1. Introduction
Bitter gourd (
Momordica charantia L.) is a tropical vine, known variously as bitter apple, bitter melon, or balsam pear, renowned for its dual significance as both a culinary delicacy and a potent medicinal plant. Its rich history spans traditional cuisines and healing practices in India, China, and Southeast Asia, where it has earned recognition for its diverse health benefits and nutritional value [
1]. Abundant in bioactive compounds distributed throughout its roots, leaves, vines, and fruits; bitter gourd stands as a reservoir of medicinally valuable constituents. These compounds, replete with health-promoting attributes, include antidiabetic, anticancer, anti-inflammatory, and antioxidant properties [
2,
3]. Notably, bitter gourd seeds have demonstrated efficacy in addressing conditions such as ulcers, liver and spleen disorders, high cholesterol, and wound healing [
4]. Given this remarkable profile, the cultivation of bitter gourd has garnered significant interest from both farmers and consumers alike, driven by the desire to optimize its growth, yield, and overall quality.
The desire to maximize bitter gourd’s growth, yield, and overall quality has led to the consideration of various strategies. Among these, amino acids (AAs) have emerged as promising candidates, given their pivotal roles in plant metabolism, physiology, and stress responses [
5]. AAs, as essential organic compounds, play integral roles in diverse biological functions, including hormone synthesis, enzymatic reactions, and nutrient transport [
6]. They serve as crucial precursors to hormone synthesis, act as signaling molecules in physiological pathways, and regulate processes such as nitrogen fixation, root development, antioxidant metabolism, and nutrient uptake efficiency [
6,
7,
8].
Although earlier research has emphasized the beneficial effects of AAs on conventional growth parameters, such as plant height, biomass, leaf area, and yield in various crops, such as ‘Washington’ navel orange (
Citrus sinensis) and wheat (
Triticum aestivum) [
9,
10], there exists a significant research gap. This gap pertains to the less-explored aspects of AAs’ influence, particularly concerning quality parameters. This involves assessing the extent to which AAs affect not only plant growth but also key quality attributes, including antioxidant activity, phenolic components, and flavonoids.
Phenolic components hold a significant place among the bioactive compounds contributing to the health-promoting attributes of bitter gourd. These phenolic compounds, recognized as key secondary metabolites in plants, are esteemed for their potent antioxidant properties and their potential to counteract oxidative stress-related ailments [
11,
12]. Additionally, bitter gourd is known to contain appreciable quantities of phenolic components including gallic acid, gentisic acid, chlorogenic acid, tannic acid, and tannins, all of which bear pharmacological significance [
13,
14]. Furthermore, it is rich in flavonoids, such as catechin and epicatechin, which is a class of bioactive compounds associated with various health benefits [
15,
16].
In light of these considerations, our study focuses on exploring the effects of foliar spray treatments with three specific AAs: Tryptophan (Trp); glutamine (Gln); and phenylalanine (Phe). The selection of these AAs is underpinned by their established roles in fundamental plant processes, their potential to enhance growth, their role as precursors of many secondary metabolites, and their contribution to stress tolerance [
17]. Trp serves as the foundation for the production of auxin, phytoalexins, alkaloids, and indole glucosinolates [
18]. Gln plays a pivotal role in the metabolism of AAs in plants. It is classified as a proteinogenic AA and serves as an N transporter and NH
3 carrier. Its roles encompass the synthesis of numerous molecules, functioning as sources of both energy and carbon, along with nitrogen backbones. This multifaceted functionality supports cellular homeostasis and the accumulation of biomass [
19]. As noted by Croteau et al. [
20], among these AAs, Phe, recognized as a pivotal component in plant growth and development, serves as a foundational component for an array of compounds. These include phenylpropanoids, flavonoids, anthocyanins, lignin, tannins, and salicylate, which collectively play indispensable roles in fostering plant growth, supporting reproduction, and fortifying the plant’s resilience against both abiotic and biotic stresses [
21]. Notably, the utilization of these compounds resulted in a substantial rise in the levels of these constituents compared to plants that were not subjected to the foliar spray treatment.
