Introduction
Biostimulants such as fulvic acid (FA) and Amino acids (AAs) are natural or synthetic substances that enhance plant growth, yield, and stress tolerance by stimulating physiological processes without acting as conventional nutrients or pesticides. In agriculture, their primary purpose is to improve nutrient-use efficiency, enhance crop quality, and mitigate the effects of abiotic stresses, offering an innovative and sustainable approach to increasing productivity while reducing environmental impact.
Humic substances, among the most prominent biostimulants, are organic compounds naturally formed through the decomposition of plant and animal matter. They are categorized into humin, humic acid, and fulvic acid (FA), standing out due to their lower molecular weight and abundance of oxygen-rich functional groups. These characteristics enable fulvic acids to pass through biological membrane micropores, unlike humic acids, which have larger molecular weights. Fulvic acids also exhibit higher total acidity, a significant number of carboxyl groups, and superior adsorption and cation-exchange capacities, allowing them to act as natural chelators that facilitate the mobilization and transport of micronutrients and pass through cell membranes. They support plant growth by enhancing photosynthesis, secretion of growth hormones, nutrient absorption, availability of pH, and boosting resistance to biotic and abiotic stresses. Fulvic acids are a mixture of weak aliphatic and aromatic organic acids that are water-soluble across a broad pH range, including acidic, neutral, and alkaline conditions. The composition of fulvic acid is highly variable and is recognized as one of the most active carbon-based chelating agents.
Amino acids (AAs), often considered the fundamental building blocks of proteins, are nitrogen-containing compounds essential for protein synthesis. They play a crucial role in metabolic processes by providing key enzymes that stimulate cell growth. They are widely recognized as effective biostimulants that significantly enhance plant growth and development. Extensive research has demonstrated their multifaceted benefits across various plant species. AAs have been shown to improve fertilizer assimilation, optimize nutrient and water uptake, and enhance photosynthetic efficiency in numerous vegetable crops. These effects collectively contribute to increased flower production, improved fruit sets, and higher fruit yield, highlighting their vital role in boosting agricultural productivity.
Results
Iceberg Lettuce Yield
The application of biostimulants significantly influenced iceberg lettuce yield in the first trial (Figure 1). The highest yield was observed with the application of vermicompost at 2 mL (VC 2 mL), reaching 12.58 kg m-2, corresponding to a 13.96% increase compared to the control treatment (11.04 kg m-2). Fulvic acid at 40 ppm (Fulvic acid 40) also resulted in a notable yield increase, achieving 12.12 kg m-2, corresponding to a 9.78% increase compared to the control. However, no significant difference was observed between Fulvic acid 40 and VC 2 mL. Other treatments, including FA 80, amino acid applications (AA 75 and Amino acid 100), and vermicompost at 1 mL (VC 1 mL), did not significantly increase compared to the control. VC 1 mL exhibited a 4.35% increase, which, although higher than the control, was not statistically significant. In the first experiment, VC 2 mL and Fulvic acid 40 demonstrated the most promising results for enhancing iceberg lettuce yield under experimental conditions.
The application of biostimulants significantly influenced iceberg lettuce yield during the second trial (Figure 2). Among the treatments, Fulvic acid 40 + VC 2 mL exhibited the highest total yield, representing a 17.94% increase compared to the control. Similarly, Fulvic acid 40 + Amino acid 100 achieved a yield corresponding to a 17.11% increase relative to the control. The yields observed in Fulvic acid 40 + Amino acid 100 (17.16 kg m-2) and Fulvic acid 40 + Amino acid 100 + VC 2 mL (17.04 kg m-2) were statistically similar. The combination treatment of Amino acid 100 + VC 2 mL produced a moderate yield, reflecting a 9.42% increase over the control. In contrast, the treatment Fulvic acid 40 + Amino acid 100 + VC 2 mL resulted in the lowest yield, indicating a 20.14% reduction compared to the control (14.55 kg m-2). Other treatments, such as Amino acid 100 + VC 2 mL, yielded 15.92 kg m-2 and significantly differed from the control and the higher-yielding treatments.
