Application of exogenous melatonin in alleviating drought stress in tobacco seedlings

Abstract

The application relates to application of exogenous melatonin in relieving drought stress of flue-cured tobacco seedlings. The application of exogenous melatonin in relieving drought stress of flue-cured tobacco seedlings is characterized by that the concentration of melatonin is 0.05-0.4 mmol / L, and the application mode includes foliar spraying or root irrigation. The drought stress is that the relative water content of soil is 25%-45%. The application period of the melatonin includes the seedling stage after transplanting. The application of the application can improve the efficient drought resistance of flue-cured tobacco, and significantly improve the water physiological state of flue-cured tobacco under severe drought conditions. The application of the application can restore the photosynthesis of flue-cured tobacco, increase the chlorophyll content, and maintain the photosynthetic efficiency. The application of the application can enhance the antioxidant capacity of flue-cured tobacco, improve the activity of the enzymatic protection system, and reduce the membrane lipid oxidation. The application of the application can improve the osmotic regulation capacity of flue-cured tobacco, enhance the cell water holding capacity, and relieve the dehydration damage.

Description

Technical Field

[0001] This invention relates to the field of tobacco cultivation technology, specifically to the application of exogenous melatonin in alleviating drought stress in flue-cured tobacco seedlings. Background Technology

[0002] Tobacco (Nicotiana tabacum) is an important economic crop in my country, and robust seedling growth is the foundation for its later yield and quality. Drought stress is a significant abiotic stress factor that restricts the normal development of tobacco seedlings, easily leading to wilting leaves, stunted growth, and limited root development, affecting the survival rate and uniformity of seedlings, and consequently impacting the quality of transplanting into tobacco fields and the final yield of tobacco leaves.

[0003] Current drought-resistant measures during the seedling stage include improving seedling substrate, implementing mist irrigation systems, and manual watering. However, these methods are cumbersome, inefficient, and resource-intensive. Therefore, developing simple, efficient, and environmentally friendly drought-resistant control methods for seedlings has become an urgent technical challenge.

[0004] Melatonin (MT), a novel plant physiologically active substance, has been extensively studied in recent years for alleviating abiotic stress in plants. It possesses strong antioxidant capabilities and can enhance plant resistance to adverse conditions such as drought by regulating plant water metabolism, scavenging reactive oxygen species, and stabilizing cell membrane systems. However, there are currently no studies or publicly reported examples of the systematic application of melatonin in tobacco seedling drought resistance. Summary of the Invention

[0005] To address the problems of growth stagnation, reactive oxygen species accumulation, and cell membrane damage in flue-cured tobacco seedlings under drought stress, this invention provides the application of exogenous melatonin in alleviating drought stress in flue-cured tobacco seedlings, thereby reducing the inhibition of physiological functions of tobacco seedlings by drought and improving drought resistance and the rate of uniform and robust seedling growth.

[0006] This invention provides the following technical solution: The application of exogenous melatonin in alleviating drought stress in flue-cured tobacco seedlings involves a melatonin concentration of 0.05–0.4 mmol / L, applied via foliar spraying or root irrigation.

[0007] Preferably, drought stress is characterized by a relative soil moisture content of 25% to 45%.

[0008] Preferably, the melatonin is applied during the seedling stage after transplanting.

[0009] Preferably, the melatonin concentration is 0.1 mmol / L. Quantitative analysis shows that foliar spraying of melatonin at a concentration of 0.1 mmol / L has the best allergic effect, which is scientifically reliable.

[0010] Preferably, the melatonin is sprayed or irrigated continuously for 3 days at the beginning of drought stress, once a day.

[0011] Preferably, the flue-cured tobacco includes K326 and Honghua Dajinyuan.

[0012] A method for alleviating drought stress in tobacco seedlings involves spraying or irrigating the melatonin described in any of the above applications continuously for 3 days at the initial stage of drought stress, once a day.

[0013] The beneficial effects of this invention are as follows: 1. The application of this invention enhances the drought resistance of flue-cured tobacco and significantly improves the water physiological state of flue-cured tobacco under severe drought conditions.

[0014] 2. The application of this invention restores the photosynthesis of flue-cured tobacco: increases chlorophyll content and maintains photosynthetic efficiency.

[0015] 3. The application of this invention enhances the antioxidant capacity of flue-cured tobacco: it improves the activity of the enzymatic protection system and reduces membrane lipid oxidation.