The influence of foliar applications of Trp, Gln, and Phe on bitter gourd’s growth, yield, and antioxidant activity has not been extensively explored. To bridge this knowledge gap, we conducted an extensive field experiment spanning two consecutive seasons. This comprehensive approach allowed us to evaluate the influence of these AAs on diverse growth parameters, including plant height, fruit length, fresh and dry plant weight, fruit yield, and branch numbers. Additionally, we explored their potential effects on antioxidant activity, total phenolic content (TPC), and total flavonoid content (TFC) in bitter gourd plants.
4. Discussion
Increasing the production of bioactive compounds in bitter gourd holds significant implications for enhancing its medicinal and nutraceutical properties. A potential approach to achieve this is by utilizing the potential of AAs, which are recognized for their role in the biosynthesis of secondary metabolites, including phenolic components and flavonoids, in various plant species.
In this study, the application of all AA treatments resulted in a significant enhancement of growth, yield, and antioxidant activity compared to the untreated plants, as indicated by our findings. Notably, the most pronounced impact on plant height was observed when Trp was applied at concentrations of 300 mg/L, followed by Gln at 300 mg/L. This positive influence of AAs aligns with similar observations documented in numerous plant species [
37,
38,
39].
Trp serves as a key precursor to the plant hormone auxin, indole acetic acid. The application of Trp at suitable concentrations can distinctly impact plant growth due to the slow and gradual release of indole acetic acid from L-Trp [
40]. Trp plays various roles in plants, including acting as an osmolyte, regulating ion transport, modulating stomatal opening, and contributing to defense mechanisms [
41]. Furthermore, plant roots can absorb Trp-derived auxins generated by rhizosphere microorganisms and transport them to the shoot, initiating physiological reactions [
42].
It is essential to acknowledge that the effect of AAs on plant growth might vary depending on factors such as the plant species, the dosage and time of application, and different climatic circumstances [
43,
44]. Nevertheless, the consistently positive outcomes observed across various studies suggest the potential of AAs as regulators of plant growth, warranting further research to comprehensively unravel their mechanisms of action and practical applications in agriculture. Spraying Trp, Gln, and Phe has been observed to have a stimulatory effect on vegetative growth in various plants, especially at high doses.
Gln plays an indispensable and multifaceted role in plant physiology and actively participates in numerous metabolic processes, especially those related to nitrogen assimilation [
45,
46]. In addition, Gln, along with arginine and asparagine, serves as one of the key AAs involved in a wide array of biochemical and metabolic reactions within plant systems. These reactions encompass tasks such as detoxification of toxins, neutralization of H
+ ions generated in ammonium-fed plants, and conferring remarkable stress tolerance [
47,
48,
49]. The outcomes of our study further underscore the pivotal role of Gln in promoting plant growth and development. This is evident in the significant increases observed in the fresh weight and first fruit diameter in bitter gourd plants sprayed with Gln at 300 followed by Trp 300 mg/L when compared to the control group.
Furthermore, the stimulating effects observed in our study, particularly at high concentrations, align with the stimulatory outcomes of foliar applications of Trp, Gln, and Phe on fruit-growth characteristics. This pattern of positive influence at higher concentrations corresponds with earlier findings in various plant species, such as
Hibiscus sabdariffa [
50],
Silybum marianum [
51], and
Lupines termis [
52], which supports our findings. The coherence between these prior studies and our results provides robust support for our findings.
The results of the present study revealed that the highest concentrations of TPC were achieved with elevated levels of both Phe 300 mg/L and Gln 300 mg/L. Phenolic compounds are recognized for their antioxidative characteristics and have been linked to a range of health advantages, including their ability to combat diseases related to oxidative stress. The observed increase in TPC suggests that the foliar application of Phe and Gln can contribute to enhancing the nutritional and health-promoting properties of bitter gourd. These findings are consistent with previous studies, such as the work conducted by Amira et al. [
47], which also underscore the effectiveness of certain amino acids in promoting the synthesis of phenolic compounds. The outcomes of our study demonstrated that the application of all the employed AAs resulted in TFC compared to the unsprayed plants. Notably, Phe at a concentration of 300 mg/L displayed the highest levels of total flavonoids. This finding is consistent with a prior study by Klimek et al., which identified Phe as the most effective AA for enhancing total flavonoid synthesis, which is in alignment with our own results [
53]. Previous research has also reported the stimulatory impact of Phe on plant growth and the production of secondary metabolite [
49], further supporting our findings. This suggests that Phe treatment at a specific concentration can positively influence flavonoid production in bitter gourd plants, followed by Glu treatment. These compounds play significant roles in various aspects of plant biology, including growth, development, and defense mechanisms against both biotic and abiotic stressors [
21]. Our results are consistent with earlier research that has documented the promotive impact of Phe on both plant growth and secondary metabolite production [
51]. The stimulatory influence of Phe on flavonoid biosynthesis, like in
Ocimum tenuiflorum and
Mentha piperita plants, has been documented by studies conducted by Jacob and Thomas [
54] and Roy et al. [
55]. Among the flavonoid compounds, quersestin was present in the highest quantity, while rutin was found in the lowest amount. These results align with a previous study by Alper and Cennet [
56], highlighting the interplay between specific plant varieties and the application of AAs through spraying in modulating phenol synthesis [
43].
