally determined that flavonoid extracts have strong scavenging abilities for ·OH, DPPH·, and hydroxyl free radicals, as well as certain reducing and antioxidative capacities, which can effectively protect cells in the body, delay aging, and prevent diseases. Therefore, flavonoid extracts may be developed as natural antioxidants. Xu et al. [38] found that Z. planispinum ‘Hanyuan’ treated at 800 µg/mL for 24 h minimizes the generation rate of Hela cells and exhibits tumor cell inhibiting activity. In summary, Z. planispinum may play a role in the development of anticancer drugs and become a major pharmaceutical raw material [39].

4.2. Research Frontier Analysis

Keyword emergence analyses can be used to track research frontiers in a field through the detection of research hotspots, as well as changes in the trends in keywords and references overa specific period of time, rather than just the frequencies of keywords [40]. The authors selected the top 25 keywords with the strongest citation outbreaks and divided the emergent words roughly into three stages according to time. The keywords from 2003 to 2013 were extraction, Zanthoxylum seed oil, polyprotein overseas Chinese association factor, saccharin, supercritical CO₂, nutrient composition, industrialization, development, Zanthoxylum oil, and cold resistance. From 2012 to 2020, the keywords included industrial development, Zanthoxylum dissitum Hemsl., prevention and control, hemp flavor substances, cultivation, and current situation. From 2020 to 2023, the main keywords were extraction process, Z. armatum, fertilization, Z. bungeanum ‘Dahongpao’, cultivation technology, gas chromatography–mass spectrometry, and orthogonal test (Figure 4).

Combined with the keyword cluster analysis, the frontier analysis revealed that future research should focus on the unified and standardized naming and adaptative mechanisms of Z. planispinum germplasm resources, targeted cultivation of Z. planispinum, extraction and processing of functional components of Z. planispinum, and construction of a Z. planispinum industrial chain.

4.2.1. Study of Unified and Standardized Nomenclature, and Adaptative Mechanisms, of Zanthoxylum Germplasm Resources

The main contents were as follows: First, to address the non-standard naming of varieties, a unified naming protocol should be used based on the growth, physiological, and ecological habits of each variety. This would more conveniently manage germplasm resources and improve the introduction of varieties, as well as the efficiency and quality of seed production. Second, leaves, branches, roots, fruits, and other organs of different prickly peppers should be collected, and their adaptative mechanisms to geographic, soil, and climate factors should be evaluated based on their phenotypic, physiological, and molecular levels using plant functional traits. Third, the outstanding traits of different prickly peppers should be screened for growth, adaptation, and quality, and their formative mechanisms should be analyzed. The key genes regulating the formation of outstanding traits should be identified, and their molecular regulatory mechanisms should be investigated. Finally, research on different excellent traits should be carried out to provide scientific support for the genetic improvement of resistance and the molecular breeding of Z. planispinum. This will enable the creation of excellent germplasm resources to serve at the national and social levels and aid in overall food security.

4.2.2. Study of the Directional Cultivation of Z. planispinum

Zanthoxylum planispinum is an important medicinal and food plant. Targeted cultivation and customized and optimized production technology for Z. planispinum are the scientific bases for its classified use and the efficient utilization of resources [41]. In the future, Z. planispinum can be categorized into fresh, dried, and seed. Varieties in each category can be further classified by their use in cosmetics, health care products, chemical industrial products, and other products, allowing for their precise positioning in each product pathway. The differences in functional ingredient requirements among different products should be determined, and management measures should be carried out accordingly, such as soil improvement, altered environmental factors [42], plastic pruning, picking period, and disease and insect control. The aim is to fully coordinate vegetative and reproductive growth, regulate the production pathways of primary and secondary metabolites, optimize the accumulation modes of key qualities, and coordinate the relationship between yield and quality. The core of directional cultivation should be based on the development of product positioning and management technology, with strong practicability and feasibility. The formation of a technological model can be used as a reference and applied to other economically important plants. Here, the model would extend the Z. planispinum industrial chain, expand the product range, and support the scientific and technological evaluation of consumer groups.