Iceberg Lettuce Growth Parameters in First Experiment
Plant Height: The Fulvic acid 80 led to a 6.3% increase in plant height compared to the control, with Fulvic acid 40 showing a 5.6% improvement. VC 2 mL also contributed positively, achieving a 2.8% increase, indicating the beneficial effects of fulvic acid and vermicompost on plant height (Table 1).
| Treatments | Plant Height (cm) | Perimeter (cm) | Head Firmness (kg m3) | Root Weight (g) |
| Control | 38.85 abc | 44.85 a | 8.24 ab | 45.40 |
| FA 40 | 41.05 a | 45.10 a | 6.88 abc | 50.00 |
| FA 80 | 41.30 a | 45.05 a | 7.73 ab | 48.50 |
| AA 75 | 37.30 bc | 40.85 b | 8.61 a | 44.70 |
| AA 100 | 33.60 d | 43.55 ab | 6.75 bc | 55.40 |
| VC 1 mL | 35.55 cd | 41.75 ab | 5.82 c | 54.60 |
| VC 2 mL | 39.95 ab | 44.00 ab | 6.67 bc | 57.20 |
Perimeter Dimension: Fulvic acid 40 exhibited a slight advantage with a 0.6% increase over the control, and Fulvic acid 80 displayed a 0.4% improvement.
Head Firmness: Amino acid 75 was the only one to increase head firmness, achieving a 4.5% improvement compared to the control. All other treatments were lower than the control, highlighting Amino acid 75 as the most effective parameter enhancement.
Root Weight: VC 2 mL achieved a notable 26% increase in root weight over the control, while Amino acid 100 provided a 22% improvement and VC 1 mL yielded a 20.2% increase. These results underscore the efficacy of vermicompost and amino acid in promoting root biomass accumulation.
Leaf Fresh Weight: The VC 2 mL resulted in the highest leaf fresh weight, with a 14% increase compared to the control. Fulvic acid 40 also showed a notable improvement, with a 10% increase, while Amino acid 100 demonstrated a 3% rise (Table 2). These results suggest that vermicompost and fulvic acid applications positively impacted leaf fresh weight.
| Treatments | Leaf Fresh Weight (g Plant−1) | Leaf Area (cm2 Plant−1) | Number of Leaves per Plant | Dry Matter (%) | Chlorophyll (SPAD) |
| Control | 372 d | 4793 b | 33.15 c | 7.31 | 16.44 ab |
| FA 40 | 409 b | 5952 ab | 34.25 bc | 7.55 | 17.86 ab |
| FA 80 | 372 d | 5791 ab | 34.80 bc | 7.63 | 17.74 ab |
| AA 75 | 371 d | 7585 a | 33.05 c | 7.41 | 15.04 b |
| AA 100 | 383 c | 6748 ab | 39.85 a | 7.68 | 21.61 a |
| VC 1 mL | 388 c | 6071 ab | 36.95 ab | 7.72 | 17.53 ab |
| VC 2 mL | 424 a | 692 ab | 37.15 ab | 7.83 | 18.46 ab |
Leaf Area: Amino acid 75 achieved the largest leaf area, showing a substantial 58% increase over the control. Amino acid 100 followed with a 41% increase, and VC 2 mL exhibited a 45% improvement.
Number of Leaves: Amino acid 100 application led to the highest leaf count, with a 20% increase relative to the control. VC 2 mL and VC 1 mL also showed positive impacts, with increases of 12% and 11.5%, respectively. This suggests that amino acids and vermicompost applications effectively enhance leaf production.
Dry Matter: In comparison to the control, dry matter content increased by 7.1%, 5.6%, 5.1%, 4.4%, 3.3%, and 1.4%, respectively, for the VC 2 mL, VC 1 mL, Amino acid 100, Fulvic acid 80, Fulvic acid 40, and Amino acid 75 treatments, although these increases were not statistically significant.
Chlorophyll Content (SPAD): The Amino acid 100 demonstrated the highest chlorophyll content, increasing SPAD values by 31% relative to the control. VC 2 mL and Fulvic acid 40 followed with 12% and 9% improvements, respectively, indicating that amino acid and vermicompost contributed positively to chlorophyll concentration.
