1 Introduction

Eggplant (Solanum melongena L.) ranks among the most economically important solanaceous vegetables, after crops like tomato and potato, and is widely cultivated across tropical and temperate regions (Oladosu et al., 2021; Shi et al., 2023). Large germplasm collections and active breeding programs underscore its global significance for food systems and markets (Oladosu et al., 2021). Demand is rising due to its culinary versatility, nutritional value, and year-round availability, yet regional production often remains below potential because of land, climate, and management constraints (Alicja et al., 2019; Rathore et al., 2022).

For both consumers and producers, yield, fruit size, shape, and external appearance are key agronomic and commercial traits (Taher et al., 2017; Alicja et al., 2019). Uniform fruit length, diameter, weight, and shape improve grading, packaging, and price, while non-uniformity leads to higher discard rates and economic loss (Wakchaure et al., 2020; Rathore et al., 2022). Environmental stresses, nutrient imbalances, and irregular fruit set frequently compromise both total yield and fruit uniformity. 

Plant growth regulators are organic compounds, distinct from nutrients, that regulate plant growth and development at very low concentrations (Bons and Kaur, 2019; Zahid et al., 2022; Amin et al., 2025). They include endogenous hormones and synthetic analogues such as auxins, gibberellins, cytokinins, ethylene, abscisic acid, brassinosteroids, jasmonates, and salicylic acid. Through modulation of cell division, elongation, flowering, fruit set, and stress responses, PGRs have become key tools in modern vegetable production systems. In vegetables, PGRs are used to improve seedling vigor, flowering, fruit set, retention, and final yield, and to alleviate abiotic stresses like drought, salinity, and temperature extremes (Wakchaure et al., 2020; Verma et al., 2024). In eggplant, foliar or floral applications of auxin-like compounds (e.g., NAA, 4-CPA, cloxyfonac), gibberellins, and other regulators can significantly increase fruit set, number of fruits per plant, and marketable yield, sometimes without changing cultural practices (Widiwurjani et al., 2021; Afrin et al., 2024). Under water-scarce conditions, PGRs such as salicylic acid, thiourea, and potassium nitrate help maintain canopy function and fruit quality, supporting yield stability (Wakchaure et al., 2020).

Multiple studies show that appropriate PGR type, concentration, and timing can enhance eggplant yield components, including plant height, leaf area, flower number, fruit number, and individual fruit weight (Wakchaure et al., 2020; Afrin et al., 2024). Yet responses are often genotype- and environment-specific, and sub-optimal doses or combinations may fail to improve final yield due to increased fruit drop or sink regulation by the plant (Alicja et al., 2019). Fruit uniformity is closely linked to processes of flower biology, fruit set, and early fruit development, all of which are hormonally regulated (Bons and Kaur, 2019; Zahid et al., 2022). Differences among flower types (e.g., long vs. short styles) in pollen tube growth, nutrient status, and endogenous hormone balance can determine which flowers set fruit and how fruits develop in size and shape. Transcriptomic analyses further highlight phytohormone-related genes as central regulators of early fruit growth and shape variation (Shi et al., 2023). However, the specific ways exogenous PGRs influence these developmental and physiological mechanisms to improve uniform fruit size and shape in eggplant remain insufficiently characterized. 

This study focuses on two central questions: first, whether plant growth regulators can enhance eggplant yield; and second, their roles and underlying mechanisms in improving fruit uniformity. Although field studies on the yield-promoting effects of plant growth regulators in eggplant are relatively abundant, research that treats uniformity as the primary evaluation criterion remains limited. Relevant evidence often needs to be inferred through an integrated assessment of fruit shape formation, floral variation, maturity synchrony, and market quality traits. Accordingly, this study combines evidence from field trial literature with relevant physiological mechanisms to provide a more systematic interpretation of the practical effects of plant growth regulators.