[0016] 4. The application of this invention improves the osmotic regulation ability of flue-cured tobacco: enhances the water-holding capacity of cells and alleviates dehydration damage.

[0017] 5. The present invention can be applied to the seedling stage, including the period before transplanting to the field and the drought during the seedling stage after transplanting, without being limited by specific time or scene, and the effect is obvious.

[0018] 6. The method of the present invention is simple to apply and low in cost: it is easy to operate and promote, and is suitable for field management. Attached Figure Description

[0019] Figure 1 The effects of exogenous melatonin on the biomass and relative leaf water content of flue-cured tobacco under drought stress are shown. a represents biomass, and b represents relative leaf water content.

[0020] Figure 2 The effect of exogenous melatonin on chlorophyll content in flue-cured tobacco leaves under drought stress.

[0021] Figure 3 The effect of exogenous melatonin on the production rates of malondialdehyde (MDA) and superoxide anions in flue-cured tobacco leaves under drought stress. a represents MDA content, and b represents the superoxide anion production rate.

[0022] Figure 4 The effect of exogenous melatonin on the content of osmotic regulators in flue-cured tobacco leaves under drought stress. a represents proline content, and b represents soluble protein content.

[0023] Figure 5The effect of exogenous melatonin on the activity of antioxidant enzymes in flue-cured tobacco leaves under drought stress. a represents superoxide dismutase activity, b represents peroxidase activity, and c represents catalase activity. Detailed Implementation

[0024] The preferred embodiments of the present invention will be described in detail below with reference to examples. The embodiments described below are some, but not all, embodiments of the present invention. The embodiments described below are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply.

[0025] Example 1 1. Materials and Methods 1.1 Test Materials The tested flue-cured tobacco variety K326 was provided by Yunnan China Tobacco Seed Co., Ltd. Melatonin was purchased from RUIBIO.

[0026] 1.2 Experimental Design The experiment was conducted in 2022 in Greenhouse No. 2 of the Experimental Farm of Southwest University. Intact coated tobacco seeds were selected and sown in seedling trays containing a tobacco seedling substrate for floating seedling cultivation. When the seedlings reached the stage of two leaves and one bud, the solution in the seedling trays was replaced with 1 / 2 Hoagland nutrient solution, and watering was carried out every three days. When the seedlings reached the stage of four true leaves, seedlings with uniform growth and no disease were selected and transplanted into pots (25 cm high, 23 cm in diameter) containing 10 kg of soil and sealed at the bottom. The potting soil was sandy loam with a maximum field capacity of 30%, a pH of 7.67, and an organic matter content of 6.69 g·kg⁻¹. -1 The available potassium content was 113.73 mg·kg⁻¹. -1 The available phosphorus content is 16.89 mg·kg⁻¹. -1 The alkaline nitrogen content was 63.72 mg·kg⁻¹. -1 The experimental treatment will be conducted when the tobacco seedlings enter the seedling stage.

[0027] The experiment adopted a two-factor randomized block design, and the melatonin (MT) concentration gradient was selected as 0, 0.05, 0.1, 0.15, 0.2 and 0.4 mmol / L. The application methods included foliar spraying (Y) and root irrigation (G), with a total of 14 treatments as shown in Table 1.

[0028] Table 1

[0029] When the flue-cured tobacco plants enter the seedling stage, drought treatment is carried out to maintain the soil moisture content at the specified drought value (θf=45%). Then, for three consecutive evenings, clean water and MT solution are sprayed and irrigated at a rate of 50 mL each to ensure that the tobacco plants can fully absorb the exogenous substances and reduce experimental errors. Three to four leaves from the top of the tobacco plant are taken to measure the corresponding physiological indicators. The sampling time is the 5th day after the treatment, and each treatment is repeated 3 times.

[0030] 1.3 Measurement Indicators and Methods 1.3.1 Determination of dry matter weight and relative water content of leaves Rinse the tobacco plants thoroughly with distilled water, blanch them in an oven at 105 ℃ for 30 min, and then dry them at 70 ℃ until constant weight. Weigh the plants. Relative water content of leaves: RWC = (fresh weight of leaves - dry weight of leaves) / (saturated weight of leaves - dry weight of leaves) 100%.

[0031] 1.3.2 Determination of chlorophyll content The chlorophyll content was determined using the 80% acetone extraction method.

