Abstract
Lagerstroemia is a genus with a wide geographical range extending from Southeast Asia to Australia, and it plays a significant role in many aspects of urban life, including supporting ecosystems, beautifying cities, and affecting human health. To gain insights into the current state of Lagerstroemia research, a bibliometric analysis was conducted using 138 publications from the Web of Science core database between 2002 and 2022. The analysis involved co-occurrence analyses of authors’ affiliated institutions, a keyword co-occurrence analysis, and a timeline view of keywords. The results of the analysis revealed three stages of Lagerstroemia research: the initial stage (2002–04); the rapid growth stage (2005–13); and the in-depth stage (2014–present). Furthermore, 335 keywords from 10 clusters were identified, which indicated that genetic diversity, breeding, and landscape utilization were the three main research areas of Lagerstroemia. Collaboration among authors mainly occurred within major natural and cultivated regions, such as China and the United States. Based on the bibliometric evaluation, future research of Lagerstroemia should focus on areas such as whole-genome sequencing, male sterility problems, resistance, ornamental characteristics, and landscape utilization guidelines. The findings of this study provide an important reference for further studies and can guide the development, innovation, and application of Lagerstroemia.
Lagerstroemia (crape myrtle) has a wide geographical range, extending from Southeast Asia to Australia. These plants have been widely used in urban landscapes because of their longevity (Wang et al. 2021), bright flowers (Anderson 2006), long-lasting blossoms, various plant habits, and wide adaptability. With ∼55 species and an increasing number of new cultivars (Gu et al. 2016), investigations of Lagerstroemia species are urgently needed. Current research is mainly focused on the genetic diversity (Cai et al. 2012; Ye et al. 2019), physiological and biochemical characteristics (Kolakul and Sripanidkulchai 2017), and cultivation management of Lagerstroemia species (De Wilde and Duyfjes 2019). However, the focus of these studies has been broad, and biases have occurred while identifying the research background through reading and induction. Knowledge mapping could solve this problem by providing a systematic analysis. It is necessary to summarize the research status of Lagerstroemia species and identify research trends and new research directions.
A bibliometric analysis is a quantitative analysis method that evaluates the developmental trend of a specific research area by analyzing publications (Vasudevan et al. 2016). This method shows the relationship between the developmental processes and structure of scientific knowledge by mapping knowledge domains, which can help researchers quickly grasp the evolutionary characteristics of a research topic, thereby greatly improving the efficiency of scientometric research and guiding follow-up research (Hou et al. 2015). Currently, Bibexcel, Histcite, SPSS, and CiteSpace are the mainstream knowledge mapping tools (Jayasree and Baby 2019). CiteSpace, a scientometric visualization software, has become the most popular software because of its wide applicability, strong dynamics, and powerful functions. An increasing number of scholars have used CiteSpace to find focused areas and trends in their research field. For example, Papadimitriu and Allinson (2022) used CiteSpace to identify the current trends and challenges of microplastics, and Duan et al. (2018) drew a knowledge map of Picea using CiteSpace software. However, only a few researchers have conducted bibliometric evaluations in the field of Lagerstroemia; therefore, a knowledge map of Lagerstroemia is needed.
This study conducted a bibliometric evaluation of 138 publications regarding Lagerstroemia over the last 20 years. The analysis and summary of research trends in this genus based on the impact of publications on the scientific community reveal the past, present, and future research of this field. The findings provide a comprehensive and systematic review of research regarding Lagerstroemia and a key reference for new researchers seeking to familiarize themselves with the Lagerstroemia literature and research trends.
Methods
Data sources.
The data source for this study was the Science Citation Index database of the Web of Science Core Collection, provided by Thomson Reuters. To select appropriate works, the following three criteria were applied: inclusion of “Lagerstroemia” in the title, abstract, or keywords; categorization within the field of plant science and horticulture; and primary research.
