1 Introduction

Loquat (Eriobotrya japonica Lindl.), a subtropical evergreen fruit tree native to south-eastern China, is economically and nutritionally important. Loquat is widely cultivated in East Asia, the Mediterranean region, and South America and is prized for its good flavor, quick ripening, and high levels of vitamins, minerals, and antioxidant compounds (Su et al., 2024). Loquat is a key southern provincial fruit crop in China's Fujian, Zhejiang, and Sichuan provinces, and an important contributor to regional agricultural economies. Loquat production, however, is still constrained by agronomic problems and year-to-year yield variability relative to other dominant fruit crops.

One of the greatest limitations to loquat production is that it bears a very low percentage of fruit set, normally ranging from 1% to 5% when it is under natural pollination (Bons et al., 2019). The inefficiency of fruiting is mostly because of physiological fruit drop, partial pollination, and ovule abortion—limitation that is aggravated by unfavorable weather and suboptimal orchard management. Consequently, fruit production and quality are negatively affected, limiting the economic yield of loquat production and making the crop less competitive in the global fruit market.

Plant growth regulators (PGRs) also emerge as potential candidates to solve the problem of loquat fruit set in recent years. PGRs are naturally occurring or synthetic chemicals that have the capability to control endogenous hormone levels and physiological processes and to initiate reproductive development. Various PGR groups including auxins, gibberellins (GAs), cytokinins, ethylene inhibitors, and abscisic acid analogs have been studied for their capacity to induce pollen germination, ovule development, nutrient transport, and prevention of physiological drop of the fruit (Huang et al., 2021; Peng et al., 2022). PGR treatment was observed to have measurable effects on improving the retention and production of loquat fruit under open-field and protected cultivation regimes (Liang and Huang, 2024).

This study explored the research progress on plant growth regulators (PGRs) in promoting loquat fruit set, including the physiological basis of fruit set, the types and functional characteristics of commonly used PGRs, their mechanisms of action, and field application techniques. The study focused on practical challenges such as dosage optimization, varietal response differences, and potential side effects. The study aims to integrate the latest research findings to provide theoretical basis and practical guidance for increasing loquat yield and achieving sustainable production.

2 Physiological Basis and Influencing Factors of Loquat Fruit Set

2.1 Characteristics of flowering and fruit setting in loquat

Loquat (Eriobotrya japonica) is an evergreen tree that flowers commonly from autumn to winter with fruit formation in late spring to early summer. Loquat fruit growth is sigmoid and has a high correlation between the length of rapid growth and final fruit size. Both seed number and fruit size have a positive correlation and seed weight with flesh and fruit weight. During the stage of rapid growth, significant physico-chemical changes occur, including color alteration, development of sugar, and loss of firmness in fruit and organic acids, which are associated with high ethylene production and with a small peak of respiration. These findings indicate that fruit ripening of loquat on the tree is a climacteric process with comparatively minor levels of ethylene production in an initial stage of development (Amorós et al., 2003; Su et al., 2024).

2.2 Major physiological and environmental factors affecting fruit set rate

Loquat fruit set is controlled by a combination of environmental and physiological factors. Growth of the fruit is regulated by hormones to a great extent, particularly by auxin and gibberellin, and by auxin particularly during the stage of fruit enlargement. Recent multi-omics analysis pinpointed candidate genes such as ETHYLENE INSENSITIVE 4 (EjEIN4) and TORNADO 1 (EjTRN1) as key regulators of fruit weight and thus implying gene regulation of set and size of the fruit (Peng et al., 2022). Mineral nutrition is required as well since biennial concentrations in leaves and soil of iron, calcium, magnesium, zinc, potassium, and nitrogen significantly affect fruit set, soluble solid content, and acidity and thus both fruit set and quality (Huang et al., 2021). Sunburn and reduced fruit set and quality may be caused by environmental stresses such as high light intensity and temperature. Heat stress induces stress response and hormone signaling pathways levels of hormones and gene expression, and this is crucial through the participation of heat shock proteins and auxin in the heat stress response (Chen et al., 2021). Agronomic practices including fruit and flower thinning have been found to improve fruit set and fruit quality, with optimal thinning intensities yielding heavier and sweeter fruits and greater yield (Mostert et al., 2024; Nordi et al., 2025) (Figure 1).

