Effect of Salicylic Acid and Humic Acid on the Yield and Antioxidant Activity of Garlic

Abstract
In this research, the effect of humic acid fertigation in two levels (0 and 20 kg.ha^-1) and salicylic acid spray in four levels (0, 0.5, 1 and 1.5 mM) on the yield, yield component, and antioxidant activity of garlic was evaluated. Based on obtained results, all salicylic acid treatments caused higher yield in comparison with control significantly, so that treatment of 1 mM salicylic acid elevated the yield more than 2 tons per hectare by comparison with control. Also humic acid led to yield increment of garlic around 5 tons per hectare by comparison with control. The highest weight of garlic was obtained in humic acid + salicylic acid 1 and 1.5 mM treatments which produced 105 and 106.66 g respectively, around 2.5 times more than control. Single treatment of salicylic acid in low and middle concentrations (0.5 and 1 mM), and high concentration of salicylic acid (1.5 mM) alongside with humic acid contributed to antioxidant capacity improvement around 20 percent more than control. Finally, it can be concluded that humic acid 20 kg.ha^-1 + salicylic acid 1.5 mM for increasing garlic yield in the open field culture is recommended.
Introduction
Garlic (Allium sativum L.) is the second most important crop among Allium species. This plant has important medicinal properties such as lowering blood cholesterol, lowering blood pressure, preventing the formation of blood platelet masses, antimicrobial, antifungal, anticancer effects, stimulating the immune system, anti-inflammatory effects, and high antioxidant properties. Many efforts have been made to increase the quantitative and qualitative properties of garlic. Among the effective treatments on plants, the role of humic substances in increasing plant growth and plant biomass has been reported frequently. Although this positive impact can be affected by the concentration, source, plant species, plant age and cultivation conditions, the direct role of humic acid in increasing the photosynthesis rate and plant growth has been well documented. Also, many studies have reported the effect of humic acid on reducing the damage caused by environmental stress, but the exact mechanism of this effect is not yet known. On the other hand, salicylic acid is another substance which is a phenolic compound in plants and acts as a hormone. Salicylic acid plays an important role in regulating plant growth and development. Also, the role of salicylic acid in the absorption of nutrients, membrane stability, water relations, stomatal function, inhibition of ethylene synthesis, and growth enhancement has been reported. Considering the positive role of humic acid and salicylic acid on the performance and physiology of plants, in this work, the simultaneous application of these two substances was investigated in garlic.
Material and Methods
This research was carried out in factorial format based on randomized complete block design (RCBD) with two factors and three replications. The first factor was humic acid fertigation in two levels (0 and 20 kg.ha^-1) and the second factor was salicylic acid spray in four levels (0, 0.5, 1 and 1.5 mM). Each of the mentioned treatments were applied in four times in May when the bulbs were grown. Evaluated parameters included yield, yield component, and antioxidant activity. Traits related to yield and its components, including yield of the whole plant (edible part with aerial part), weight of single bulb, weight of clove, length and width of clove and number of cloves were calculated at the end of the experiment. Diphenylpicrylhydrazyl stable radical or DPPH was used to measure total antioxidant activity. Total phenol and total flavonoid contents were measured as antioxidant substances and dietary value properties. The comparison of means was done using Duncan's multiple range test at the probability level of one and five percent.
Results and Discussion
Based on obtained results, all salicylic acid treatments caused higher yield in comparison with control significantly, so that treatment of 1 mM salicylic acid elevated the yield more than 2 tons per hectare by comparison with control. Also, humic acid led to yield increment of garlic around 5 tons per hectare by comparison with control. The highest weight of garlic was obtained in humic acid + salicylic acid 1 and 1.5 mM treatments which produced 105 and 106.66 g respectively, around 2.5 times more than control. Single treatment of salicylic acid in low and middle concentrations (0.5 and 1 mM), and high concentration of salicylic acid (1.5 mM) alongside with humic acid contributed to antioxidant capacity improvement around 20 percent more than control. In general, it can be concluded that the optimal doses of salicylic acid can have a positive effect on photosynthesis and ultimately on growth. Because of its water retention properties, humic acid can help the movement of elements in the soil, increase soil microbial population, release some nutrients, improve soil properties, increase soil organic matter, and can be used to increase crop yield. Because of this, the combination of these two factors (salicylic acid and humic acid) could lay positive effects on different aspects of garlic growth and development. It seems that the increase in antioxidant capacity following the treatment of high concentration of salicylic acid occurs when some substances, including nutritional elements, are also available, and the presence of humic acid can respond to this need.
Conclusion
It can be concluded that humic acid 20 kg.ha^-1 + salicylic acid 1.5 mM for increasing garlic yield in the open field cultivation can be recommended. This combined treatment showed a high antioxidant activity as well, so humic acid 20 kg.ha^-1 + salicylic acid 1.5 mM was the best treatment both in quantitative and qualitative aspects.