Iceberg Lettuce Growth Parameters in Second Experiment
Plant height: The combined Fulvic acid 40 + Amino acid 100 yielded the highest plant height, with an increase of 6.4% compared to the control. Fulvic acid 40 + VC 2 mL was closely followed, achieving a 6.2% increase, and Amino acid 100 + VC 2 mL showed a 1.3% improvement over the control. (Table 3).
| Treatments | Plant Height (cm) | Perimeter (cm) | Head Firmness (kg m-3) | Root Weight (g) |
| Control | 29.42 b | 38.85 bc | 9.47 b | 80.70 b |
| FA 40 + AA 100 | 31.30 a | 45.45 a | 8.96 b | 80.70 b |
| FA 40 + VC 2 mL | 31.25 a | 42.15 ab | 8.75 b | 95.00 a |
| AA 100 + VC 2 mL | 29.80 ab | 42.80 ab | 8.85 b | 87.00 ab |
| FA 40 + AA 100 + VC 2 mL | 29.20 b | 37.55 c | 22.17 a | 72.10 c |
Perimeter: Fulvic acid 40 + Amino acid 100 demonstrated a 17% increase in perimeter dimension over the control, while Amino acid 100 + VC 2 mL achieved a 10.2% increase. The Fulvic acid 40 + VC 2 mL combination showed an 8.5% enhancement compared to the control.
Head firmness: In terms of head firmness, the Fulvic acid 40 + Amino acid 100 + VC 2 mL substantially increased, providing a 134.2% improvement over the control, making it the most effective treatment in this parameter. Other treatments, including Fulvic acid 40 + Amino acid 100, Fulvic acid 40 + VC 2 mL, and Amino acid 100 + VC 2 mL, demonstrated values below the control.
Root weight: Fulvic acid 40 + VC 2 mL yielded the highest root weight, with a 33.9% increase compared to the control. The Amino acid 100 + VC 2 mL combination also showed a substantial increase of 22.7%, and Fulvic acid 40 + Amino acid 100 demonstrated a 13.8% enhancement.
Leaf fresh weight: In the second trial, the combinations led to a higher increase in leaf fresh weight than in the first trial (Table 4). Specifically, the Fulvic acid 40 + VC 2 mL and Fulvic acid 40 + Amino acid 100 produced 17–18% higher leaf fresh weight than the control.
| Treatments | Leaf Fresh Weight (g Plant−1) | Leaf Area (cm2 Plant−1) | Number of Leaves per Plant | Dry Matter (%) | Chlorophyll (SPAD) |
| Control | 490 b | 10,943 bc | 33.90 ab | 9.92 c | 20.45 |
| FA 40 + AA 100 | 574 a | 12,005 b | 35.70 ab | 11.16 bc | 19.67 |
| FA 40 + VC 2 mL | 579 a | 13,580 a | 35.60 ab | 11.04 bc | 17.80 |
| AA 100 + VC 2 mL | 537 ab | 10,492 c | 37.15 a | 13.48 ab | 20.02 |
| FA 40 + AA 100 + VC 2 mL | 392 c | 10,308 c | 32.20 b | 14.26 a | 19.97 |
Leaf area: The Fulvic acid 40 + VC 2 ml treatment increased leaf area by 24.1%, while the Fulvic acid 40 + Amino acid 100 treatment resulted in a 9.7% increase compared to the control.
Number of leaves: The Amino acid 100 + VC 2 mL combination yielded the highest leaf count, with a 9.6% increase compared to the control. This increase is more significant than observed in the first trial, suggesting that combinations were more supportive of leaf production than single biostimulant applications.
Dry matter: For dry matter content, the most substantial increases were observed in the Fulvic acid 40 + Amino acid 100 + VC 2 mL and Amino acid 100 + VC 2 mL treatments, with enhancements of 43.8% and 35.9%, respectively, compared to the control.
Chlorophyll content (SPAD): Chlorophyll content did not show statistically significant differences among treatments.
Macro-Nutrient Profiles of Iceberg Lettuce
In the first trial of iceberg lettuce cultivation under soilless culture, significant variations in macro-nutrient concentrations were observed among treatments with different biostimulants (p < 0.05) (Table 5). Nitrogen concentration was highest in VC 2 mL, showing a substantial increase of 37.4% compared to the control. VC 1 mL and Fulvic acid 40 followed, with increases of 23.0% and 15.7%, respectively. Phosphorus levels reached their maximum with VC 2 mL, surpassing the control by 19.2%. Fulvic acid 80 showed a minor increase of 11.5%, while Fulvic acid 40 minimally contributed to phosphorus enhancement. Potassium concentration peaked with Fulvic acid 80, achieving a rise of 57.6% over the control. Strong performances were also observed with VC 2 mL and Amino acid 75, which recorded increases of 45.4% and 42.4%, respectively. The highest calcium content was recorded with Fulvic acid 40, representing an improvement of 18.0% relative to the control. Fulvic acid 80 followed closely with a 15.0% increase, and Amino acid 75 contributed moderately to calcium enhancement. Magnesium concentrations exhibited the most significant increase with Amino acid 75, rising by 144.3% compared to the control. Fulvic acid 80 and Fulvic acid 40 also demonstrated notable improvements, with increases of 102.5% and 69.6%, respectively.
