The effect of humic acid foliar application on some morphophysiological and biochemical characteristics of spearmint (Mentha spicata L.) in drought stress conditions

Introduction Spearmint is one of the plants that attracted the attention of researchers because of its economic and pharmaceutical importance. This plant is one of the most important and most famous species of mint which disperses in different parts of the world. One of the scarce resources that is affected by rainfall is water. The effect of water stress depends on the duration, durability and amount of deficiency. One of the most important factors limiting the growth of plants around the world is drought, which is the most common environmental stress. One of the most effective mechanisms that is used by plant in water stress condition is osmotic regulation. The accumulation of proline and soluble sugars (as the most important osmolite) in regulation of osmotic is done to dominate the negative effects of drought stress. Results of researches have shown that drought stress has effective effect on the growth and biochemical characteristic of plants, and it differs among different plants. Optimum plant nutrition is one of the effective methods for plant resistance in drought conditions. Humic acid is one of the most important fertilizers in the agriculture. It creates more space for water penetration by modifying physical soil characteristic and improving soil granulations. Humic acid increases the activity of photosynthesis by increasing the activity of the Robisco enzyme. To consider the high importance of spearmint as a valuable vegetable and medicinal plant, the aim of this study was to estimate the resistance of spearmint to different levels of drought stress and evaluate the effects of foliar application humic acid as a moderator factor in drought stress condition. Materials and methods A pot research was conducted in factorial experiment in a completely randomized design with 3 replications to investigate the effects of humic acid as a growth factor and moderator of the effect of drought stress, on Mentha spicata L. at the Faculty of Agriculture, Ferdowsi University of Mashhad. The first factor was three levels of drought stress (30, 70 and 100 % FC) and the second factor was four levels of humic acid (0, 100, 500 and 1000 mgL-1). At the firstly, spearmint rhizomes with 10 cm length were prepared with at least 2 buds. In the middle of March, about 4 rhizomes were cultivated in depth of 5 cm of pots soil. Foliar application of humic was used at three stages (one week before drought stress and one and three weeks after applying drought stress) on the plant surface. We didn’t use any humic acid on the control treatment. About 4 weeks after humic acid application, the morphological, physiological and biochemical traits were measured at flowering stage. Results Vegetative traits such as plant height, number of branches, leaf length, leaf width and fresh and dry weight of leaf and stem showed a significant decrease with increasing drought stress (70% of field capacity) in compared to control treatment. Foliar application of humic acid, especially 1000 mgL-1 improved these traits in compared to control plants in drought stress condition. Also, the mean comparison of the interaction treatments showed that the highest amounts of photosynthetic pigments was observed at 70% FC and application of 1000 mgL-1 of humic acid and the least one was belonged to control treatment and application of 500 mgL-1 of humic acid. The highest antioxidant activity of leaf extract (80.66%) and total phenol (0.87 mg/g fresh weight) were observed in plants treated with 1000 mgL-1 humic acid and 70% FC. The highest relative water leaf content was obtained at non-stress conditions (control) and the foliar application of 1000 mgL-1 of humic acid. Conclusion This study showed that spearmint is very susceptible to drought stress, which could not withstand drought stress of 30% FC even for a short time. In general, means comparison of results showed that drought stress significantly reduced the morphological traits of spearmint. Also, the interaction treatments showed that the using of 1000 mgL-1 humic acid had the greatest effect on the growth indices. The highest amounts of photosynthetic pigments was observed at 70% FC and foliar application of 1000 mgL-1 of humic acid and the lowest one was belonged to control treatment of humic acid and without stress condition. Also, the highest levels of total phenol and antioxidant activity were observed in 70% FC and 1000 mgL-1 humic acid treatment. Therefore it can be stated that spearmint is high susceptible plant to drought stress because of sever effects of drought stress on its growth characteristics and physiological processes. The foliar application of humic acid, especially at high concentration (1000 mgL-1) can be effective in regulating the osmotic potential under slightly drought stress conditions. So, it can be used as an organic fertilizer in sustainable agriculture.