2 Types of Plant Growth Regulators Commonly Used in Eggplant Production

2.1 Auxin regulators and their roles

In eggplant production, the most widely used and common plant growth regulators are still auxin-based compounds, especially IAA, NAA, and 2,4-D. The most direct function of these regulators is to increase the probability of fruit set and, to some extent, reduce flower and fruit drop when pollination is unstable, low-temperature stress occurs, or floral organs develop poorly. Chen et al. (2022) showed that the main purpose of spraying 2,4-D at the flower bud stage in eggplant is usually to reduce floral abscission and promote fruit set. At the molecular level, the SmARF family in eggplant responds significantly to 2,4-D treatment, indicating that the role of exogenous auxin is not simply to “promote fruiting”, but rather to regulate and reshape a whole set of developmental signaling pathways.

NAA has received considerable attention in production practice, partly because of its fruit-setting effect and partly because of its ability to improve fruit shape. Field trial results show that the commonly effective and relatively stable concentration of NAA is usually around 40 ppm. Although the exact value may vary somewhat among cultivars and seasonal conditions, the overall direction of its effect remains consistent. NAA treatment usually increases the effective fruiting rate of long-styled and medium-styled flowers, while also improving leaf photosynthetic capacity and PSII efficiency, which is ultimately reflected in increased fruit number and yield (Moniruzzaman et al., 2014). Amin et al. (2025) tested 40, 50, 60, and 70 ppm NAA together with a control, and the results indicated that 40 ppm was the best treatment: plant height reached 73.73 cm, branch number 9.20, leaf number 97, single-fruit weight 186.67 g, fruit number per plant 10.11, yield per plant 1.31 kg, and estimated yield 41.9 t/ha. This treatment was applied at the 50% flowering stage and again 20 days later. These findings indicate that exogenous auxin regulators can indeed improve fruit set and yield, but their effectiveness depends on an appropriate flowering-stage window and a reasonable concentration; otherwise, excessive hormone application may easily lead to malformed fruits or fruit drop at later stages.

2.2 Gibberellins and cytokinins

Gibberellins, especially GA₃, function in eggplant production more like regulators with an “amplifying effect”, as their action is often expressed through simultaneous increases in flower number, fruit number, and individual fruit size. Field trial results showed that treatment with GA₃ at 50 ppm increased the number of fruits per plant to 18.56, compared with 11.34 in the control. Yield per plant increased from 1.38 kg to 1.58 kg, representing an increase of about 14.5%, while the increase in fruit number reached 63.7%. These results indicate that the role of GA₃ in eggplant is not limited to promoting fruit enlargement, but also has a significant effect on the fruit-bearing structure of the plant (Kropi, 2018). GA₃ at 75 ppm is also frequently included in optimal treatment combinations, as it can not only advance the time to 50% flowering, but also further increase fruit number per plant and total yield (Pradeepkumar et al., 2020).

Compared with auxins and GA₃, there is relatively less direct field evidence for cytokinin application in eggplant, but its role in regulating early cell division and buffering stress has become fairly clear. Studies have shown that 6-BA treatment can alleviate the decline in chlorophyll content, reactive oxygen species accumulation, and membrane lipid peroxidation caused by low-temperature stress, while increasing the activities of antioxidant enzymes such as SOD, POD, CAT, APX, and GR. In other words, cytokinins may not show as direct an effect on yield improvement as GA₃, but they play a strong foundational role in seedling uniformity, maintenance of plant vigor, and the eventual formation of uniform fruit set. Further molecular evidence shows that the SmRR family in eggplant is closely associated with cytokinin signal transduction, and some of these genes also respond sensitively to IAA and stress conditions. This suggests that cytokinins do not act independently, but instead participate together with auxins in regulating the developmental rhythm of the plant (Chen et al., 2016).

