Affected by climate change and human activities, soil salinization is intensifying globally, leading to a decline in land productivity. It is reported that by 2050, 50% of the world’s arable land may be affected by salinization. High concentrations of salt ions in salinized soil reduce plants’ photosynthetic efficiency and nutrient absorption capacity, cause physiological and metabolic disorders, inhibit growth and development, and ultimately result in crop yield reduction. Under salt stress, plants often face osmotic stress, ionic toxicity, and oxidative stress: osmotic stress reduces root water absorption capacity, triggering “physiological drought”; ionic toxicity interferes with physiological metabolism, affecting ion absorption, photosynthesis, and protein synthesis; at the same time, salt stress disrupts the balance between the production and scavenging of reactive oxygen species (ROS), leading to their excessive accumulation in cells, inducing membrane lipid peroxidation, and even damaging the cell membrane system. Although plants can alleviate salt stress by upregulating enzymatic or non-enzymatic scavenging systems, accumulating secondary metabolites, and regulating ion distribution, their self-regulatory capacity is limited. Therefore, developing new fertilizers or regulators to improve plant salt tolerance is of great significance for the development and utilization of salinized soil and the improvement of crop yield and quality.
Domestic and foreign studies have shown that after amino acid fertilizer is applied to soil, it can not only promote root nutrient absorption but also enhance crop resistance and improve fruit quality. Under salt stress, soaking seeds with amino acid fertilizer can increase the activity of protease and amylase in pakchoi seeds, maintain the proline content and antioxidant enzyme activity in seedling leaves, and promote seed germination and seedling salt tolerance; foliar spraying of mixed amino acid fertilizer can improve the function of the photochemical PSII system in grape leaves, enhance photosynthesis, and regulate the soluble sugar and titratable acid content of fruits.
Rhamnolipid is a surfactant with antibacterial properties, non-toxicity, and good water solubility, which has been applied in environmental remediation and agricultural fields. Recent studies have found that rhamnolipid can reduce cell surface tension, promote plant nutrient absorption, and also serve as a carbon source to provide a material basis for the proliferation of soil microorganisms, alleviating adversity by adjusting plant root structure and morphology. Previous studies by the research group have confirmed that rhamnolipid can improve the photosynthesis and antioxidant defense capacity of tomatoes under salt stress, thereby increasing yield and salt tolerance. However, research on the combined application of amino acid fertilizer and rhamnolipid to improve plant salt tolerance is still rare and urgently needs in-depth exploration.
Tomato (Solanum lycopersicum L.) is a widely cultivated cash crop and a model species for plant research. However, the closed facility environment, excessive application of chemical fertilizers, and unreasonable cultivation and irrigation measures are likely to cause soil compaction and secondary salinization, leading to microbial community imbalance, insufficient nutrient supply, and reduced soil productivity. Soil salinization has become one of the main limiting factors for the high-quality and high-yield cultivation of tomatoes in facilities. Therefore, this study used Provence tomatoes as test materials to explore the effects of different ratios of amino acid fertilizer and rhamnolipid on tomato yield, quality, and salt tolerance under moderately salinized soil conditions.
1.Materials and Methods
(1) Test Materials Related to Amino Acid Fertilizer and Rhamnolipid
The tested tomato variety was Provence. The amino acid content in the amino acid liquid fertilizer was 105 g·L⁻¹, mainly composed of alanine, glycine, and lysine, with a trace element (Fe, Mn, Cu, Zn) content of 23 g·L⁻¹ and a pH value of 5.5. The concentration of the rhamnolipid solution was 200 g·L⁻¹.
(2) Overview of the Research Area for Amino Acid Fertilizer and Rhamnolipid Tests
The test was conducted from December 2022 to June 2023 at Shayuan Farm, Dali County, Weinan City, Shaanxi Province (109°83′E, 34°69′N). The test area was 0.73 hm², and the tested soil was light loam, belonging to moderately salinized soil. The basic properties of the soil are shown in Table 1.
