One way to advance onion production involves the application of biostimulants (BTs) of plant growth. Laboratory studies have revealed that those substances stimulate various physiological processes in the model plant Arabidopsis spp., positively impacting its growth and development. Given this assumption, there is a necessity to conduct field experiments with onions using various BTs to ascertain their practicality for adoption by farmers.
Currently, the most commonly used BTs include seaweed extract (Seaweed Extract), humic substances (HSs) such as humic and fulvic acids (HFAs), and microorganisms such as Trichoderma spp.. BTs play a significant role in reducing biotic and abiotic stresses, which can lead to a decreased synthesis of plant secondary metabolites.
The application of Seaweed Extract-based BTs was shown to increase the yield of lettuce (Lactuca sativa), peppers (Capsicum annuum), and tomatoes (Solanum lycopersicon). However, in trials with shallots (Allium cepa Aggregatum group), no Seaweed Extract effect on yield was observed. In tomato, Seaweed Extract enhanced the number and size of xylem and phloem cells in stems, potentially influencing the transport of mineral elements throughout the plant. It was also found that Seaweed Extract improves the expression of genes encoding enzymes involved in carbon fixation during the Calvin cycle.
In peppers and yarrow (Achillea millefolium), the application of HSs increased flavonoid content, whereas in onions, higher dry matter content was observed after HS treatment. The stimulative effect of HSs is explained by an increase in the activity of proton pumps on the plasma membrane and tonoplast of plant cells. Additionally, HSs enhance the synthesis of phenylalanine and tyrosine ammonia lease, which are involved in the process of phenolic synthesis.
In onions from sets, HSs reduced the content of phenolics and antioxidant activity. Conversely, in the case of pepper fruits, there was an increase in phenolic levels. This research highlights the complexity of the effects of HS on the distribution of secondary metabolites in different plant organs (i.e., bulbs or fruit).
In vegetable production, fungi from the Trichoderma genus can be utilized as BTs. In studies involving lettuce and tomatoes, Trichoderma spp. has been shown to increase yields. The application of Trichoderma spp. also led to increased sugar and protein content in Chinese cabbage (Brassica campestris ssp. chinensis var. utilis), and in lettuce, it enhanced phenolic content. The stimulative effects of Trichoderma spp. are primarily attributed to its release of auxins, peptides, and various organic compounds that stimulate plant metabolism.
Given that onions are a crucial component in numerous dishes and salads worldwide, there is a need to enhance their production to enable farmers to achieve high yields and consumers to enjoy the high nutrition and sensory qualities of onions. Considering the existing research on onions, many studies have explored the impact of BTs on onions produced from seedlings and cultivated in pots. However, there remains a notable gap in scientific data regarding the impact of BTs on onions directly seeded in the field over a minimum of two years. This knowledge is vital to ensure the practical applicability of results for farmers.
Yield of First-Class Onion Bulbs
The application of BTs significantly influenced the yield of first-class onions over the two years of the experiment (Figure 1).
In 2021, the highest yield was observed in treatment BT3 (37.49 t/ha), representing a 22.8% increase compared to C (30.54 t/ha). Additionally, treatment BT2 (33.10 t/ha) and BT3 (37.49 t/ha) increased the yield of first-class onion bulbs by 20.1% and 8.4%, respectively.
During 2022, the highest yield of first-class onions was recorded in plot BT3 (89.90 t/ha), while the lowest was in plot C (72.95 t/ha), with a statistically significant difference of 18.7%. In the same year, the application of BT2 (85.36 t/ha) and BT3 (89.90 t/ha) led to yield increases of 18.0% and 24.3%, respectively, compared to treatment C.
Dry Matter Content
In this study, BTs significantly influenced dry matter content in onion bulbs (Table 1).
| Biostimulants | Year 2021 | Relative Change (%) | Year 2022 | Relative Change (%) |
|---|---|---|---|---|
| C | 9.72 ± 0.12 a | / | 10.07 ± 0.03 c | / |
| BT1 | 9.56 ± 0.12 a | −1.64 | 9.73 ± 0.02 d | −3.37 |
| BT2 | 9.32 ± 0.00 b | −4.11 | 10.69 ± 0.08 a | +6.15 |
| BT3 | 8.94 ± 0.05 c | −8.02 | 10.55 ± 0.25 a | +4.76 |
In 2021, the highest dry matter content was recorded in treatment C (9.72%), while the lowest was recorded in BT3 (8.94%), with their difference being statistically significant. In the same year, biostimulants BT1 and BT2 significantly decreased dry matter content compared to C.