[0032] 1.3.3 Determination of antioxidant properties The activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), malondialdehyde (MDA) content, and superoxide anion (·O2) were determined using the nitroblue tetrazolium colorimetric method, guaiacol colorimetric method, ultraviolet absorption method, thiobarbituric acid method, and hydroxylamine method, respectively. - ) Generation rate.

[0033] 1.3.4 Determination of osmotic adjustment substance indicators The contents of proline (Pro) and soluble protein (SP) were determined by the azirone colorimetric method and the Coomassie brilliant blue method, respectively.

[0034] 1.4 Comprehensive evaluation of the effect of exogenous melatonin on alleviating drought stress in flue-cured tobacco First, a correlation analysis was conducted on the changes of 10 indicators under different concentrations of melatonin in flue-cured tobacco under drought stress to compare the correlations between the indicators. Then, principal component analysis was used to reduce the dimensionality of the 10 indicators under different treatments of flue-cured tobacco and screen out comprehensive indicators for alleviating drought stress. Finally, the membership function method was used to conduct a comprehensive evaluation of drought stress relief.

[0035] 1.4.1 Membership function values ​​of each processing comprehensive index .

[0036] In the formula: X i X represents the i-th comprehensive index; min X represents the minimum value of the i-th comprehensive index; max This represents the maximum value of the i-th comprehensive index.

[0037] 1.4.2 Weights of each processing comprehensive index .

[0038] In the formula: ω i P represents the importance and weight of the i-th comprehensive indicator within its respective comprehensive indicator set; i This represents the contribution rate of the i-th comprehensive indicator for each treatment.

[0039] 1.4.3 Comprehensive evaluation of the mitigation of drought stress by various treatments .

[0040] Z represents the comprehensive evaluation value for alleviating drought stress obtained from the comprehensive index evaluation under different concentrations of melatonin treatment.

[0041] 1.5 Data Processing Statistical analysis was performed on the data using Excel 2016 and SPSS 25.0. Duncan's test was used for significance testing (P=0.05), and Pearson's test was used for correlation analysis. Graphs were generated using Origin software.

[0042] 2 Results and Analysis 2.1 Effects of exogenous melatonin on tobacco biomass and relative leaf water content under drought stress like Figure 1 As shown, compared with the control (CK) treatment, the biomass and relative leaf water content (RWC) of flue-cured tobacco under drought stress (D) decreased by 22.51% and 12.42%, respectively, both showing significant differences (P≤0.05). Compared with the D treatment, different application methods and concentrations of MT could increase the biomass and RWC content of flue-cured tobacco under drought stress. With the increase of exogenous MT concentration, the biomass and RWC of flue-cured tobacco under drought stress showed a trend of first increasing and then decreasing. Among them, the biomass and RWC of DG treatment increased by a maximum of 12.59% and 3.64% respectively compared with the D treatment, while the maximum increase of DY treatment was 16.88% and 6.76%, respectively, with significant differences (P≤0.05).

[0043] 2.2 Effects of exogenous melatonin on chlorophyll content in flue-cured tobacco leaves under drought stress like Figure 2As shown, compared with the control (CK) treatment, the chlorophyll content of flue-cured tobacco leaves under drought stress (D) was significantly reduced by 13.84% (P≤0.05). Compared with the D treatment, different application methods and concentrations of MT could increase the chlorophyll content of flue-cured tobacco under drought stress. With the increase of exogenous MT concentration, the chlorophyll content of flue-cured tobacco under drought stress showed a trend of first increasing and then decreasing. Specifically, under the DG treatment, the chlorophyll content of flue-cured tobacco in treatments T0, T1, T2, T3, T4, and T5 increased by 0.08%, 0.37%, 2.19%, 2.92%, 5.84%, and 5.55%, respectively, while under the DY treatment, the increases were 0.01%, 2.99%, 7.45%, 6.50%, 5.92%, and 3.51%, respectively.