| Treatments | N | P | K | Ca | Mg |
| Control | 2.30 b | 0.26 b | 2.71 c | 1.94 ab | 0.79 c |
| FA 40 | 2.66 ab | 0.20 c | 3.34 abc | 2.29 a | 1.34 b |
| FA 80 | 2.57 ab | 0.23 bc | 4.27 a | 2.23 a | 1.60 b |
| AA 75 | 2.36 b | 0.23 bc | 3.86 ab | 1.68 b | 1.93 a |
| AA 100 | 2.37 b | 0.24 bc | 3.00 bc | 1.67 b | 0.40 d |
| VC 1 mL | 2.83 ab | 0.25 bc | 3.53 abc | 1.50 bc | 0.34 d |
| VC 2 mL | 3.16 a | 0.31 a | 3.94 ab | 1.10 c | 0.53 cd |
In the second trial of lettuce cultivation under soilless conditions, the macronutrient concentrations showed significant differences among treatments, with some combinations outperforming others (Table 6). Nitrogen concentration reached its highest levels in Fulvic acid 40 + VC 2 mL, demonstrating a considerable improvement of 37.4% over the control. Fulvic acid 40 + Amino acid 100 and Amino acid 100 + VC 2 mL followed closely, showing similar enhancements in nitrogen levels. Phosphorus concentration was most elevated in Fulvic acid 40 + VC 2 mL, achieving an increase of 69.2% compared to the control. Fulvic acid 40 + Amino acid 100 ranked second with a 65.4% increase, while the Amino acid 100 + VC 2 mL combination also performed well, with a moderate enhancement. Although statistically not significant, potassium concentration peaked in Fulvic acid 40 + VC 2 mL, which improved by 43.5% compared to the control. Amino acid 100 + VC 2 mL followed with an increase of 43.2%, and Fulvic acid 40 + Amino acid 100 ranked third with a 33.2% improvement. Fulvic acid 40 + VC 2 mL recorded the highest concentration for calcium, reflecting an improvement of 68.6% over the control. Fulvic acid 40 + Amino acid 100 followed with a 50.0% increase, while Amino acid 100 + VC 2 mL showed an enhancement of 48.5%. Magnesium levels were highest in Amino acid 100 + VC 2 mL, marking a 92.4% increase over the control. Fulvic acid 40 + Amino acid 100 contributed a 69.6% improvement, while Fulvic acid 40 + VC 2 mL achieved a rise of 65.8%.
| Treatments | N | P | K | Ca | Mg |
| Control | 2.67 bc | 0.42 ab | 3.61 | 1.03 ab | 0.79 a |
| FA 40 + AA 100 | 3.03 ab | 0.43 a | 3.70 | 0.97 ab | 0.70 ab |
| FA 40 + VC 2 mL | 3.16 a | 0.44 a | 3.89 | 1.14 a | 0.67 ab |
| AA 100 + VC 2 mL | 3.00 a ab | 0.40 ab | 3.88 | 0.96 ab | 0.73 a |
| FA 40 + AA 100 + VC 2 mL | 2.50 b c | 0.36 b | 3.44 | 0.64 b | 0.50 b |
Antioxidant Contents of Iceberg Lettuce
In the first trial of iceberg lettuce cultivation under soilless culture, significant differences were observed in antioxidant parameters (Table 7). Vitamin C content was highest in VC 1 mL, showing a 33.9% increase compared to the control. VC 2 mL ranked second with a 28.1% improvement, followed by Amino acid 100, which increased Vitamin C levels by 25.6%. The application of biostimulants resulted in an increase in total phenol content compared to the control for the following treatments: Fulvic acid 40 (25.34%), Amino acid 100 (8.96%), and VC 2 mL (7.54%). Other treatments did not show an increase in total phenol content. In terms of total flavonoid content, the treatments that exhibited an increase compared to the control were Fulvic acid 40 (60.64%), Amino acid 100 (31.38%), VC 2 mL (29.26%), and AA 75 (15.96%). Other treatments did not increase total flavonoid content.