2.3 Other types of regulators

The role of ethylene and its inhibitors in eggplant cultivation is more closely associated with two aspects: “preventing abscission” and “delaying senescence”. The former mainly occurs before fruit formation, as ethylene generally exerts a certain inhibitory effect on fruit set; the latter is mainly expressed during the postharvest stage, when ethylene accelerates fruit senescence and softening. Sharif et al. (2022) reported that the ethylene inhibitor 1-MCP can promote parthenocarpy in some fruit vegetables. In postharvest treatment of eggplant, application of 1-MCP at 5–10 μL/L can significantly delay fruit softening, inhibit the activity of cell wall hydrolytic enzymes, and extend shelf life. These findings suggest that although ethylene inhibitors are not the main regulatory tools for increasing yield in the field, they are of great value in maintaining marketable uniformity and extending the marketing period.

New regulators in the brassinosteroid group are better understood from the perspective of yield stability rather than yield maximization. Studies have shown that under low-temperature stress or during recovery from chilling injury, treatment with 0.1 μM 24-epibrassinolide can reduce the accumulation of MDA, H₂O₂, and superoxide anions in eggplant seedlings, while increasing the activities of enzymes related to the AsA-GSH cycle. This type of regulator has shown certain potential in promoting early recovery of growth, maintaining leaf photosynthesis, and supporting subsequent uniform fruit enlargement (Wu et al., 2015). However, based on currently available public research evidence in eggplant, the conclusion that brassinosteroids can directly increase yield under field conditions is still less consistent than for GA₃ and NAA. Therefore, a more practical application strategy is to regard them as important supplementary tools for regulating plant growth, stabilizing yield, and maintaining quality during stress-prone seasons.

3 Effects of Plant Growth Regulators on Eggplant Yield

3.1 Regulation of flowering and fruit set

The starting point of eggplant yield formation lies in flowering, and many production problems also first emerge during the flowering stage. A high proportion of heterostylous flowers, together with factors such as low temperature, weak light, and fluctuations in water availability, may frequently result in the phenomenon of “flowering without fruit set” (Figure 1). A study on 13 eggplant genotypes showed that the proportion of long-styled and medium-styled flowers, which possess normal fruit-setting ability, ranged from 43.60% to 75.62%, whereas the proportion of short-styled flowers ranged from 20.47% to 45.51%, and these short-styled flowers essentially lacked fruit-setting ability (Khaleghi et al., 2021). This indicates that eggplant itself exhibits clear differences in floral organ type, which is also an important reason why exogenous plant growth regulator treatments applied during flowering often produce obvious effects. NAA and related treatments can improve floral characteristics and promote initial fruit set; in essence, they increase the fertilization success rate of flowers with fruiting potential.

From the perspective of flowering progress, the regulatory effect of GA₃ is usually more direct. Experimental results showed that treatment with GA₃ at 75 ppm advanced the time to 50% flowering to 51.62 days, compared with 58.87 days in the control, representing an advancement of 7.25 days or about 12.3% (Dewangan and Jangre, 2024). In the same experiment, this treatment produced 4.01 flowers per cluster, indicating that GA₃ can not only accelerate flowering but also help improve flowering quality. This change is of considerable practical significance, because once flowering becomes more synchronized, the subsequent fruit-setting process and harvest period also tend to become more concentrated, thereby creating favorable conditions for yield formation and field management.

3.2 Effects on fruit growth and dry matter accumulation

Once fruit set has been completed, subsequent yield formation mainly depends on two key processes: whether cell division is sufficient and whether cell expansion proceeds smoothly. Comparatively speaking, GA₃ tends to play a stronger role in promoting cell elongation and fruit enlargement, whereas auxin regulators often participate in both the cell division and cell expansion stages. Studies have shown that exogenous auxin promotes fruit length increase mainly through the coordinated action of longitudinal cell division and cell expansion, while also enhancing the activity of pathways related to GA biosynthesis and cell wall biosynthesis (Zhao et al., 2025). Through combined morphological and transcriptomic analyses of long-fruited and round-fruited eggplant types, Shi et al. (2023) found that clear fruit-shape differences had already appeared before flowering, indicating that the initiation of fruit differentiation occurs earlier than traditionally assumed. By the sixth day after flowering, the fruits had entered a stage of rapid enlargement. Transcriptomic data further showed that many plant hormone-related genes were already upregulated on the day of flowering, among which SmARF1 maintained consistently high expression, suggesting that auxin signaling plays a crucial role in the early initiation of fruit development. The study also identified multiple differentially expressed genes (DEGs) related to the SUN, YABBY, and OVATE families, among which SmOVATE5 showed a negative regulatory effect, meaning that it suppressed fruit growth. These results clearly indicate that the regulatory window for fruit development has already opened before flowering and during the early flowering stage, and that the timing of regulation is relatively early.