The search was conducted in Apr 2022, and a total of 138 studies were retrieved. All relevant data, including the author, title, abstract, keywords, category, source publications, and cited references, were exported in plain text format and further processed using CiteSpace 5.8.r2 (Chen 2006) under the Web of Science data analysis option.
Methods.
The scientific visualization software package, CiteSpace, developed and provided by Chen (2006), was used for the cluster analysis and social network analysis. The following types of analyses embedded in CiteSpace were conducted to investigate the literature regarding Lagerstroemia: analysis of authors and collaborators; co-occurrence analysis of authors’ affiliations; keyword co-occurrence analysis; and timeline view of keywords in Lagerstroemia research. All data for this study were analyzed using software (Microsoft Excel 2020; Microsoft Corporation, Redmond, WA, USA) and visualization software (CiteSpace 5.8.r2) (Chen 2006).
Results
Author co-citation analysis and organization co-citation analysis.
An author co-citation analysis, which reflects collaboration and associations between authors in a specific field, was first proposed by Mccain et al. (1987). The co-author map of Lagerstroemia mainly includes a network of all the scholars whose research field is mainly focused on resource investigation and diversity analysis of crape myrtle in China, with Qixiang Zhang, Huitang Pan, and Ming Cai being the most influential. Another influential network consists of Mengmeng Gu, Bin Wu, Gary W. Knox, Hongmin Qin, Runshi Xie, and Cecil T. Pounders, who participated in the breeding and evaluation of new Lagerstroemia cultivars (Fig. 1). Additionally, a network that focuses on the genetic diversity of crape myrtle is also gradually developing, with Cuihua Gu, Li Ma, and Kai Chen as the most influential scholars. Among these authors, Qixiang Zhang, Huitang Pan, and Ming Cai from Beijing Forestry University were ranked the top three with the most publications, with 12, 11, and 8, respectively. Jia Wang, Tangren Chen, Cecil Pounders, and Dan He also published more than three articles (Fig. 2). These authors are the core scholars in the field of Lagerstroemia and have a close cooperative relationship with each other.
Author co-citation map of Lagerstroemia research.
Citation: HortScience 58, 8; 10.21273/HORTSCI17155-23
Organization co-citation map of Lagerstroemia research.
Citation: HortScience 58, 8; 10.21273/HORTSCI17155-23
Co-cited literature analysis.
A co-citation analysis can reveal relationships between publications that share similar topics or frameworks. Building a co-citation network of references is helpful for analyzing the research direction, identifying developmental trends, extracting fundamental knowledge, and providing a reference for future research.
Highly cited literature regarding Lagerstroemia mainly focuses on the evaluation of genetic diversity. Cabrera (2004) developed an educational Lagerstroemia website that highlights the history, biology, taxonomy, uses, culture, and management of cultivars in the nursery and landscape. Pounders et al. (2007) used simple sequence repeats (SSRs) to evaluate interspecific hybrids between Lagerstroemia indica and L. speciosa and provided SSR loci for future research of species diversity. Wang et al. (2010) used SSRs to reveal relationships among 37 cultivars, mainly derived from Lagerstroemia indica, L. fauriei, and their interspecific hybrids (Fig. 3). These three works are the most frequently cited studies with high academic impact, thus reflecting the fundamental research in the Lagerstroemia-related research fields.
The co-cited literature map of Lagerstroemia research.
Citation: HortScience 58, 8; 10.21273/HORTSCI17155-23
Keyword co-occurrence analysis.
Keywords are essential components of academic works because they provide concise and condensed summaries of the leading research content of a scholarly article. The frequency of keywords reflects the current hot topics of research and the centrality of keywords in the co-occurrence network. To identify the research hotspots of Lagerstroemia plants, CiteSpace was used to generate a keyword occurrence map (Fig. 4). Keywords that appeared more than three times were extracted and sorted into a table (Table 1). The co-occurrence analysis revealed that a total of 335 keywords formed 10 clusters, with the five largest clusters being Lagerstroemia speciosa, air pollution effect, leaf epidermal polyphenol, crape myrtle, and density-dependence. The most frequently occurring keywords were growth, leaf, plant, cultivar, identification, hybrid, and genetic diversity.