| Treatments | Vitamin C (mg 100 g FW−1) | Total Phenols (mg GA 100 g FW−1) | Total Flavonoids (mg RU 100 g FW−1) |
| Control | 5.40 d | 41.39 ab | 188 b |
| FA 40 | 6.37 c | 51.88 a | 302 a |
| FA 80 | 6.57 bc | 39.08 b | 182 b |
| AA 75 | 6.76 abc | 41.19 ab | 218 b |
| AA 100 | 6.78 abc | 45.10 ab | 247 ab |
| VC 1 mL | 7.23 a | 32.94 b | 178 b |
| VC 2 mL | 6.92 ab | 44.51 ab | 243 ab |
In the second trial in the spring, the combination treatments demonstrated notable increases in antioxidant content compared to the control (Table 8). For vitamin C, the highest increase was observed with the Fulvic acid 40 + Amino acid 100 + VC 2 mL treatment, which showed a 17.16% improvement, followed by Fulvic acid 40 + Amino acid 100 (15.06%), Fulvic acid 40 + VC 2 mL (9.11%), and Amino acid 100 + VC 2 mL (4.20%). Regarding total phenol content, Fulvic acid 40 + Amino acid 100 + VC 2 mL exhibited the most significant increase at 52.54%, while Fulvic acid 40 + Amino acid 100 and Amino acid 100 + VC 2 mL also showed substantial increases of 40.50% and 43.06%, respectively. The Fulvic acid 40 + VC 2 mL treatment resulted in a 22.28% increase. For total flavonoid content, Fulvic acid 40 + Amino acid 100 + VC 2 mL again led with a 52.38% increase, followed by Fulvic acid 40 + Amino acid 100 at 46.32%, Amino acid 100 + VC 2 mL at 42.42%, and Fulvic acid 40 + VC 2 mL at 35.50%.
| Treatments | Vitamin C (mg 100 g FW−1) | Total Phenols (mg GA 100 g FW−1) | Total Flavonoids (mg RU 100 g FW−1) |
| Control | 5.71 d | 57.71 c | 231 c |
| FA 40 + AA 100 | 6.57 ab | 81.08 ab | 338 ab |
| FA 40 + VC 2 mL | 6.23 bc | 70.57 bc | 313 b |
| AA 100 + VC 2 mL | 5.95 cd | 82.56 ab | 329 ab |
| FA 40 + AA 100 + VC 2 mL | 6.69 a a | 88.03 a | 352 a a |
Leaf Quality Parameters of BRIX, Acidity, pH, and EC Values of Iceberg Lettuce
In the first trial of iceberg lettuce cultivation under soilless conditions, the effects of biostimulants on brix, acidity, pH, and EC values were evaluated. In both trials, the Brix values in lettuce leaves did not show any statistically significant differences among the treatments. Among these parameters, pH showed statistically significant differences (p < 0.05), while other parameters exhibited no variation (Table 9). pH levels showed significant variation, with the highest values recorded in Fulvic acid 80, Fulvic acid 40, and the control, all maintaining statistically similar levels. The lowest pH levels were observed in the VC 1 mL and Amino acid 100, significantly reduced compared to the control.
| Treatments | Brix (%) | pH | EC (µS cm−1) |
| Control | 1.77 | 6.64 abc | 780 b |
| FA 40 | 1.55 | 6.66 ab | 1150 a |
| FA 80 | 1.60 | 6.69 a | 1182 a |
| AA 75 | 1.30 | 6.73 a | 1081 a |
| AA 100 | 1.40 | 6.39 bc | 1123 a |
| VC 1 mL | 1.42 | 6.37 c | 966 ab |
| VC 2 mL | 1.90 | 6.68 a | 1123 a |
Table 9. Effects of biostimulants on some quality parameters of iceberg lettuce in the first trial. FA = Fulvic acid, AA = Aminoacid, VC = Vermicompost.