Field data correspond well with the physiological processes described above. Treatment with GA₃ at 50 ppm not only increased the number of fruits per plant, but also raised total dry matter accumulation to 802.40 g per plant, compared with 702.90 g per plant in the control. Results from combination treatments further showed that NAA 40 ppm + GA₃ 50 ppm increased single-fruit weight to 180.48 g, while yield per plant reached 2.91 kg (Kropi, 2018). This finding points to a key issue: if only fruit number increases without a simultaneous promotion of fruit enlargement, there is a risk of producing “too many small fruits with only limited improvement in total yield.” By contrast, when NAA and GA₃ are applied together, both fruit number and single-fruit weight can be increased, thereby more effectively enhancing final yield.

3.3 Effects on yield components

In terms of yield components, the number of fruits per plant and single-fruit weight are the two core factors determining eggplant yield level. Because different studies vary considerably in cultivar type, cultivation conditions, and ecological environment, the absolute values obtained in different experiments are not suitable for simple horizontal comparison. However, the general pattern of change is relatively consistent. Research has shown that treatment with GA₃ at 50 ppm increased fruit number per plant from 11.34 to 18.56, while yield per plant rose from 1.38 kg to 1.58 kg (Kropi, 2018). Another study compared the effects of 25, 50, and 75 ppm GA₃ with several micronutrient treatments, and the results indicated that, under those experimental conditions, GA₃ at 25 ppm produced the best improvement in yield traits, especially in fruit number per plant, single-fruit weight, and yield per plant, all of which were superior to the control. This finding highlights an important fact: GA₃ does indeed have the capacity to improve eggplant yield, but its optimum concentration is not fixed; rather, it is jointly influenced by varietal characteristics and ecological conditions. In other words, 75 ppm may perform best in some experiments, whereas under other conditions 25 ppm may produce better results. This indicates that the yield-enhancing effect of GA₃ is objectively real, but its optimal dosage is clearly context-dependent (Bhattarai et al., 2021).

Evaluation of yield stability must also be considered under stress years or unfavorable environmental conditions. Field studies combining water-deficit stress with plant growth regulator treatments provide particularly representative evidence. As irrigation volume gradually decreased from the recommended level, marketable fruit yield rapidly declined to 86%, 74%, 50%, 30%, 12%, and 8% of the control level. However, after the application of regulators such as salicylic acid (SA), potassium nitrate, and thiourea, yield still increased further by 7.3% to 22.7%, while water productivity improved to 5.50–6.77 kg/m³, compared with only 5.16 kg/m³ in treatments without regulators (Wakchaure et al., 2020). These results show that the more practical value of plant growth regulators in eggplant production lies not only in further increasing yield under high-yield conditions, but more importantly in reducing the extent of yield loss under stress or difficult seasons, thereby enhancing the stability of yield formation.

4 Effects of Plant Growth Regulators on Fruit Uniformity

4.1 Regulation of fruit shape and size uniformity

In many cases, poor market appearance quality in eggplant is not caused by low average yield, but rather by excessive variation in fruit length, thickness, and curvature. Fruit shape uniformity is closely associated with hormonal balance within the plant. The study by Zhao et al. (2025) provides a relatively clear explanation of this process: exogenous auxin can regulate fruit shape-related genes such as SmOVATE, SmSUN, and IQD, and together with GA-related and cell wall biosynthesis pathways, promote longitudinal cell division and elongation. This indicates that whether a fruit is slender or straight is not determined solely by the genetic background of the cultivar; exogenous regulators may also amplify or reduce such variation. When regulation is appropriate, the distribution of fruit length tends to become more concentrated, whereas excessive regulation may induce excessive elongation or malformed fruits.