Keyword co-occurrence map of Lagerstroemia publications.
Citation: HortScience 58, 8; 10.21273/HORTSCI17155-23
Frequency of keywords from Lagerstroemia publications.
Based on a keyword co-occurrence analysis, the current research focus on Lagerstroemia is mainly centered around genetic diversity, breeding, and landscape utilization, with a particular emphasis on genetic relationship analysis, cultivar identification, and stress physiology. These research hotspots should be considered important areas for future investigations of Lagerstroemia. Among all Lagerstroemia species, L. speciosa has been the subject of the most extensive research, with a large body of work already accumulated in the areas of breeding and landscape utilization. Because of its high landscape value, this species is likely to remain a popular research topic in the years to come.
Keyword timeline analysis.
The timeline of keywords provides a clear picture of how the research field of Lagerstroemia has evolved and progressed over time. Specifically, the Lagerstroemia research hotspots initially centered around L. speciosa and gradually expanded to encompass other species and their adaptability, stress responses, species distribution, and genetic diversity (Fig. 5).
The keyword timeline of Lagerstroemia publications.
Citation: HortScience 58, 8; 10.21273/HORTSCI17155-23
Overall, the research of Lagerstroemia can be approximately divided into three stages. The initial stage (2002–04) consists of 16 publications without a clear research focus. The growing stage (2005–13) comprises a total of 53 publications with a focus on density-dependence, L. speciosa, fossil records, leaf epidermal polyphenols, and air pollution effects. The current developmental stage (2014–present) includes 69 publications that are primarily focused on the gene level. Comparative analysis, apical dominance, gene function, isolation, and control were research hotspots.
Discussion
Genetic diversity of Lagerstroemia.
Since the beginning of the 21st century, scholars have placed significant focus on the genetic diversity of Lagerstroemia. Rinehart et al. (2015) used SSRs to estimate the genetic diversity among Lagerstroemia species in the United States. Several authors (Cai et al. 2011, 2012; He et al. 2019; Liu et al. 2013; Xu et al. 2017; Ye et al. 2019) evaluated the genetic diversity among Chinese crape myrtle species using amplified fragment length polymorphisms, morphological characteristics, and SSRs, as well as expressed sequence tag SSR markers. They also analyzed the complete chloroplast genomes of six Lagerstroemia species (Gu et al. 2019). The phylogenetic relationships among 22 Lythraceae species, including 13 Lagerstroemia species, through comparative analyses of chloroplast genomes was investigated (Rinehart and Pounders 2010; Zheng et al. 2020), resulting in ten potential molecular markers for future applications.
These studies of the genetic diversity of Lagerstroemia have primarily aimed at cultivar identification and DNA fingerprinting databases, with SSRs being the most widely used technology over the past 20 years. However, with the development of molecular biotechnology, whole-genome sequencing, advanced genetic markers, and gene mapping of Lagerstroemia should be the focus of future studies. Moreover, the plant materials used in these studies were predominantly Lagerstroemia indica, L. speciosa, L. subcostata, L. fauriei, and their hybrids. It is of practical significance to broaden the research of genetic diversity to include other Lagerstroemia species and cultivars.
Breeding of new Lagerstroemia cultivars.
Breeding of Lagerstroemia has primarily focused on hybridization between Lagerstroemia taxa with high ornamental value and pest resistance (Knox and Norcini 1991). The breeding objectives are to achieve high resistance to cold, drought, diseases, and insects, as well as large flowers, multicolored leaves, and diverse habits. L. indica, L. speciosa, L. subcostata, and L. fauriei have been widely selected as parents. The United States and China are the countries with the most extensive breeding research.