In the second trial of iceberg lettuce under soilless culture, statistically significant differences were observed for pH (p < 0.05) and EC (p < 0.05). At the same time, brix and acidity values did not show significant variations (Table 10). pH levels demonstrated significant differences among treatments. The highest pH values were recorded in Fulvic acid 40 + VC 2 mL and Fulvic acid 40 + Amino acid 100 + VC 2 mL, maintaining slightly higher levels than the control. Conversely, Fulvic acid 40 + Amino acid 100 and Amino acid 100 + VC 2 mL recorded lower pH values, indicating a notable reduction. Electrical conductivity values were significantly affected by the treatments. In comparison to the control, the highest increases in EC values were observed with the FA 80, Fulvic acid 40, and Amino acid 100 treatments, showing increases of 51.5%, 47.4%, and 44.0%, respectively. These results indicate that fulvic acid and amino acid applications significantly enhanced the EC levels of the growing medium. The control recorded one of the lowest EC values among all treatments.
| Treatments | Brix (%) | pH | EC (µS cm−1) |
| Control | 2.25 | 6.56 ab | 1344 b |
| FA 40 + AA 100 | 2.50 | 6.43 b | 1697 ab |
| FA 40 + VC 2 mL | 2.30 | 6.62 a | 1569 ab |
| AA 100 + VC 2 mL | 2.50 | 6.46 ab | 1952 a |
| FA 40 + AA 100 + VC 2 mL | 2.54 | 6.60 a | 1618 ab |
Nitrate Content in Iceberg Lettuce
In the first trial of iceberg lettuce cultivation, the nitrate reduction capacity of biostimulants was evident, with specific treatments significantly decreasing nitrate accumulation in leaf tissues (Figure 3). The lowest nitrate content was recorded in Amino acid 100 (690 mg/kg), demonstrating a remarkable reduction of 39.1% compared to the control (1133 mg/kg). This highlights Amino acid 100 as the most effective biostimulant in mitigating nitrate accumulation, making it highly desirable for enhancing lettuce quality. Among the vermicompost, VC 2 mL (1078 mg/kg) and VC 1 mL (1085 mg/kg) achieved moderate reductions of 4.9% and 4.2%, respectively, compared to the control. While these reductions are less pronounced, they still reflect the potential of vermicompost to limit nitrate levels. Conversely, FA-based, including Fulvic acid 80 (1238 mg/kg), Fulvic acid 40 (1229 mg/kg), and Amino acid 75 (1189 mg/kg), showed increased nitrate content, with levels exceeding the control. These results suggest that FA-based are less effective or may even promote nitrate accumulation under the conditions tested. Overall, the findings demonstrate that the nitrate-reducing capacity of biostimulants varies significantly, with Amino acid 100 emerging as the most effective treatment for reducing nitrate content in lettuce leaves, a highly desirable outcome for improved quality and safety.
In the second trial of iceberg lettuce cultivation, biostimulants demonstrated varying capacities to reduce nitrate content in leaf tissues, highlighting their potential to enhance lettuce quality (Figure 4). The lowest nitrate content was recorded in Fulvic acid 40 + Amino acid 100 + VC 2 mL (275 mg/kg), representing a significant reduction of 56.2% compared to the control (628 mg/kg). This treatment emerged as the most effective in mitigating nitrate accumulation, underscoring the synergistic effects of combining multiple biostimulants. Amino acid 100 + VC 2 mL also showed a substantial reduction in nitrate content, with a 44.9% decrease compared to the control. This highlights its effectiveness in reducing nitrate levels while maintaining leaf quality. In contrast, Fulvic acid 40 + Amino acid 100 (646 mg/kg) and Fulvic acid 40 + VC 2 mL (596 mg/kg) recorded nitrate contents similar to or slightly higher than the control, indicating limited or no nitrate-reducing capacity under the tested conditions. The results demonstrate that combining biostimulants reduces nitrate levels in lettuce leaves. Fulvic acid 40 + Amino acid 100 + VC 2 mL is particularly effective, making it a desirable option for achieving high-quality lettuce with reduced nitrate content.
Discussion
The exogenous application of biostimulants, including amino acids, vermicompost, and fulvic acid, represents an innovative and eco-friendly agricultural approach that complements mineral fertilization, enhances plant productivity, promotes growth, increases yield, and improves product quality.
Effects of Biostimulants on Plant Growth Parameters and Iceberg Lettuce Yield
The findings in this study reveal that biostimulants enhanced lettuce performance, promoting growth parameters, chlorophyll levels, and nutrient absorption. Vermicompost achieved the most substantial increases in root biomass and leaf area. In contrast, combinations of amino acids and fulvic acid effectively reduced nitrate content while increasing antioxidants and dry matter accumulation. The combined application of Fulvic acid 40 + VC 2 mL resulted in an 18.12% increase in iceberg lettuce yield compared to the control. At the same time, the combination of Fulvic acid 40 + Amino acid 100 led to a 17.4% increase.