Figure 1 Relationship between fruit thinning intensity and fruit size and yield (Adopted from Nordi et al., 2025)

2.3 Types and mechanisms of flower and fruit drop in loquat

Flower and fruit abscission in loquat are caused by physiological and environmental factors. Physiological drop is more associated with abnormal cell division during the early stages of development and hormonal imbalances, particularly with auxin, gibberellin, and ethylene. Environmental aspects are also responsible, with genes like EjBZR1 playing a negative role in cell expansion and fruit growth and thus influencing the fruit drop tendency (Su et al., 2021). Environmental stresses like high temperature and nutritional deficiency also induce flower and fruit drop via the disruption of hormonal regulation and cell activities (Huang et al., 2021). Understanding of these mechanisms will further be helpful in devising methods for enhancing fruit set and yield in loquat.

3 Common Types and Functional Characteristics of Plant Growth Regulators

3.1 Auxins

Auxins are regulators of plant growth involved in the regulation of cell elongation, root formation, and organogenesis. Auxins are the central hormones in coordinating the plant response to external stimuli, and they constantly interact with other hormones in the regulation of synthesis, transport, and signal transduction. Auxins also play a role in fruit set, development, and abscission and typically synergize or antagonize other hormones to regulate plant growth and stress (Mazzoni-Putman et al., 2021; Thapa et al., 2024).

3.2 Gibberellins

Gibberellins are mostly involved in triggering stem elongation, seed germination, and flowering. Gibberellins are responsible for vegetative to reproductive transition growth and participate in fruit development and ripening. Gibberellins interact with other hormones such as auxins and abscisic acid and control growth and developmental processes, and their use may enhance fruit set and quality (Waadt et al., 2022; Jain et al., 2023).

3.3 Cytokinins

Cytokinins are shoot growth and cell division promoters. Cytokinins are the primary regulators of organogenesis, and they also delay leaf senescence and control mobilization of nutrients. Cytokinins act in conjunction with auxins to control root and shoot growth and can potentially control abiotic stress responses by impacting hormonal crosstalk as well as gene expression (Thapa et al., 2024).

3.4 Ethylene regulators

Ethylene is a gaseous hormone which controls fruit ripening, senescence, and abscission. Ethylene regulators suppress or stimulate ethylene action and can impact processes such as flower opening, fruit ripening, and stress responses. Ethylene opposes the action of abscisic acid and co-operates with auxins and cytokinins in various physiological activities (Jain et al., 2023).

3.5 Abscisic acid and other novel growth regulators

Abscisic acid (ABA) is a key stress-response hormone that mediates stomatal closure, seed dormancy, and drought and salt tolerance (Li, 2024). ABA interacts in a complex manner with auxin, cytokinin, and ethylene to regulate growth in response to stress. Other recently discovered growth regulators, such as brassinosteroids, jasmonic acid, salicylic acid, and strigolactones, also play significant roles in plant growth and stress adaptation, which tend to include intricate signaling processes and communication with classical hormones (Sabagh et al., 2021; Zahid et al., 2023; Ochatt, 2024) (Figure 2).

Figure 2 Role of selected PGRs on plant growth and development (Adopted from Sabagh et al., 2021)

4 Mechanisms of Plant Growth Regulators in Enhancing Loquat Fruit Set

4.1 Regulation of hormonal balance and ovule development

Plant growth regulators such as auxins, gibberellins, cytokinins, and abscisic acid play a central role in regulating endogenous hormone levels to facilitate ovule production and fruitful reproduction. Exogenously applied PGRs may control the hormone balance to initiate division and differentiation within the reproductive organs, which maintains ovule viability and fertilization. For example, paclobutrazol changes the gibberellin, abscisic acid, and cytokinin levels, which lead to improved fruit set and quality because of its influence on hormonal regulation and inducing carbohydrate storage in young fruits (Bons and Kaur, 2019).