[0044] 2.3 Effects of exogenous melatonin on the production rates of malondialdehyde and superoxide anions in flue-cured tobacco leaves under drought stress like Figure 3 As shown, compared with the control (CK) treatment, the production rates of malondialdehyde (MDA) and superoxide anions (·O2) in flue-cured tobacco leaves under drought stress (D) were significantly higher. - The levels of MDA and O2 in tobacco leaves increased by 64.42% and 28.71% respectively, both showing significant differences (P≤0.05). Compared with treatment D, different application methods and concentrations of MT all reduced MDA and O2 in flue-cured tobacco leaves under drought stress. - The content of MDA and O2 in tobacco leaves increased with increasing exogenous MT concentration. - The contents of all showed a trend of first decreasing and then increasing, with the content of MDA and O2 in flue-cured tobacco leaves increasing under DG treatment. - Compared with treatment D, the maximum reduction in content was 7.87% and 9.17%, and the differences were significant (P≤0.05). The maximum reduction in content in treatment DY was 10.97% and 12.20%, and the differences were significant (P≤0.05).

[0045] 2.4 Effects of exogenous melatonin on the content of osmotic regulators in flue-cured tobacco leaves under drought stress like Figure 4 As shown, compared with the control (CK) treatment, the contents of soluble protein (SP) and proline (Pro) in flue-cured tobacco leaves under drought stress (D) were significantly increased by 10.98% and 172.27%, respectively (P≤0.05). Compared with the D treatment, different application methods and concentrations of MT could increase the contents of SP and Pro in flue-cured tobacco leaves under drought stress. With the increase of exogenous MT concentration, the contents of SP and Pro in flue-cured tobacco leaves under drought stress showed a trend of first increasing and then decreasing. Among them, the DG treatment showed the largest increase in the contents of SP and Pro in flue-cured tobacco leaves compared with the D treatment, reaching 8.40% and 64.67%, respectively, with a significant increase in Pro content (P≤0.05). The DY treatment showed the largest increase, reaching 12.53% and 37.82%, respectively, with significant differences (P≤0.05).

[0046] 2.5 Effects of exogenous melatonin on the activity of antioxidant enzymes in flue-cured tobacco leaves under drought stress like Figure 5 As shown, compared with the control (CK) treatment, the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in flue-cured tobacco leaves under drought stress (D) were significantly increased by 49.30%, 38.87%, and 30.71%, respectively (P≤0.05). Compared with the D treatment, different application methods and concentrations of MT could increase the activities of SOD, POD, and CAT in flue-cured tobacco leaves under drought stress. With the increase of exogenous MT concentration, the activities of antioxidant enzymes in flue-cured tobacco leaves under drought stress showed a trend of first increasing and then decreasing. Among them, the maximum increase in SOD, POD, and CAT activities in flue-cured tobacco leaves under the DG treatment was 22.43%, 12.34%, and 7.08% compared with the D treatment, with a significant increase in SOD activity (P≤0.05). The maximum increase in DY treatment was 12.33%, 30.32%, and 24.51%, with significant differences (P≤0.05), and the increases in POD and CAT activities were also significant (P≤0.05).

[0047] 2.6 Correlation analysis of exogenous melatonin on various indicators of flue-cured tobacco leaves under drought stress First, a correlation analysis was conducted on the changes of 10 indicators under different application measures and different concentrations of melatonin in flue-cured tobacco under drought stress. Table 2 shows that 19 pairs of indicators were highly significantly correlated (P<0.01), and 13 pairs were significantly correlated (P<0.05). Specifically, biomass was highly significantly positively correlated with chlorophyll content, RWC, and Pro (correlation coefficients of 0.825, 0.735, and 0.841, respectively), highly significantly negatively correlated with MDA (-0.745), and significantly positively correlated with SP, POD, and CAT (correlation coefficients of 0.734, 0.686, and 0.614, respectively). It was also significantly positively correlated with ·O2. - It showed a significant negative correlation (-0.729).

[0048] Table 2

[0049] Note: This indicates a highly significant correlation at the P<0.01 level. This indicates a significant correlation at the P<0.05 level.