From the perspective of practical production, both NAA and GA₃ can improve fruit shape, but their regulatory emphasis differs. NAA tends to stabilize early fruit set and young fruit development, whereas GA₃ more clearly promotes fruit enlargement and longitudinal elongation. Patel et al. (2022) showed that under combined NAA and GA₃ treatment, fruit length, fruit diameter, and single-fruit weight all increased, and this improvement occurred synchronously rather than merely as simple fruit elongation accompanied by insufficient lateral growth. This phenomenon is highly meaningful in commercial production, because fruit grading is most negatively affected by unevenness such as “some fruits being too long while others are too thin”, whereas combined regulation is usually more effective than a single regulator in narrowing the range of fruit-shape variation. It should be noted that field studies directly evaluating eggplant uniformity using the coefficient of variation of fruit shape are still relatively limited. Therefore, the judgment of “uniformity” here is mainly inferred from the simultaneous improvement of fruit shape-related traits.

4.2 Regulation of fruit developmental synchrony

Another important aspect of fruit uniformity is whether the developmental rhythm remains relatively synchronized. If the flowering time within the same batch differs substantially, the subsequent fruit-setting and ripening processes usually also become clearly dispersed. The role of GA₃ in promoting earlier flowering and increasing the number of flowers per cluster already reflects a certain “synchronizing” effect. Combined with studies on heterostylous flowers in eggplant, it can be seen that when the proportion of long-styled and medium-styled flowers increases, early effective fruit set tends to become more concentrated, and the subsequent fruit developmental window also becomes more uniform (Dewangan and Jangre, 2024). This has strong practical value in production: for manual harvesting, it reduces the frequency of repeated picking rounds; for protected cultivation, it also facilitates the unified scheduling of water and fertilizer management as well as pest and disease control measures.

Uniformity of ripening is also closely related to whether “lagging fruits” appear under stress conditions. Once adverse environments such as drought, salinity stress, or low temperature cause some fruits to suspend development, clear stratification often emerges within the whole fruit batch. Although regulators such as SA, 6-BA, and EBR may not show yield-promoting effects as directly as GA₃, they can maintain chlorophyll content, relative water content, membrane stability, and antioxidant capacity, thereby reducing the risk of growth stagnation during critical developmental stages (Mady et al., 2023). Under such conditions, fruit development is more likely to remain synchronized, and the ripening period also tends to become more concentrated. Although this effect may not be highly dramatic, it has considerable practical value in improving fruit uniformity.

4.3 Quality traits related to marketability

From the perspective of market evaluation, whether eggplant fruits are “uniform” is usually reflected in four major indicators: fruit size, color, firmness, and surface defects. Existing studies on marketability consistency have largely focused on these aspects. In the field trial conducted by Wakchaure et al. (2020), marketable fruit quality was defined in terms of average fruit weight, fruit diameter, sphericity, and firmness, and the study clearly showed that these indicators were jointly affected by irrigation level and plant growth regulator treatment. Research on the genetic basis of appearance traits has also shown that peel anthocyanin composition, surface texture, and fruit surface appearance directly affect the commercial value of eggplant. This suggests that so-called “uniformity” is not an abstract concept, but one that is ultimately reflected in grading standards and market price.

Among these traits, peel color is especially critical under protected winter production conditions. Weak light environments easily lead to problems such as uneven pigmentation, whitening, and blotchy coloration. Luo et al. (2023) reported that low light reduces the visual quality and commercial value of eggplant peel, whereas materials that can maintain good coloration under low-light conditions are particularly valuable for commercial production. Further studies have shown that peel color at different developmental stages is jointly determined by a series of metabolites and regulatory genes. In other words, if plant growth regulators can stabilize the overall physiological status of the plant, or maintain anthocyanin accumulation during the later stages of fruit development in combination with cultivar characteristics, then improved uniformity may ultimately be expressed as more consistent fruit coloration and more stable appearance quality.