In 2022, the highest dry matter content in onions was observed in BT2 (10.69%), significantly exceeding that of C. Additionally, the increase in dry matter was influenced by BT3 (10.55%), while BT1 significantly reduced dry matter content compared to C.
Comparing both years, it is evident that there was a higher dry matter content in 2022 compared to 2021.
Total Phenolic Content
The application of BTs significantly reduced the phenolic content in onions over the two years of the study (Table 2).
| Biostimulants | Year 2021 | Relative Change (%) | Year 2022 | Relative Change (%) |
|---|---|---|---|---|
| C | 1066.06 ± 1.01 a | / | 795.28 ± 2.71 e | / |
| BT1 | 1047.08 ± 4.60 b | −1.78 | 744.47 ± 1.03 f | −6.38 |
| BT2 | 873.27 ± 2.15 c | −18.08 | 640.70 ± 2.00 h | −19.43 |
| BT3 | 960.88 ± 4.35 d | −9.86 | 731.93 ± 4.08 g | −7.96 |
In the first year (2021), the application of BT2 notably decreased the phenol level (18.08%) compared to C, and the difference was significant. Additionally, BT1 and BT3 reduced the phenolic content in bulbs by 1.78% and 9.86%, respectively, compared to C.
In 2022, the highest phenolic content was observed in C (795.28 mg/100 g), while the lowest was in BT2 (640.70 mg/100 g), with a significant difference of 19.43%. The application of BT1 and BT3 reduced the phenolic content by 6.38% and 7.96%, respectively, compared to C.
Observing all treatments across both years, it is evident that the phenolic content was significantly higher in 2021 compared to 2022.
Flavonoid Content
The flavonoid content in onions varied based on the application of BTs over the two years of the experiment (Table 3).
| Biostimulants | Year 2021 | Relative Change (%) | Year 2022 | Relative Change (%) |
|---|---|---|---|---|
| C | 358.07 ± 2.61 b | / | 244.15 ± 1.12 e | / |
| BT1 | 418.53 ± 10.26 a | +16.88 | 265.71 ± 3.14 d | +8.83 |
| BT2 | 271.31 ± 10.18 d | −24.22 | 293.99 ± 4.04 c | +20.41 |
| BT3 | 417.97 ± 2.45 a | +16.72 | 238.24 ± 4.52 e | −2.42 |
In 2021, the highest flavonoid content was measured in BT1 (418.53 mg/100 g), representing a 16.88% increase compared to C, and the difference was significant. In the same year, BT3 significantly increased flavonoids (+16.72%), while BT2 significantly decreased them (-24.22%) compared to C.
During 2022, the highest flavonoid content was found in plot BT2 (293.99 mg/100 g), indicating a 20.41% increase compared to C (244.15 mg/100 g). Additionally, the application of BT1 (265.71 mg/100 g) significantly increased the flavonoid level by 8.83%, while BT3 (238.24 mg/100 g) reduced flavonoids by 2.42% compared to C.
Antioxidant Statu
In this study, BTs had varied effects on the antioxidant activity of onions, as determined by the DPPH test (Table 4).
| Year | Biostimulants | DPPH | Relative Change (%) | FRAP | Relative Change (%) | ABTS | Relative Change (%) |
|---|---|---|---|---|---|---|---|
| 2021 | C | 561.86 ± 1.75 b | / | 494.03 ± 0.97 b | / | 1489.22 ± 9.53 b | / |
| BT1 | 611.10 ± 4.33 a | +8.76 | 508.04 ± 1.51 a | +2.83 | 1508.9 ± 7.60 a | +1.32 | |
| BT2 | 419.54 ± 4.31 e | −25.33 | 327.65 ± 2.40 f | −33.67 | 1245.34 ± 5.42 f | −16.37 | |
| BT3 | 483.10 ± 2.77 c | −14.01 | 418.09 ± 2.80 c | −15.37 | 1333.43 ± 8.29 d | −10.46 | |
| 2022 | C | 486.93 ± 1.43 c | / | 344.90 ± 2.75 e | / | 1284.88 ± 2.67 e | / |
| BT1 | 430.74 ± 2.46 d | −11.53 | 377.54 ± 1.05 d | +9.46 | 1367.75 ± 6.54 c | +6.44 | |
| BT2 | 336.66 ± 3.57 g | −30.86 | 233.84 ± 1.80 h | −32.20 | 1010.10 ± 2.05 h | −21.33 | |
| BT3 | 361.84 ± 2.3 f | −25.68 | 290.71 ± 0.70 g | −15.71 | 1150.32 ± 2.98 g | −10.47 |
For the DPPH test in 2021, the highest results were observed in BT1 (611.10 mg/100 g), representing an 8.76% increase compared to C (561.86 mg/100 g). In the same year, BT2 (419.54 mg/100 g) and BT3 (483.10 mg/100 g) decreased antioxidant activity by 25.33% and 14.01%, respectively.