4.2 Promotion of pollen germination and pollen tube growth

PGRs, particularly auxins and gibberellins, are said to induce pollen germination and pollen tube growth that is fundamental in successful fertilization and fruiting. With ideal hormonal conditions, these regulators facilitate the elongation of pollen tubes within the style, enhancing ovule fertilization possibilities and the development of fruit growth (Gill et al., 2023; Singh et al., 2024).

4.3 Inhibition of physiological fruit drop

Among the key benefits of PGR application is reducing physiological fruit drop. Auxins and gibberellins are predominantly used in an attempt to minimize pre-harvest fruit fall by maintaining the hormonal cues in the direction of fruit retention. The hormones reverse abscission signals, strengthening fruit attachment, and enhancing the rate of fruit retention, leading to enhanced yields (Suman et al., 2017). Ethylene regulators can be used to slow down abscission and enhance the life of fruit retention on the tree (Singh et al., 2024).

4.4 Influence on carbon-nitrogen metabolism and nutrient transport

PGRs control carbon-nitrogen metabolism and nutrient transport, which have significant roles in fruit development and quality. For instance, paclobutrazol enhances the accumulation of carbohydrates and stimulates the transport of nutrients to developing fruit, resulting in increased fruit size, weight, and quality. This is achieved through modifying the metabolic activities of the plant and increasing water and nutrient utilization efficiency, which favors extended fruit development under various environmental conditions (Desta and Amare, 2021; Zahid et al., 2023).

5 Application Strategies of Plant Growth Regulators for Improving Loquat Fruit Set

5.1 Spraying and smearing techniques

The effectiveness of plant growth regulators (PGRs) in enhancing loquat fruit set is highly dependent on the timing and concentration of application. Spraying is most effective when performed at critical developmental stages, such as the onset of flowering, early fruit set, and during the rapid fruit enlargement phase. For example, the application of synthetic auxins like 3,5,6-TPA at 15 mg/L during the early fruit growth stage or one month later significantly increased fruit size and accelerated ripening, with a 10% increase in fruit diameter compared to controls (Reig et al., 2016). Similarly, forchlorfenuron (a cytokinin-type PGR) applied at 20 mg/L at 24 and 38 days after bloom resulted in optimal improvements in fruit weight and quality (Zhang et al., 2025). The concentration must be carefully optimized, as excessive doses can lead to phytotoxicity or abnormal fruit development, while suboptimal concentrations may yield negligible effects (Desta and Amare, 2021; Kumar et al., 2023; Zhang et al., 2025).

Smearing techniques involve the direct application of PGRs to specific plant tissues, such as inflorescences or young fruits, to maximize local absorption and minimize wastage. For loquat, smearing is typically performed on the panicles or young fruitlets during pre-anthesis or immediately after fruit set. This targeted approach ensures that the PGRs are delivered precisely where hormonal regulation is most needed, promoting cell division and fruit retention. Technical guidelines recommend using a fine brush or cotton swab to apply the solution evenly, avoiding runoff and ensuring thorough coverage. Smearing is particularly useful for high-value cultivars or in research settings where precise dosing is required.