[0050] 2.7 Principal component analysis of the comprehensive evaluation of exogenous melatonin's role in alleviating drought stress in flue-cured tobacco Principal component analysis was performed using SPSS 25.0 to reduce the dimensionality of 10 indicators from different treatments of flue-cured tobacco. Using a cumulative variance contribution rate ≥80% and an eigenvalue ≥1 as the criterion, three principal components were extracted (as shown in Table 3), with a cumulative contribution rate of 89.827%. These three principal components can be used to summarize and analyze the ability of melatonin to alleviate drought stress in flue-cured tobacco. The first principal component showed the largest contributions from RWC, MDA, and O2. - The indicators POD and CAT are mainly related to membrane lipid peroxidation and the activity of antioxidant protective enzymes. In the second principal component, biomass, chlorophyll, SP, and Pro contribute significantly and can be defined as photosynthetic factors and osmotic regulators. In the third principal component, SOD contributes significantly, working with antioxidant enzymes in the first principal component to inhibit membrane lipid peroxidation and scavenge reactive oxygen species; both can be defined as antioxidant factors. Therefore, antioxidant factors, osmotic regulators, and photosynthetic factors can be selected as comprehensive indicators for evaluating the role of melatonin in alleviating drought stress in flue-cured tobacco.

[0051] Table 3

[0052] 2.8 Comprehensive Evaluation of the Membership Function of Exogenous Melatonin in Alleviating Drought Stress in Flue-cured Tobacco The membership function method was used to comprehensively evaluate the treatments of flue-cured tobacco. The membership function values, weights, and comprehensive evaluation values ​​Z of each treatment under the three principal components were calculated according to the formulas in 1.4.1 (1), 1.4.2 (2), and 1.4.3 (3) (Table 4). As shown in the table, the mitigation effect of exogenous melatonin on drought stress in seedling flue-cured tobacco, in descending order of Z value, is as follows: DYT2>DYT3>DYT4>DGT4>DGT2>DGT3>DGT5>DYT5>DYT1>DGT1>DGT0>DYT0>D.

[0053] Table 4

[0054] Note: The character G indicates root application, the character Y indicates leaf application, and the character D indicates drought; T0, T1, T2, T3, T4, and T5 represent melatonin concentrations of 0, 0.05, 0.1, 0.15, 0.2, and 0.4 mmol / L, respectively.

[0055] 3. Discussion Biomass accumulation is a comprehensive reflection of a plant's resistance to drought stress and an important indicator for evaluating its resilience. Drought can lead to water metabolism imbalances in plants, resulting in decreased plant water content. The relative water content of leaves more accurately reflects the true extent of drought damage. Chloroplasts are crucial sites for photosynthesis in plants, and their photosynthetic pigments are highly sensitive to environmental changes. Under drought stress, the accumulation of reactive oxygen species and membrane lipid peroxides in plants can degrade chlorophyll, leading to a decrease in chlorophyll content and severely impacting photosynthesis. In this study, drought stress significantly reduced the biomass, relative leaf water content, and chlorophyll content of flue-cured tobacco. Under drought stress, exogenous melatonin increased the biomass, relative leaf water content, and chlorophyll content of flue-cured tobacco to varying degrees, indicating that exogenous melatonin can improve leaf water status, alleviate damage to the photosynthetic system, enhance the absorption and utilization of light energy by leaves, and thus promote biomass accumulation, effectively mitigating the growth inhibition of seedling flue-cured tobacco caused by drought stress. Foliar application was more effective than root application, and irrigation with melatonin was more effective than foliar spraying in improving photosynthetic characteristics, accelerating the accumulation of photosynthetic products, and increasing soybean yield. This is inconsistent with the results of this study, which may be related to different crop varieties, growth habits, and their own endogenous melatonin content.