Studies have consistently shown that the application of AAs through foliar spraying at various concentrations can have a notable and favorable impact on the overall quantity of phenols, essential oil content, and their composition in different plant species. For instance, in the case of
Ocimum basilicum plants, previous research has demonstrated that foliar spraying with AAs led to an increase in phenol content and the essential oil yield, along with modifications in their composition [
27]. In another study by Farhan et al. [
28] the application of Phe through foliar spraying at a concentration of 150 mg/L on
Anethum graveolens L. resulted in a notable enhancement in herb-oil yield and increased TFC compared to untreated plants. Similarly, Poorghadir et al. [
57], found that foliar spraying with Phe at a concentration of 1.5 g/L had the highest efficacy in augmenting the essential oil content, particularly carvacrol and gamma-terpinene, in
Satureja hortensis plants. Remarkably, the application of these compounds significantly elevated the levels of these constituents compared to untreated plants. These studies collectively underline the positive influence of AAs on phenol content, essential oil yield, and composition, offering valuable insights for improving the phytochemical profile of various plant species.
Interestingly, our results show variations in the quantities of different compounds based on the AA treatments. Notably, the application of Phe at 300 mg/L led to significant alterations in the chemical composition of the extract, resulting in increased quantities of certain phenolic and flavonoid compounds. This suggests that the foliar application of AAs can effectively modulate the synthesis of specific compounds in bitter gourd, potentially contributing to its health-promoting attributes. The implications of our findings hold significance for both agricultural practices and human health. The ability to enhance the TPC, TFC, and antioxidant properties of bitter gourd through foliar spray treatments with AAs offers promising prospects for sustainable agriculture. These treatments have the potential to improve crop yield and quality, thereby contributing to the overall productivity of bitter gourd cultivation. Furthermore, the enriched chemical composition of bitter gourd can enhance its nutritional value and health-promoting properties, further emphasizing the importance of the present study.
5. Conclusions
The present study sheds light on the potential impact of AA foliar applications on the growth, yield, and antioxidant properties of the bitter gourd plant. Recognized for its pharmacological benefits, bitter gourd has shown promise for further enhancement through strategic AAs use. Key findings highlight that Trp at 300 mg/L had the most substantial effect on increasing plant length, with Phe at the same concentration yielding notable results. Additionally, remarkable improvements in various yield parameters, including fresh and dry plant weights, fresh weight of the first fruit, fruit number per plant, fresh weight of fruits per plant, and total fruit yield per hectare, were observed with Trp at 300 mg/L and Gln at 300 mg/L, emphasizing their practical significance for yield enhancement. Furthermore, Phe, at a dosage of 300 mg/L, exhibited the highest levels of TPC and TFC, along with potent scavenging activity against 2,2-diphenyl-1-picrylhydrazyl, underscoring its potential as an inducer of antioxidant compounds. These outcomes reveal the promising potential of Trp, Gln, and Phe foliar sprays to optimize bitter gourd growth, yield, and the concentration of specific antioxidant compounds. The practical implications of our research are noteworthy, offering a strategic approach for enhancing bitter gourd cultivation. This has far-reaching implications for sustainable agricultural practices and contributes to the agricultural and pharmaceutical sectors by elevating both the nutritional and medicinal attributes of this remarkable crop.