4.2.3. Study of the Extraction and Processing of the Functional Components of Z. planispinum

The contents of the functional components of Z. planispinum are relatively rich, which provides the plant with a significant competitive market advantage. Qian et al. [43] found that the volatile oil, alkaloids, and other compounds contained in Zanthoxylum extract show good repelling and killing activity against parasites, Plasmodium, and other warehouse pests. Some scholars have also developed Z. planispinum as a natural fumigant for the storage of cultural relics and paintings and applied it to the protection of traditional cultural relics, which has become the practical basis for the development and orientation of Z. planispinum biomass products. Santos et al. [44] found that 6-methoxyseselin, a compound derived from Zanthoxylum, has good pharmacokinetic characteristics and a high lung permeability factor, and it is expected to be a candidate for novel anti-asthma drugs. These research findings demonstrate the potential of Zanthoxylum in the development and processing of pharmaceutical products. Furthermore, it is an easily cultivable medicinal plant.

For the extraction and processing of such components, it is suggested that the following research be carried out and improved: First, the differences in the functional components of different Z. planispinum varieties from diverse regions and plantings should be identified, and their respective advantages in component contents should be investigated to provide basic information for directional cultivation, product development, and market diversion. Second, specific extraction and processing parameters for different functional components should be studied, their effects on component retention and stability evaluated, and extraction efficiency improved. Third, the effects and mechanisms of different extraction and processing methods on quality should be determined to provide a theoretical reference for production applications. Fourth, optimized processing and production techniques should be combined to obtain higher-quality end products and integrate production, supply, and sales aspects.

4.2.4. Research on the Construction of a Z. planispinum Industrial Chain

The Z. planispinum industrial chain includes germplasm creation, standardized cultivation, functional component mining, product processing, and other aspects. At present, these are all being studied, but not in a unified, systematic manner. In the future, the following should be investigated: the interaction mechanisms among different links of the industrial chain, aspects of theoretical research, technology research and development, experimental demonstrations, transformation, and product promotion. Additionally, a mutually supportive industrial development pattern should be formed to improve the efficiency of industrial operations. Food chain and food web theories should be used as references to evaluate the conversion efficiency of the industrial factor flow, reduce the losses in industrial ecosystems from material cycles and energy flows, and achieve environmental friendliness and ecological security. In addition, the role of the Sichuan pepper industry in rural revitalization should be determined, including the driving mechanisms and pathways, and a model of industrial revitalization should be established to promote rural revitalization, which in turn furthers the development of a high-quality social economy.

All of these aspects are related to, and promote, each other. They are not isolated from each other. Thus, these elements should be seen as united, and the system should be seen as a whole. Theoretical research and practical applications should be organically integrated, paying attention to the feedback and response mechanisms of different links and establishing an information dissemination pathway. In this manner, integrated research can be realized, the operational efficiency of the industry can be fully improved, and the healthy, stable, and sustainable development of the industry can be promoted.

5. Conclusions

Through a literature analysis, it was determined that from 2020 to 2022, there were many Chinese publications on Z. planispinum compared with the number of English publications, but publications in both languages showed upward trends, indicating that this field is growing in popularity. The Chinese institutions where most of the research was conducted included Northwest A&F University, Southwest University, Sichuan Agricultural University, and Guizhou Normal University. The analysis determined that the current Chinese authors and institutions do not collaborate, suggesting that institutions should promote exchanges and cooperation to advance the development of the Sichuan pepper industry.

At present, the research and development of Chinese germplasm resources mainly relies on Sichuan, Shaanxi, Yunnan, and other provinces with large planting areas, and the research on and development of varieties are diverse. For efficient management, a gene bank of germplasm resources should be established to facilitate selection and breeding research. The current cultivation-related management technology is very mature, but owing to economic reasons, economizing on labor and costs has gradually become the preferred cultivation management mode pursued by people. The various extraction processes and methods must be optimized for the high-quality development of the Sichuan pepper industry. Flavonoids have various biological and pharmacological effects, and they are potential future pharmaceutical raw materials. Thus, the optimization of the flavonoid extraction process from Z. planispinum is a current research focus. At present, some achievements have been made, but increasing research on extraction technology and the pharmacological effects of Z. bungeanum is still necessary.

The unified and standardized naming of Z. planispinum germplasm resources will help reduce misidentification errors. It will also aid in exploring the adaptative mechanisms of specific Z. planispinum varieties, including their adaptability to local conditions and environments, which will promote the growth of Z. planispinum. The directional cultivation of Z. planispinum should be investigated, and a system should be established according to product positioning development and management technology. The formation of a technical model will be conducive to the cultivation of suitable Z. planispinum varieties and can provide scientific and technological support for the development of the Z. planispinum industry. Because the conditions for extracting different functional components vary, there is a need to develop extraction and processing techniques suitable for their specific properties. The construction of an industrial chain will help increase economic benefits and improve management efficiency while minimizing unnecessary losses.