(Table 1 Soil base indicators in the test area)
(3) Experimental Design of Amino Acid Fertilizer and Rhamnolipid
A total of 8 treatments were set up in the test: 1. No chemical fertilizer application (CK); 2. Conventional fertilization (CF); 3. Conventional fertilization + amino acid fertilizer (CFA); 4. Conventional fertilization + rhamnolipid (CFR); 5. Conventional fertilization + amino acid fertilizer + rhamnolipid (ratio 50∶1, CFAR1); 6. Conventional fertilization + amino acid fertilizer + rhamnolipid (ratio 50∶2, CFAR2); 7. Conventional fertilization + amino acid fertilizer + rhamnolipid (ratio 50∶4, CFAR3); 8. Conventional fertilization + amino acid fertilizer + rhamnolipid (ratio 50∶5, CFAR4). The specific fertilizer input for each treatment is shown in Table 2.The plot area was 66 m² (6.6 m×10 m), the planting density of tomatoes was 57000 plants·hm⁻² with a plant spacing of 35 cm and row spacing of 50 cm, and each treatment was repeated 3 times. The fertilization method was drip irrigation fertilization, with an irrigation volume of 250~270 m³·hm⁻² each time; NPK compound fertilizer was applied as base fertilizer, and high-potassium compound fertilizer was topdressed 4 times; amino acid fertilizer and rhamnolipid were applied with water during the expansion period of the first to fourth ear fruits.
(Table 2 Fertilizer inputs in each treatment)
(4) Determination Indicators of Amino Acid Fertilizer and Rhamnolipid Tests
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Growth and Photosynthetic Indicators
The fresh weight of plants, chlorophyll content (TYS-4N chlorophyll meter), net photosynthetic rate, and transpiration rate (Li-6400 portable photosynthesis system) were measured at the fruit setting stage of the first ear.
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Physiological Indicators
At the fruit setting stage of the first ear, the peroxidase (POD) activity (guaiacol method), malondialdehyde (MDA) content (thiobarbituric acid method), superoxide anion (O₂⁻) production rate (nitroblue tetrazolium colorimetric method), proline content (sulfosalicylic acid colorimetric method), and the content of sodium and potassium ions in leaves (HNO₃-HClO₄ digestion + atomic absorption spectrophotometer method) were measured.
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Yield and Quality
The total tomato yield was counted during the harvest period; the soluble sugar (sulfuric acid-anthrone colorimetric method) and vitamin C content (2,6-dichlorophenol indophenol titration method) of fruits were measured at the ripening stage of the first ear.
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Root Nutrients
The total nitrogen, total phosphorus, and total potassium contents of roots were determined.
(5) Data Analysis of Amino Acid Fertilizer and Rhamnolipid Tests
Excel and SPSS 24 software were used for data processing, single-factor analysis of variance and least significant difference test (P<0.05) were used to analyze differences, and Origin 2021 software was used for plotting.
2.Results and Analysis
(1) Effects of Amino Acid Fertilizer and Rhamnolipid on Tomato Growth and Photosynthetic Characteristics
Compared with conventional fertilization, the single application of amino acid fertilizer or rhamnolipid had no significant effect on tomato growth and photosynthetic characteristics; the combined application of the two significantly increased plant fresh weight, chlorophyll content, net photosynthetic rate, and transpiration rate, with the CFAR3 treatment being the best. Among them, compared with CFA and CFR treatments, the CFAR3 treatment significantly increased plant fresh weight by 30.6% and 37.7%, chlorophyll content by 16.7% and 21.2%, net photosynthetic rate by 41.4% and 43.1%, and transpiration rate by 37.1% and 41.2%, respectively.
(Fig.1 Effects of amino acid fertilizer and rhamnolipid on tomato biomass and photosynthetic characteristics)
(2) Effects of Amino Acid Fertilizer and Rhamnolipid on Oxidative Damage in Tomato Leaves
The combined application of amino acid fertilizer and rhamnolipid could significantly alleviate oxidative damage to tomato leaf cells. Compared with the single application of amino acid fertilizer (CFA), the MDA content of CFAR1~CFAR4 treatments decreased by 15.2%, 24.1%, 26.6%, and 25.3% respectively; CFAR2~CFAR4 treatments significantly reduced the O₂⁻ production rate, among which the CFAR3 treatment was the lowest, decreasing by 26.6% compared with CFA.
(Fig.2 Effects of amino acid fertilizer and rhamnolipid on oxidative damage in tomato leaves)
(3) Effects of Amino Acid Fertilizer and Rhamnolipid on Tomato Salt Tolerance
Compared with the single application of amino acid fertilizer or rhamnolipid, the combined application of the two significantly increased POD activity and proline content, and decreased leaf Na⁺/K⁺. The CFAR3 treatment had the best effect, which significantly increased POD activity by 31.4% and 38.8%, and proline content by 20.6% and 25.8% compared with CFA and CFR treatments, respectively; at the same time, it significantly decreased Na⁺/K⁺ by 43.0% and 42.4%.