[0056] Under normal conditions, plant cells produce free radicals, reactive oxygen species (ROS) such as hydroxyl radicals (·OH) and singlet oxygen (·OH). 1 O2), superoxide anion radicals (·O2) - The production and scavenging of oxygen-containing non-free radical hydrogen peroxide (H2O2) are in dynamic equilibrium. Under drought conditions, this dynamic equilibrium is disrupted, leading to the accumulation of large amounts of ROS in plants. This causes loss of selective permeability of the cell membrane, leakage of intracellular electrolytes and organic matter, and membrane lipid peroxidation and defatting, resulting in damage to the membrane system and harm to crops. Among these, malondialdehyde and O2 are particularly problematic. - It can be used to represent the degree of lipid peroxidation in plant cell membranes. In this study, drought stress affected the MDA content and ·O2 content in flue-cured tobacco leaves. - The significant increase indicates damage to the leaf membranous system; under drought stress, exogenous melatonin reduced leaf MDA content and ·O2 to varying degrees. - This indicates that exogenous melatonin can reduce the accumulation of reactive oxygen species in flue-cured tobacco under drought stress, alleviate oxidative damage to cell membranes, and thus improve the drought resistance of flue-cured tobacco. MDA and ·O2 - There was a highly significant positive correlation (r=0.741, P<0.01), meaning that MDA content and ·O2 were significantly positively correlated. - There may be a synergistic relationship between them. To prevent the accumulation of reactive oxygen species (ROS) from damaging plant cells, plants possess many antioxidant mechanisms, including enzymatic and non-enzymatic protection systems. SOD, POD, and CAT are important components of the enzymatic protection system. SOD can reduce reactive oxygen species (ROS) levels by 60-70%. -The reaction generates O2 and H2O2. POD participates in the scavenging of H2O2, and CAT reacts with H2O2 via heme iron of the enzyme to generate iron peroxide active bodies, thereby scavenging excess reactive oxygen species (ROS) under plant stress. In this study, drought stress significantly increased the activities of SOD, POD, and CAT in flue-cured tobacco leaves, indicating that the tobacco's own antioxidant enzyme protection system begins to scavenge excess ROS under drought stress. Under drought stress, exogenous melatonin increased the activities of SOD, POD, and CAT in leaves to varying degrees, indicating that exogenous melatonin can enhance the scavenging capacity of ROS by increasing the activity of antioxidant enzymes, thereby maintaining the metabolic balance of the intracellular ROS system, maintaining the stability and activity of the plasma membrane, and alleviating the damage caused by drought to flue-cured tobacco.

[0057] Drought stress increases cell osmotic potential, preventing plants from absorbing water and causing water deficiency in plant tissues. Accumulating osmotic regulators such as proline and soluble protein can alleviate the damage caused by drought stress and enhance the plant's water retention capacity. In this study, the contents of soluble protein (SP) and proline (Pro) in flue-cured tobacco leaves were significantly increased under drought stress, and exogenous melatonin treatment further increased the contents of SP and Pro in the leaves. This indicates that exogenous melatonin treatment, by accumulating osmotic regulators, maintains cell osmotic potential, enhances cell water absorption capacity, reduces cell water loss, thereby protecting cell activity and improving the drought resistance of flue-cured tobacco. This also indirectly explains the increase in the relative water content of leaves.

[0058] Plant drought resistance is influenced by multiple physiological factors, and alleviating drought stress in plants also requires the combined action of multiple physiological factors. Correlation analysis in this study revealed highly significant correlations among 19 pairs of indicators (P<0.01) and significant correlations among 13 pairs (P<0.05). However, the high correlation among indicators may lead to overlap in the information extracted by different indicators. Principal component analysis (PCA), however, can transform multiple complex indicators into several independent new comprehensive indicators without loss of original information, thus reducing the information duplication caused by the one-sidedness of single indicators. While PCA has been used to screen drought resistance indicators in various plants such as sorghum, wheat, and rice, it is not commonly used as a comprehensive indicator for screening exogenous substances to alleviate drought stress in plants. In this study, three principal components can be used to summarize and analyze the ability of melatonin to alleviate drought stress in flue-cured tobacco. Among them, antioxidant factors, osmotic regulatory factors, and photosynthetic factors can serve as comprehensive indicators for melatonin's ability to alleviate drought stress in flue-cured tobacco. By comprehensively evaluating different treatments using membership functions, the results showed that the optimal treatment for alleviating drought stress in tobacco seedlings was foliar spraying with 0.10 mmol / L melatonin.

[0059] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. The application of exogenous melatonin in alleviating drought stress in tobacco seedlings, characterized in that, The melatonin concentration is 0.05–0.4 mmol / L, and the application methods include foliar spraying or root irrigation.

2. The application according to claim 1, characterized in that, Drought stress is characterized by a relative soil moisture content of 25% to 45%.

3. The application according to claim 1, characterized in that, The application period for melatonin includes the seedling stage after transplanting.

4. The application according to claim 1, characterized in that, The melatonin concentration was 0.1 mmol / L.

5. The application according to claim 1, characterized in that, The melatonin was sprayed or irrigated continuously for 3 days at the beginning of drought stress, once a day.

6. The application according to claim 1, characterized in that, Flue-cured tobacco varieties include K326 and Honghua Dajinyuan.

7. A method for alleviating drought stress during the seedling stage of flue-cured tobacco, characterized in that, The melatonin described in any of claims 1 to 6 is sprayed or irrigated continuously for 3 days at the beginning of drought stress, once a day.

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