In 2022, compared to C (486.93 mg/100 g), the plots where BT1 (430.74 mg/100 g), BT2 (336.66 mg/100 g), and BT3 (361.84 mg/100 g) were applied showed a significant decrease in the DPPH test results by 11.53%, 30.86%, and 25.68%, respectively.
The application of BTs also influenced the results of the FRAP test in onions (Table 4). In 2021, BT1 (508.04 mg/100 g) significantly increased the FRAP test results compared to C (494.03 mg/100 g). However, BT2 (327.65 mg/100 g) and BT3 (418.09 mg/100 g) significantly decreased antioxidant activity by 33.67% and 15.37%.
During 2022, BT1 (377.54 mg/100 g) significantly increased the FRAP test by 9.46% compared to C (344.90 mg/100 g). Similar to the previous year, BT2 (233.84 mg/100 g) and BT3 (290.71 mg/100 g) decreased FRAP test results by 32.20% and 15.71%, respectively.
BTs had varying effects on the antioxidant activity of onions measured using the ABTS test (Table 4). In 2021, treatment BT1 (1508.91 mg/100 g) significantly increased the ABTS test result compared to C (1489.22 mg/100 g). The application of BT2 (1245.34 mg/100 g) and BT3 (1333.43 mg/100 g) significantly decreased antioxidant activity by 16.37% and 10.46%, respectively.
In the following year (2022), treatment BT1 (1367.75 mg/100 g) increased the ABTS test result by 6.44% compared to C (1284.88 mg/100 g). However, BT2 (1010.10 mg/100 g) and BT3 (1150.32 mg/100 g) significantly decreased ABTS test antioxidant activity by 21.33% and 10.47%, respectively.
Figure 2 illustrates the relationship between phenolic content and antioxidant tests (DPPH, FRAP, and ABTS). Throughout 2021 and 2022, a noticeable linear increase in antioxidant activity was observed with the rise in phenolic content in onion bulbs.
Discussion
Onions stand out as one of the most important vegetable plants, playing a significant role in human nutrition. The primary objective of onion production is to achieve a high yield of first-class bulbs, given their premium value in domestic and international markets.
Onions are sensitive to changes in meteorological conditions during bulb formation. The optimal temperature during this phase, as indicated by the same authors, is 22.0 ℃, occurring approximately 60 days after sowing. In line with this, the bulb formation period in 2021 occurred in the third decade of May, with a temperature of 16.9 ℃, while in 2022, during this phase, it occurred in the second decade of May, with a recorded temperature of 21.3 ℃.
Humidity above 60% adversely affects bulb formation. In this experiment, in the third decade of May 2021, the registered air humidity was 69.5%, while in 2022, in the second decade of May, it was 49.3%. Additionally, in 2022, higher levels of humus, N, and P were measured compared to the soil in 2021, further explaining the significantly higher yield in 2022.
In 2021 and 2022, the application of Seaweed Extract significantly increased the yield of first-class onions. The stimulating effect of Seaweed Extract indicated that in peppers and tomatoes, it enhances the expression of genes responsible for the synthesis of auxins, gibberellins, and cytokinins. The effect of Seaweed Extract can also be explained by an increase in the number and size of xylem and phloem cells in the tomato stem, which can have a favorable impact on the transport of mineral elements through the plant.
Humic and fulvic acids increased bulb yield, but it was statistically significant. The beneficial effect of HS has been observed in tomatoes. The effect of HS can be described by an increase in the activity of proton pumps on the plasma membrane and tonoplast of plant cells.
In vegetable production, fungi from the genus Trichoderma spp. can be used as a BT. In 2021 and 2022, Trichoderma spp. significantly increased the yield of first-class onions. The stimulative effect of Trichoderma spp. is mainly based on its ability to release auxins, peptides, and various organic compounds that can enhance plant metabolism.