(Fig.3 Effects of amino acid fertilizer and rhamnolipid on salt tolerance of tomato)
(4) Effects of Amino Acid Fertilizer and Rhamnolipid on Nutrient Contents in Tomato Roots
There was no significant difference in the total nitrogen content of roots between the single application of amino acid fertilizer and rhamnolipid and their combined application; the total phosphorus content of the CFAR3 treatment was significantly higher than that of the CFR treatment by 17.0%, and the total potassium content was significantly higher than that of the CFA and CFR treatments by 13.9% and 13.6%, respectively; there was no significant difference in the total nitrogen, total phosphorus, and total potassium contents of roots among different ratio treatments.
(Fig.4 Effects of amino acid fertilizer and rhamnolipid on nutrient contents in tomato root)
(5) Effects of Amino Acid Fertilizer and Rhamnolipid on Tomato Yield and Quality
The single application of amino acid fertilizer or rhamnolipid had no significant effect on tomato yield, soluble sugar, and vitamin C content; the combined application of the two significantly improved them, with the CFAR3 treatment being the best. Compared with CFA and CFR treatments, the CFAR3 treatment increased tomato yield by 38.5% and 47.2%, soluble sugar content by 14.2% and 14.6%, and vitamin C content by 18.2% and 19.5%, respectively.
(Fig.5 Effects of amino acid fertilizer and rhamnolipid on yield and quality in tomato)
(6) Path Analysis of Main Physiological Indicators Related to Amino Acid Fertilizer and Rhamnolipid and Yield
The fresh weight of plants had the greatest direct effect on tomato yield (direct path coefficient 0.3420), while proline had the smallest direct effect (0.0240). POD activity had the highest correlation coefficient with yield (0.9640) and the greatest indirect effect (total indirect path coefficient 0.9944); superoxide anion, MDA content, and Na⁺/K⁺ had negative indirect effects on yield, with total indirect path coefficients of -0.6796, -0.7190, and -0.8923, respectively.
(Table 3 Path analysis of various physiological indicators and yield under salt stress)
3.Discussion
Salt stress leads to the excessive accumulation of reactive oxygen species (ROS) in plants, triggering membrane lipid peroxidation and damaging the cell membrane structure. The increase in POD activity and proline content can enhance plants’ antioxidant capacity and osmotic regulation capacity, thereby alleviating salt damage. In this study, both the single application of amino acid fertilizer and rhamnolipid could increase the POD activity and proline content of tomatoes, and decrease the O₂⁻ production rate and MDA content, among which amino acid fertilizer had a better effect; the combined application of the two further strengthened this effect. It is speculated that rhamnolipid can enhance the permeability of root cell membranes and promote the absorption and utilization of amino acids by tomatoes.
Under salt stress, the excessive accumulation of Na⁺ in plant cells disrupts ionic homeostasis, and maintaining Na⁺/K⁺ balance is the key to improving salt tolerance. This study found that the combined application of amino acid fertilizer and rhamnolipid could significantly reduce the Na⁺/K⁺ in tomato leaves. It is speculated that this may be achieved by expanding the vacuolar sequestration of Na⁺ in root cells, reducing the transport of Na⁺ to leaves, and promoting the absorption of K⁺ to alleviate ionic toxicity.
In addition, amino acid fertilizer can provide quick-acting nutrients and carbon sources for tomatoes and regulate metabolic processes; rhamnolipid can serve as a carbon source, improve soil nutrient availability and water retention capacity. The combined application of the two can synergistically improve the chlorophyll content and photosynthetic capacity of tomatoes, laying a foundation for the improvement of yield and quality. Soluble sugar and vitamin C are core indicators of fruit quality. The combined application treatment significantly increases their contents by promoting photosynthesis and the expression of key enzymes, improving fruit quality.
4.Conclusion
Under the condition of conventional fertilization in moderately salinized soil, both the single application of amino acid fertilizer and rhamnolipid can enhance the salt tolerance of tomatoes, among which amino acid fertilizer has a better effect; the combined application of the two can produce a synergistic effect, further improve tomato growth, photosynthetic characteristics, and nutrient absorption, and alleviate salt stress damage. Overall, the combination of 300 L·hm⁻² amino acid fertilizer and 24 L·hm⁻² rhamnolipid has the best effect, which can significantly improve the salt tolerance, yield, and fruit quality of tomatoes, and provide technical support for the high-quality and high-yield cultivation of tomatoes in salinized soil.
Dora Agri supplies high-quality amino acid fertilizer and rhamnolipid products, which can effectively improve crop salt tolerance, increase yield, and enhance quality. Customers with needs are welcome to contact us for details, and we support free sample collection to assist agricultural production and scientific research practice.