5 Physiological and Molecular Mechanisms

5.1 Hormonal signaling pathways

Plant growth regulators (PGRs) influence eggplant yield and uniformity by reshaping hormonal networks, carbon allocation, and stress responses that control fruit set, growth, and stability. Auxin and gibberellin (GA) act as primary drivers of fruit initiation, cell division and expansion, often sufficient to induce parthenocarpy when applied exogenously (Fenn and Giovannoni, 2020; He and Yamamuro, 2022; Su et al., 2025). Crosstalk occurs through direct interaction between auxin‐responsive ARF/IAA proteins and GA repressor DELLA proteins, which co-regulate genes for hormone metabolism and fruit growth, integrating auxin and GA signals into a unified control of fruit set and early enlargement (Hu et al., 2018; He and Yamamuro, 2022). Cytokinin cooperates with auxin and GA to enhance parthenocarpic fruit set in cucumber, with high cytokinin and GA but low abscisic acid (ABA) characterizing highly parthenocarpic genotypes (Zhao et al., 2025).

These hormones coordinate transcriptional programs: auxin–GA complexes modulate feedback genes in their own pathways and activate fruit growth-related genes, while cytokinin-responsive type-B response regulators and auxin-regulated ARFs mediate broad transcriptional reprogramming during fruit development (Fenn and Giovannoni, 2020). Shifts in ABA and ethylene further remodel gene expression at maturation and under stress, influencing fruit size and development patterns (Waadt et al., 2022; Thilakarathne et al., 2025).

5.2 Metabolic and cellular processes

PGRs indirectly govern carbohydrate allocation by altering sink strength in developing fruits. Sugar transporters and sugar–hormone integration ensure that sink organs such as fruits receive sufficient carbohydrates, with sugars acting as both substrates and signals that interact with auxin and cytokinin pathways (Wingler and Henriques, 2022; Guo et al., 2023). Under carbon restriction, marked declines in cytokinins and downregulation of cytokinin biosynthesis genes coincide with reduced expansin expression and fruit weight, showing that cytokinins drive not only cell division but also cell wall loosening and expansion (Nardozza et al., 2020).

At the cellular level, auxin and GA jointly promote cell division and subsequent expansion in early fruit development across multiple species, while cytokinins modulate both proliferation and elongation through expansin-linked cell wall relaxation (He and Yamamuro, 2022). Sugar-auxin crosstalk further integrates metabolic status with cell cycle activity, chromatin state and auxin-regulated gene expression, ensuring that cell division and differentiation proceed only when carbohydrate supply is adequate (Sabagh et al., 2022).

5.3 Stress responses and hormonal regulation

Abiotic stresses such as drought, salinity, heat and flooding disrupt endogenous hormone balances, compromising reproductive development and yield stability (Waadt et al., 2022; Baral et al., 2025). PGRs—endogenous or applied—mitigate these effects by reconfiguring hormonal networks: ABA, salicylic acid, ethylene and jasmonates primarily activate defense and osmotic adjustment, while auxin, GA and cytokinins maintain growth, with extensive crosstalk between stress and growth pathways (Sabagh et al., 2022; Baral et al., 2025).

GA-inhibiting triazoles such as paclobutrazol alter gibberellin, ABA and cytokinin levels, reducing excessive vegetative growth, enhancing carbohydrate accumulation, improving water status and strengthening tolerance to abiotic stress while supporting fruit number and quality (Desta and Amare, 2021; Sabagh et al., 2021). Under low sugar availability or environmental stress, sugar signaling promotes ABA and ethylene accumulation and disrupts auxin transport, driving fruit abscission; once sugar status improves, rising cytokinin and GA levels restore cell division and expansion and stabilize fruit set (Waadt et al., 2022; Zhao et al., 2025). Thus, PGR-mediated adjustment of hormonal networks links stress physiology directly to yield stability and fruit uniformity.

