Introduction
Drought stress as a non-biological stress in nature has several dimensions and affects plants at different levels. During prolonged drought stress, the amount of water in the plant cells decreases and causes thorogenesis, which eventually leads to plant death. The concentration of solutions in the cytosol and extracellular matrices increases due to drought stress in plants and cell development decreases as a result, which leads to growth inhibition and failure to reproduce. Subsequently, the plant withers with the accumulation of abscisic acid and compatible osmolytes such as proline. Overproduction of reactive oxygen species and formation of scavenging compounds of radicals such as ascorbate and glutathione take place at this stage. Drought stress also affects stomatal closure, physiological traits, gas exchange, reduced transpiration, and reduced carbon assimilation (photosynthesis). Drought stress also affects stomatal closure, physiological traits, gas exchange, transpiration reduction, and carbon assimilation reduction (photosynthesis). Negative effects on mineral metabolism and nutrition (absorption and transport of nutrients) reduce leaf area and change the distribution of assimilates between organs.

Organic matter is essential for fertility and preservation of soil biological, chemical and physical properties. Recently, much attention has been paid to the possibility of using organic matter to improve plant growth. Humic substances (humic acid and folic acid) are among the organic substances that are used as growth regulators under suitable irrigation conditions and drought.
In recent years, the number of studies on melatonin in plants has increased significantly. The wide range of functions in different organisms indicates the potential of this substance in plant physiology. Its role as an anti-stress agent against stressors such as water, salinity, low temperature and high temperature, UV and toxic chemicals has been analyzed. The antioxidant role and growth promoter (among other melatonin functions in the plant) is very important. Melatonin is a polar molecule with diverse physiological and cellular functions. Melatonin has received a great deal of attention in recent years, as research has shown that melatonin is widely found in the leaves, roots, stems, fruits, and seeds of all plant species.
To date, numerous applications of fulvic acid and melatonin in agriculture, food industry and animal sciences have been proposed, but unfortunately few studies have been conducted on the simultaneous use of fulvic acid and melatonin in agriculture, which necessitates research in this area. Therefore, considering the economic importance and increasing demand for small fruits, especially strawberries, in this study, the effect of fulvic acid and melatonin on the biochemical properties of strawberries under water stress was investigated.
Materials and methods
This experiment was performed in Bostanabad city (East Azerbaijan province) in Iran. Strawberry seedlings (Kamarosa cultivar) were obtained from a commercial strawberry growing greenhouse in Urmia in October and transferred to the greenhouse in cultivation trays. Laboratory studies were performed in the laboratories of the Department of Horticulture and Soil Science, Faculty of Agriculture, Mohaghegh Ardabili University. This experiment was performed as a factorial experiment in a completely randomized design with 3 factors in 4 replications. Factors include water stress at three levels of 100 (full irrigation (control), 25 and 50%, maintainable water drain), fulvic acid at three levels (control, 200 and 400 mg/l) and melatonin at three levels (control, 75 and 150 μM). After complete establishment and cooling of plants, melatonin and fulvic acid were applied to the treatments in three stages through foliar application. The first stage was performed after complete establishment of the plant (15 days after planting), the second stage was performed 10 days after the initial time of treatment and the third stage was performed 10 days after the second stage of foliar application. It should be noted that distilled water was used to treat the plants in the control group.
In order to apply water stress to the plants, at first the soil is completely dried (to decrease the effect of moisture on the weight of the soil in the pot), then the same amount of coarse sand was poured on the floor after weighing to create sufficient drainage in the pots. The pots were then placed on a scale (accurate to one tenth of a gram) and the soil mixture was poured into all of them. Then one of the pots was selected randomly and the soil inside it was completely submerged and covered with plastic and allowed the gravity water to come out. Because this part of the soil moisture is useless for the plant and must be drained. The pot was weighed and taken daily for a week (until the weight of the pot was fixed) after the soil moisture reached the field capacity. Then the soil inside the pot was emptied in a special container and the mass of the container along with the soil inside it was recorded and dried in an oven at 105 ° C for 24 hours. After removing from the oven, the soil with the container was weighed again and the difference between these two masses (before the oven and after the oven) showed the mass of water (Mw) of the soil in the pot in the last stage of the experiment. The mass of the container was deducted from the mass of the soil sample along with the container after the oven and the mass of dry soil (Ms) was calculated. Finally, the mass moisture content (θm) was obtained using the following equation.