Approximately 55 species of Lagerstroemia have been discovered worldwide, extending from Southeast Asia to Australia. With rich diversity in flower and foliage colors and habits ranging from miniature shrubs to large trees (Liu et al. 2008), there is great potential to breed new cultivars. Hybridization between L. indica and L. speciosa has been effective for improving flower size, but their siblings are usually sterile. Future research may focus on fertility recovery, embryo rescue, and polyploid breeding. Pseudorobitis gibbus and Eriococcus lagerstroemiae are major insects that damage Lagerstroemia. Improving resistance to these two pests should also be a focus of breeding. Given the high landscape value of Lagerstroemia, selecting cultivars with better ornamental features and higher resistance should be a priority.
Landscape utilization of Lagerstroemia.
Lagerstroemia, which is native to China (Egolf 1967), was introduced to Europe in the 16th century, and to the southeastern United States in the 17th century (De Wilde and Duyfjes 2016). It is now one of the most popular and widely used ornamental plants worldwide. Research of the landscape utilization of Lagerstroemia mainly focuses on cultivar evaluation and comparative studies involving other woody plants in terms of landscape efficacy.
Willem et al. (2016) surveyed the resources of Lagerstroemia in Indochina and found that flowering and fruiting materials were key factors in species classification. Marwah et al. (2021) surveyed producers’ preferences for crape myrtle and found that they were more concerned with crape myrtle pest resistance than family/individual operations. Scholars also conducted various comparative analyses, including variety collection, evaluation, resistance, and adaptability analyses, and concluded their significant differences (Kjelgren et al. 2008).
Compared with other woody ornamental plants, an increasing number of studies have found that Lagerstroemia plants have significant advantages regarding drought resistance (Rahman et al. 2021), carbon storage (Ahmad et al. 2019), and air pollution control (Wu et al. 2008). Wu et al. (2008) also found that crape myrtle was suitable for reforestation in stone fields. Ghafari et al. (2020) found that L. indica had ecological potential and is suitable for urban beautification.
Lagerstroemia has both higher ornamental value and important ecological benefits. We should strengthen the systematic study of Lagerstroemia landscape applications, especially for the development of planning and design guidelines for urban landscapes. Additionally, we should increase its applications in ecological restoration, such as mine field restoration and ecological restoration.
Conclusion
Based on 138 publications from the Web of Science database between 2002 and 20 Apr 2022, we have summarized the research status and conducted an in-depth analysis of Lagerstroemia using CiteSpace. Our study systematically sorted research hotspots and trends, which provide scientific support for optimizing the basic theory and application system of Lagerstroemia. The following conclusions were reached: author and affiliation co-citation analyses indicated that collaboration among authors mainly occurred within the same geographical region, but cooperation between different regions should be strengthened in the future; highly cited literature regarding Lagerstroemia mainly focused on evaluation and genetic diversity, thus reflecting academic influence and the research foundation of Lagerstroemia-related research fields; the keyword co-occurrence analysis of Lagerstroemia revealed that research focused on genetic diversity, breeding, and landscape utilization, with 335 keywords forming 10 clusters; and Lagerstroemia-related research can be approximately divided into the initial stage (2002–04), growth stage (2005–13), and developmental stage (2014–present). Research has gradually transitioned from simple to diversified and gene-related research, with a focus on the development of molecular technology. Therefore, with respect to genetic diversity, whole-genome sequencing of Lagerstroemia and genetic diversity beyond L. indica, L. speciosa, L. subcostata, and L. fauriei should be areas of focus. During the breeding of new cultivars, we should focus on addressing the male sterility problem between L. indica and L. speciosa, improving resistance to Pseudorobitis gibbus and Eriococcus lagerstroemiae, and selecting better ornamental traits. Finally, in terms of landscape applications, we should develop Lagerstroemia landscape design guidelines and explore their potential applications in ecological restoration.
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