5.2 Mixing and combination of different regulators

The combined application of auxins and gibberellins has been shown to produce a synergistic effect on fruit set and development in loquat. Auxins (such as NAA or its synthetic analogs) promote cell division and fruit initiation, while gibberellins (such as GA₃) stimulate cell elongation and the formation of parthenocarpic fruits (Jiang et al., 2016; An et al., 2020; Jiang et al., 2020; He and Yamamuro, 2022). Studies have found that GA3 treatment significantly increases the fruit set rate of triploid loquats, with fruits in the treated group retained and developing after three weeks, whereas those in the control group largely abscised (Figure 3). Transcriptomic and proteomic analyses revealed that GA-induced fruit set is accompanied by the upregulation of auxin biosynthesis genes and cell division-related genes, reflecting the synergistic interaction between these hormones (Jiang et al., 2016; Jiang et al., 2020). Experiments have demonstrated that mixing NAA, CPPU (a cytokinin), and GA3 at specific concentrations (e.g., CPPU 40 mg/L + GA₃ 50 mg/L + NAA 16 mg/L) can significantly improve fruit enlargement and quality, with the ratio and timing of regulator application being critical to maximizing the synergistic effect.

Figure 3 Development of the triploid loquat at 3 weeks after gibberellin (GA) treatment (Adopted from Jiang et al., 2016) Image caption: A: GA treatment; B: Control (Adopted from Jiang et al., 2016)

Cytokinins, such as forchlorfenuron (CPPU), are often combined with nutrient solutions to further enhance fruit set and quality. Cytokinins promote cell division and expansion, while nutrient supplementation ensures that developing fruits have adequate resources for growth (Aremu et al., 2020; Surya et al., 2021; Zhang et al., 2025). Forchlorfenuron application at 20 mg/L, for instance, not only increased fruit weight but also improved sugar-to-acid ratio and bioactive compound accumulation in loquat (Zhang et al., 2025). The integration of cytokinins with balanced nutrient sprays (including magnesium, zinc, and boron) has been shown to optimize fruit nutritional quality and reduce physiological disorders (Ali et al., 2021; 2024). Such combined strategies are particularly effective in orchards facing nutrient limitations or environmental stress.

5.3 Safety and standardized management

While PGRs offer substantial benefits, improper use can result in phytotoxicity, fruit deformities, or residue accumulation. Phytotoxic effects are often dose-dependent and can manifest as leaf burn, fruit drop, or abnormal growth patterns (Desta and Amare, 2021; Jain et al., 2023). To mitigate these risks, it is essential to adhere to recommended safety intervals between application and harvest, ensuring that residue levels remain within safe limits for human consumption (Kumar et al., 2023). Regular monitoring and residue analysis are advised, especially when using synthetic PGRs or in export-oriented production systems.

The adoption of standardized operational procedures is crucial for the safe and effective use of PGRs in loquat cultivation. Guidelines should specify the appropriate PGR type, concentration, timing, application method (spraying or smearing), and safety precautions (Desta and Amare, 2021; Jain et al., 2023; Kumar et al., 2023). Field training for workers, use of personal protective equipment, and record-keeping of application details are recommended best practices. Additionally, periodic field trials and extension services can help update growers on the latest research findings and regulatory requirements, promoting responsible and sustainable PGR use in loquat orchards.

6 Current Issues and Research Challenges

6.1 Dosage and safety concerns

PGR effect on loquat was found to be extremely dose-dependent in experiments, and the plant response vs. dosage relationship is not always linear. Under- and over-doses generate sub-optimal or toxic effects, which make it difficult to determine safe and optimal dosages for growth stages and organs. This diversity underscores the need for proper dosage standards to avoid unwanted impacts on plant health and fruit quality (Surya et al., 2020; Desta and Amare, 2021; Surya et al., 2021).

6.2 Regional adaptability differences due to variety and climate factors

Loquat varieties and PGR sensitivity vary significantly depending on genetic origin and environmental factors. Climatic conditions of the region, irrigation system, and stress severity (e.g., drought stress, frost stress) influence loquat plant response to PGR application. Regional differences in this context make it challenging to attempt overall application guidelines and highlight the need for ongoing location- and variety-dependent investigations (Gugliuzza et al., 2020; Wang et al., 2021).

6.3 Potential side effects on fruit development and quality

While PGRs can induce growth and stress tolerance, there is potential for unforeseen side effects on fruit form, nutritional content, and overall quality. For example, some treatments could increase some of the growth qualities but negatively affect others, e.g., leaf or fruit form, or not have a significant impact on all the desired qualities. The ignoring of potential negative effects on postharvest quality and fruit development remains a significant concern (Surya et al., 2020).