Biostimulants are rich in growth regulators, including humic substances, cytokinins, and auxins, contributing to increases in root and leaf biomass, plant height, leaf number, and shoot and root dry weight. Canellas et al. and Taha et al. reported that the application of fulvic acid significantly enhanced lettuce growth parameters compared to the control. The foliar application at 3.1 g m-2 resulted in a 126% increase in fresh weight, 127% in dry weight, and 30% in plant height. Similarly, irrigation application at 3.1 g m-2 increased by 105% in fresh weight, 103% in dry weight, and 14% in plant height. These results demonstrate the effectiveness of fulvic acid, mainly through foliar application, in promoting lettuce growth. Lüdtke et al. showed that the application of Fulvic acid + NPK significantly improved lettuce growth compared to the control. By day 48, the number of leaves increased by 51%, plant diameter by 40%, plant height by 33%, and root length by 45%. Additionally, the aerial fresh mass of lettuce reached approximately 280 g/plant under Fulvic acid + NPK treatment, representing a 180% increase compared to the control. These findings highlight the potential of Fulvic acid + NPK to enhance overall lettuce growth, contributing to both shoot and root development and biomass production.
Fulvic acid application plants increased the net photosynthetic rate and chlorophyll fluorescence characteristics, including initial fluorescence (Fo), steady-state fluorescence (Fs), maximum fluorescence (Fm), variable fluorescence (Fv), photochemical quenching coefficient (qP), and the effective photochemical quantum yield of PSII (Fv’/Fm’) and promoted photosynthesis. In addition, fulvic acids enhance photosynthesis by modulating signaling pathways associated with plant hormones. They improve essential physiological parameters, including transpiration rate, stomatal conductance, water-use efficiency, intercellular CO2 concentration, chlorophyll content, and relative water content. These findings underscore the significant potential of fulvic acids to enhance plant growth and productivity by optimizing critical physiological processes.
Al-Karaki and Othman demonstrated that in soilless cultivation using a tuff-zeolite medium, foliar application of an amino acid biostimulant at a concentration of 4 mL/L significantly increased yields, with a 26% improvement in iceberg lettuce and a 50% increase in romaine lettuce compared to the control. Amino acids are building blocks for protein biosynthesis and precursors for various biosynthetic pathways and key regulators in signaling processes and stress responses. Their role in promoting a more robust root system enhances water and nutrient uptake efficiency, ultimately driving better plant growth and yield. Haghighi et al. further reported that amino acid application improved the fresh and dry weights of cabbage shoots and the fresh weight of roots by enhancing key physiological processes such as photosynthesis, transpiration, and stomatal conductance. These improvements were linked to enhanced nitrogen uptake and assimilation, contributing to more efficient CO2 assimilation and the effective transport of soluble sugars via the phloem to sink tissues. The biostimulatory effects of amino acids are also associated with activating critical enzymes, such as nitrate reductase and glutamine synthase, which are pivotal in nitrogen metabolism. These findings underscore the multifaceted benefits of amino acid biostimulants in improving plant growth and productivity, particularly under soilless culture systems.
Effects of Biostimulants on Mineral Nutrient Contents
Biostimulants, particularly fulvic acid, amino acids, and vermicompost, play a significant role in enhancing the mineral content of plants. Biostimulants primarily stimulate root development, increasing the plant’s nutrient uptake capacity. According to Yassen et al., the application of 0.95 kg m-2 vermicompost + 150 mL L-1 vermiwash significantly improved the N, P, K, Ca, and Mg contents in the head of lettuce compared to the control. The increases were 46%, 43%, 45%, 16%, and 62%, respectively. These results demonstrate the effectiveness of this optimal vermicompost dose in enhancing both the macronutrient and micronutrient composition of lettuce heads, contributing to their overall nutritional quality. A more robust root system enables the efficient absorption and transport of nutrients to plant tissues. Moreover, biostimulants like vermicompost enhance nutrient availability by providing organic compounds and beneficial microorganisms that facilitate nutrient uptake. Additionally, biostimulants influence hormone balance and signaling pathways to support nutrient mobilization. Elevated cytokinin levels promote nutrient transport to source tissues, such as leaves, enhancing plant growth.