6 Application Strategies of PGRs in Eggplant Production

6.1 Application methods and timing

Plant growth regulators can substantially improve eggplant yield, stress tolerance, and fruit quality, but their benefits depend strongly on application method, timing, and dose. Foliar spraying is the most common method in eggplant, enabling rapid absorption and relatively precise timing around key stages such as vegetative growth, flowering, and early fruit set (Figure 2). Foliar application of α-tocopherol, ZnO nanoparticles, salicylic acid, potassium nitrate, thiourea, and biostimulants (garlic extract, vermicompost tea, yeast extract) enhanced growth, water status, antioxidant activity, and yield under both optimal and drought conditions (Semida et al., 2021; Akram et al., 2023). Foliar PGRs are usually applied with surfactants to improve cuticular penetration and uniform coverage (Dick and VanderWeide, 2025).

Root-zone or substrate applications are preferred for some systemic regulators such as paclobutrazol, which is more effective when applied to the growth medium than as a spray because of longer contact and uptake time (Desta and Amare, 2021). Seed priming with PGRs (e.g., α-tocopherol, guvermectin) can enhance early vigor and later yield response, representing a complementary strategy to foliar use (Liu et al., 2022; Akram et al., 2023).

Optimal timing is crop- and regulator-specific. In fruit crops, foliar PGRs applied at full bloom or shortly after flowering markedly influence fruit set, size, and quality (Aryal and Alférez, 2025; Baldissera et al., 2025). In eggplant, applications at vegetative and pre- or post-transplant stages, as well as around flowering and early fruit set, were most effective for stimulating canopy growth, maintaining water status, and improving yield and fruit traits under water stress (Ali et al., 2019; Wakchaure et al., 2020). Repeated applications may increase responses but excessive frequency can cause growth inhibition or oxidative damage.

6.2 Dosage optimization and combination use

PGRs exhibit clear dose–response relationships: low to moderate concentrations often stimulate growth and yield, whereas high doses can induce phytotoxicity or yield decline (Ali et al., 2019; Semida et al., 2021; Akram et al., 2023). In eggplant, moderate foliar levels of α-tocopherol or ZnO nanoparticles maximized growth and fruit yield under drought, while higher doses or over-frequent botanical sprays increased lipid peroxidation and reduced growth (Akram et al., 2023). Similar patterns are reported for auxins, gibberellins, and cytokinins in cucurbits and tree fruits, where recommended ppm ranges are critical to avoid negative effects on fruit quality or return bloom (Sabir et al., 2021; Baldissera et al., 2025).

Combination treatments can produce synergistic effects by targeting complementary hormonal pathways. In cucumber, combined auxin and gibberellin improved vegetative growth and fruit yield more than either alone (Gosai et al., 2020). In apple, combinations of cytokinin (BA) and auxin (NAA) increased yield and the proportion of large fruits beyond single applications (Baldissera et al., 2025). In eggplant under deficit irrigation, mixtures of salicylic acid, potassium nitrate, thiourea, or commercial biostimulants improved canopy traits, water productivity, and fruit quality compared with untreated controls, with some regulators more effective under specific stress intensities (Wakchaure et al., 2020). These findings support careful factorial trials in eggplant to identify synergistic PGR combinations and avoid antagonistic or redundant effects.

6.3 Safety and environmental considerations

Despite agronomic benefits, PGR use raises concerns about residues in edible tissues and broader environmental impacts. Surveys in vegetables have detected multiple endogenous-type PGR residues (auxins, gibberellins, cytokinins) in a high proportion of market samples, with gibberellins sometimes exceeding maximum residue limits set by European, US, and Japanese regulations (Le et al., 2020; Zhou et al., 2025). Reviews highlight mammalian toxicities (hepatic, renal, reproductive, carcinogenic) associated with specific synthetic PGRs, emphasizing the importance of dose-response analysis and rigorous risk assessment (Zhou et al., 2025).

In soils, PGRs undergo adsorption, desorption, hydrolysis, photolysis, and microbial degradation, and their persistence and mobility determine risks to non-target organisms and groundwater (Chen et al., 2022). In some production systems, misuse and overuse have led to declining product quality and dual contamination of crops and cultivation environments, prompting calls for stricter registration, residue limits, and monitoring (Zhang et al., 2020; Zhou et al., 2025).