6.4 Lack of molecular-level understanding of regulatory mechanisms

In contrast, there is scant information on the molecular bases of the physiological and biochemical responses of PGRs in loquat (Tranbarger and Tadeo, 2025). Gene expression and signal transduction mechanisms have started to be explored in more recent investigations on PGR-induced stress responses, but overall knowledge at the molecular level is sporadic. This knowledge gap limits the possibility for optimization of PGR application and design of targeted interventions for improved fruit set and stress resistance (Sabagh et al., 2021; Wang et al., 2021).

7 Future Research Directions and Application Prospects

7.1 Screening and development of efficient and low-toxicity new growth regulators

Current research is laying a lot of emphasis on the synthesis of new PGRs with low toxicity and high activity. Synthetic researches are being attempted towards synthesizing new classes of PGRs such as phenylurea derivatives and phytohormone functional analogues with increased bioactivity and reduced toxicity. Encapsulation technology and micro/nano-formulations have also been explored to enhance stability, reduce the environmental impact, and enable controlled release, reducing toxicity and enhancing efficacy (Campos et al., 2023; Yan et al., 2023; Chen et al., 2024).

7.2 Integration with loquat genomics and hormone signaling pathways

Intersecting PGR treatment with genomic and transcriptomic research is a direction of the future. Current studies have made headways in deciphering the molecular mechanism of PGR activity in loquat, such as modification of hormone signaling pathway and stress response gene. For instance, transcriptome profiling characterized candidate genes and transcription factors of hormone signaling and stress response, which can be used as a basis for precision breeding and targeted application of PGR (Wang et al., 2021; Dongariyal et al., 2024).

7.3 Construction of precision regulation models and integration with digital agriculture

The future of PGR use is in precision agriculture where computer models and digital technologies can more accurately tailor timing, rate, and combinations of PGRs. Advances in metabolomics, genomics, and digital sensing technologies can bring predictive modeling of PGR response, which can facilitate site-specific and cultivar-specific management. The integration will optimize fruit set, yield, and quality and reduce resource utilization and environmental footprint (Gugliuzza et al., 2020; Zhang et al., 2025).

7.4 Comprehensive regulation strategies based on coordinated improvement of fruit set, yield, and quality

Systems approach is needed, incorporating PGRs with other agronomics (e.g., fertilization, irrigation) and employing molecular information to steer increases in fruit set, yield, and quality. Multi-component approaches, e.g., combining PGRs with fertilizers or biostimulants, have been documented to show synergistic action on loquat growth and stress tolerance. Multi-factorial experiments and systems-level optimization are to be accorded importance in the future for high-quality and sustainable production (Surya et al., 2020; Surya et al., 2021; Campos et al., 2023).

8 Concluding Remarks

Modern research shows that PGRs such as auxins, gibberellins, cytokinins, and chemicals such as paclobutrazol are very effective in controlling fruit set, yield, and quality of loquat and other fruit trees. The regulators promote internal physiological processes, suppress fruit drop, and reverse the detrimental effects of abiotic stresses such that fruit retention and yield are enhanced.

The application of PGRs has become an essential component of modern fruit production, offering a practical means to boost productivity and fruit quality. By optimizing hormonal balance, PGRs not only increase fruit number and weight but also improve nutritional and market value, making them invaluable for efficient and sustainable loquat cultivation.

The application of PGRs has become an essential component of fruit production today, offering a feasible measure to enhance productivity and fruit quality. PGRs not only increase fruit quantity and weight but also improve nutritional and market value by regulating hormonal balance, making them indispensable for productive and sustainable loquat cultivation.

Acknowledgments

The authors thank Ms. Wang for her support and assistance in data collection and material compilation. The authors also thank the two anonymous reviewers for their careful review of the manuscript.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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