Measurement and amount of Mw according to the relationship between humidity (mass moisture (θm)) at that stage, was obtained after obtaining moisture in the last stage of the experiment and adding the mass difference of the pots in each time period. (Given that the difference in the mass of the pots on each successive day indicates the amount of water evaporated and the amount of dry soil at each stage is constant). By obtaining the mass of the pot and the mass moisture, a linear relationship between them was drawn by Excel software. The moisture content of the pots was calculated by the regression relationship with the mass of the pots at the stress levels.
Results
Analysis of variance of data showed that the effect of drought stress and its interactions with fulvic acid and melatonin treatment on yield, RWC, electrolyte leakage and antioxidant enzymes activity were statistically significant. The interactions between treatments (drought stress, fulvic acid and melatonin) were statistically significant and only the triple effects of catalase were not statistically significant. The results showed that the drought satress (25 and 50%, maintainable water drain) without the application of fulvic acid and melatonin ecreased yield, RWC and increased electrolyte leakage. But application of fulvic acid and melatonin increased the yield and RWC under drought stress conditions. Furthermore, the application of fulvic acid and melatonin reduced the oxidative damage (electrolyte leakage) in the plants through increased enzymatic activity under drought stress conditions. Generally, the highest and lowest yield was obtained in control plants (full irrigation) with foliar spraying with melatonin (150 µM) and 50%, maintainable water drain which were 693.54 and 504.78 g, respectively. The highest and lowest ascorbate peroxidase activity was obtained in 50%, maintainable water drain with foliar spraying melatonin (75 µM) and control plants (full irrigation) with foliar spraying melatonin (75 µM) and fulvic acid (400 mg/l) which were 2.929 and 1.426 µmol min-1 gFW-1, respectively. The highest and lowest peroxidase activity was obtained in 50%, maintainable water drain with foliar spraying melatonin (75 µM) and fulvic acid (200 mg/l) and control plants (full irrigation) with foliar spraying fulvic acid (400 mg/l) which were 2.411 and 0.58 µmol min-1 gFW-1, respectively. The highest and lowest superoxide dismutase activity was obtained in 50%, maintainable water drain with foliar spraying melatonin (150 µM) and fulvic acid (400 mg/l) and control plants (full irrigation) with foliar spraying fulvic acid (200 mg/l) which were 96.93 and 36.18 µmol min-1 gFW-1, respectively. The highest and lowest level electrolyt leakage leaves also were observed in 50%, maintainable water and control plants (full irrigation) with foliar spraying melatonin (150 µM) and fulvic acid (200 mg/l) which were 96.93 and 36.18 respectively. The highest and lowest relative water content (RWC) were obtained control plants (full irrigation) with foliar spraying with melatonin (150 µM) and 50%, maintainable water drain which were 95.35 and 53.92%, respectively. The highest and lowest catalase activity was obtained in 50%, maintainable water drain with foliar spraying fulvic acid (400 mg/l) and control plants (full irrigation) which were 0.7108 and 0.4946 µmol min-1 gFW-1, respectively.
Conclusion
Overall, the results of the present study suggest that the use of melatonin and fulvic acid may be an effective approach to improve the yield of strawberry plants under drought stress. Melatonin and fulvic acid helped to strengthen the antioxidant enzymes system of strawberry plants, which reduced the oxidative damage. Also, foliar application of melatonin and fulvic acid had a protective role on strawberry plants grown under drought stress and its optimal concentration will be useful in increasing drought tolerance in many crops.