Sustainable PGR use in eggplant should therefore prioritize: adherence to registered products and label doses; minimal effective application frequency; preference for lower-risk or biogenic regulators and biostimulants where possible; and integration with cultural and irrigation management to reduce dependence on chemical inputs (Akram et al., 2023; Liu et al., 2024). Development of high-throughput residue testing and clearer maximum residue limits can support safer adoption while maintaining the yield and uniformity benefits sought in commercial eggplant production (Le et al., 2020; Zhou et al., 2025).

7 Concluding Remarks

A synthesis of the available studies shows that plant growth regulators can indeed improve eggplant yield and, to some extent, enhance fruit uniformity, although different types of regulators differ in their primary functions. For example, GA₃ is more effective at improving flowering quality, increasing fruit number, and promoting fruit enlargement; NAA is more beneficial for stabilizing fruit set, reducing flower and fruit drop, and supporting fruit shape formation; whereas 6-BA, SA, and EBR are better suited to maintaining growth continuity under stress conditions and reducing developmental differences among fruit batches. For commercial production, the most practical benefits lie in a more stable fruiting process, more uniform fruit shape, and a more concentrated ripening period. Mechanistically, these effects are not independent of one another. Auxins, gibberellins, and cytokinins collectively participate in fruit initiation, cell division, and enlargement, while central carbon metabolism, cell wall synthesis, and stress-related antioxidant systems determine whether these hormonal signals can ultimately be translated into successful fruit elongation and stable fruit development. Precisely because this regulatory network is complex, the same regulator often shows considerable variation in performance across different cultivars, seasons, and protected cultivation conditions.

At present, the most prominent issue is whether the effects of plant growth regulators can be reproduced consistently. On the one hand, existing field trials on plant growth regulators in eggplant are still strongly region-specific, with substantial differences in cultivar type, seasonal conditions, and cultivation management background, making it difficult to apply optimal dosages and treatment timing directly across production systems. On the other hand, studies on fruit uniformity are clearly fewer than those on yield. Many papers report only fruit number, single-fruit weight, and total yield, while giving much less attention to indicators such as coefficients of variation, marketable grading proportion, and ripening synchrony index. As a result, assessments of consistency often remain at the level of indirect inference. At the molecular level, although a number of key regulatory factors have been identified, including SmARF, SmRR, SmOVATE5, and SmMYB113, there are still relatively few studies that fully connect the chain from “exogenous regulator–signal transduction-fruit shape uniformity-commercial grading.” In particular, direct field evidence for the roles of cytokinins, ethylene inhibitors, and brassinosteroids in regulating eggplant uniformity remains limited, indicating that this field still has substantial room for expansion.

Future research may be advanced from three main directions. First, greater emphasis should be placed on the study of hormonal interactions, rather than continuing to focus primarily on the effects of single compounds. Existing studies have already shown certain advantages of combined NAA and GA₃ treatment, and future work could incorporate auxins, gibberellins, cytokinins, and brassinosteroids into a unified temporal framework for systematic investigation. Second, the evaluation system for fruit uniformity should be further improved. Instead of judging treatment effects only by mean values, comprehensive assessment should include indices such as fruit length variation, dispersion of single-fruit weight, ripening concentration, and commercial grading rate. Third, the application of plant growth regulators should be increasingly integrated with precision regulation and smart agriculture technologies. As fruit recognition and counting technologies in greenhouse production continue to mature, it should become entirely feasible to further combine flowering-stage recognition, environmental monitoring, and site-specific spraying technologies, thereby shifting regulator application from traditional experience-based operation to precise intervention based on key developmental stages.

Acknowledgments

The authors would like to express their sincere gratitude to Ms. Zhang for her assistance in organizing the literature materials. The authors also extend special thanks to the two anonymous peer reviewers for their comprehensive evaluation 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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