Al-Karaki and Othman demonstrated that in the soilless culture of iceberg lettuce, the foliar application of an amino acid-based biostimulant led to notable increases in leaf mineral concentrations compared to the control. Specifically, for iceberg lettuce, nitrogen (N) content increased by approximately 29%, phosphorus (P) by 25%, potassium (K) by 22%, and magnesium (Mg) by 53%. Lüdtke et al. demonstrated that the application of Fulvic acid + NPK significantly improved the macronutrient and micronutrient contents of lettuce compared to the control. N, P, K, Ca, and Mg contents increased by 11%, 14%, 67%, 15%, and 2%, respectively. These findings underscore the effectiveness of Fulvic acid + NPK in enhancing plant nutrient levels, contributing to the nutritional quality of lettuce. Biostimulants also improve photosynthesis, supporting energy metabolism and facilitating active nutrient transport and ion uptake in roots. Amino acids contribute to increased carbohydrate production, meeting the energy demands of roots and improving nutrient acquisition. Vermicompost supports beneficial microbial populations, further enhancing the bioavailability of essential minerals.
Effects of Biostimulants on Iceberg Lettuce Quality and Antioxidant Contents
In this study, biostimulants, particularly fulvic acid, amino acids, and vermicompost, have proven effective in enhancing vitamin content, antioxidant capacity, phenolic compounds, and flavonoid levels in lettuce. Plants treated with amino acids exhibited higher phenolic content and antioxidant activity, which can be partially attributed to the localized beneficial effects of amino acids on plant processes. Amino acids are precursors to a wide range of secondary metabolites and are closely linked to synthesizing proteins, hormones, and carbohydrates. Haghighi et al. demonstrated that the exogenous application of AAs enhances the nutritional profile of cabbage, including increased levels of phenols, total protein, proline, and essential amino acids such as glutamic acid, glutamine, and asparagine, along with improved antioxidant capacity.
Abdel-Baky et al. demonstrated that applying fulvic acid significantly increased total carbohydrate content, crude protein, and essential minerals. Additionally, it elevated the levels of key amino acids such as arginine, lysine, phenylalanine, and tryptophan. The foliar application of fulvic acid significantly enhanced secondary metabolites, including total phenolic content, anthocyanins, and total carotenoids, underscoring its potential to improve quality attributes.
Effects of Biostimulants on Iceberg Lettuce Nitrate Content
In the second spring trial, biostimulant applications significantly decreased nitrate content in lettuce leaf tissues. This is consistent with the findings of Coronel et al., who reported that soilless lettuce cultivation enhances NR activity, making it a promising method for producing lettuce with reduced nitrate levels. Biostimulants promote plant growth by increasing amino acid concentrations, which are directly utilized in protein biosynthesis. Elevated carbohydrate levels in the leaves further support this process by enhancing nitrogen assimilation through the nitrate pathway. Carbohydrates provide the carbon skeletons required for converting reduced nitrate (ammonia) into amino acids, facilitating protein synthesis and overall growth. Additionally, applying exogenous amino acids has reduced nitrate concentrations in cabbage leaves, suggesting that externally supplied amino acids may regulate nitrate uptake.
Conclusions
This study demonstrates the significant potential of biostimulants such as fulvic acid, amino acids, and vermicompost in enhancing the growth, yield, and nutritional quality of soilless-grown iceberg lettuce. Applying these eco-friendly biostimulants, individually and in combination, improved key plant parameters, including biomass production, nutrient uptake, and antioxidant content, while reducing nitrate levels in lettuce leaves. These findings underscore the value of biostimulants as sustainable solutions for modern agriculture, reducing dependency on conventional fertilizers and mitigating environmental impacts. Biostimulants are a promising approach for advancing sustainable soilless culture systems and addressing the growing global demand for high-quality, nutrient-rich vegetables.
Biostimulants provide sustainable solutions for soilless systems by enhancing nutrient-use efficiency and amplifying the effectiveness of conventional fertilizers. Future research should aim to optimize biostimulant application strategies by exploring novel biostimulants, innovative combinations, and optimal dosing regimens to maximize their efficacy and investigate their long-term benefits for sustainable soilless culture production.
Dora Biostimulant products are designed based on natural active ingredients to support plants when they need specific physiological responses. Biological stimulants are developed through a lot of research and innovation, aiming to bring maximum vitality, yield, and quality to crops. See more details of Dora organic amino acid fertilizer series and Dora Fulvic Acid and Dora Potassium Humate.