Dry matter content is a crucial indicator of the quality of onions. Onions with higher dry matter content exhibit slowed life processes, primarily water loss and respiration. In practical terms, such onions can be stored for an extended period in warehouses. The application of Seaweed Extract reduced dry matter content during the two years of the study. This can be explained by the fact that Seaweed Extract can enhance N uptake in onions. An increased N content relative to carbon content in cells signifies a lower fiber (cellulose) content, indicating a higher water content in cells.
On plots where Trichoderma spp. was applied, there was a significant increase in dry matter content over both years. Similarly, in a study with Chinese cabbage (Brassica campestris ssp. chinensis var. utilis), the application of Trichoderma spp. significantly increased the content of protein and sugars, which represent components of dry matter.
Phenolics are bioactive compounds that significantly contribute to the taste and aroma of onions. The synthesis of secondary metabolites (e.g., phenolics), occurs when the plant is exposed to stress conditions. In this study, the application of all BTs led to a reduction in phenolic content in the bulbs compared to the C. This can be explained by the ability of BTs to mitigate the impact of abiotic stress, resulting in a reduction in the accumulation of secondary metabolites, such as phenolics.
The application of Humic and fulvic acids reduced the phenolic content in onions, reduced phenolics in onion sets using HS. However, in pepper fruit, the use of HS increased phenolics. This may be related to the fact that the effect of HS depends on the part of the plant being studied.
In a study on tomatoes, the application of Trichoderma spp. reduced the phenolic content in the fruit. Similarly, the application of Trichoderma spp. reduced phenolic content in onion bulbs in this study.
Flavonoids are essential bioactive compounds that significantly impact the taste of onions and have numerous health benefits due to their potent antioxidant properties. Antioxidants are essential for protecting the human body from harmful radicals, thereby reducing the risk of various diseases. As critical secondary metabolites, flavonoids play a significant role in plant defense against different biotic and abiotic stresses. The application of Seaweed Extract significantly increased flavonoid content in 2021 and 2022.
Similar results have been documented in onions, carrots (Daucus carota), and beans (Phaseolus vulgaris) following Seaweed Extract application. This can be explained by the fact that Seaweed Extract enhances the expression of genes involved in the synthesis of enzymes participating in the phenylpropanoid pathway, a process essential for flavonoid synthesis.
On plots where Humic and fulvic acids was used in 2022, there was a 20.41% increase in flavonoid levels. This can be attributed to the fact that HS improves the synthesis of phenylalanine and tyrosine ammonia lyase, enzymes involved in the flavonoid synthesis process. The application of HS on plots with pepper and yarrow (Achillea millefolium) has been shown to increase flavonoid content.
The application of Trichoderma spp. reduced flavonoid content in directly seeded onions fertilized with 150 kg N/ha. However, in this study, during 2021, the use of the Trichoderma spp. increased flavonoid levels by 16.72%. Consistent with this, recent findings indicate that Trichoderma spp. activates the phenylpropanoid pathway in flavonoid synthesis.
Antioxidant activity in vegetables plays a crucial role in reducing the risk of chronic diseases, including cardiovascular diseases, anemia, and cancer. Antioxidants protect human cells from damage caused by free radicals and reactive oxygen species. In 2021, on plots where Seaweed Extract was applied, the antioxidant activity of onions significantly increased, as confirmed by DPPH, ABTS, and FRAP tests. The increased flavonoid content can explain this, as flavonoids exhibit strong antioxidant properties.
Humic and fulvic acids and Trichoderma spp. reduced the antioxidant activity of onions, a trend confirmed over the two years of the study. This reduction could be ascribed to the decreased phenolic content in these treatments, as phenolics have significant antioxidant properties. This is further supported by a linear relationship between phenolics and antioxidants (DPPH, FRAP, and ABTS), which was found in this study.
Conclusions
The application of Seaweed Extract and Trichoderma spp. significantly increased bulb yield.
On plots treated with Seaweed Extract, Humic and fulvic acids, and Trichoderma spp., phenolic content was significantly reduced. The application of Seaweed Extract significantly increased flavonoids and antioxidant activity (FRAP and ABTS) in both years.
Based on the findings, the recommendation would be to use Seaweed Extract and Trichoderma spp. to achieve high bulb yields in the production of onions from direct seed sowing, while Seaweed Extract is recommended for enhancing antioxidant capacity. Additionally, the application of BTs has environmental significance as they are made from natural substances that cannot harm the environment.
Various BTs significantly influence bulb yield and phytochemical content in onions grown through direct seeding. We supply raw materials and you own formulations. See more details of Dora Biostimulants.