Evaluation of Growth, Yield and Morphological and Physiological Characteristics of Five Mint (Mentha spp.) Species in the Aeroponic System under Greenhouse

The aeroponic system is a promising technique for the future of agriculture, growing plants in an air or mist environment without the use of soil or an aggregate medium. In aeroponic, plants are suspended in a closed or semi-closed environment by spraying the plant's dangling roots and lower stem with an atomized or sprayed, nutrient-rich water solution (30). Increased aeration of nutrient solution delivers more oxygen to plant roots in aeroponic in compare to conventional hydroponic systems, stimulating growth and helping to prevent pathogen formation (1). Cultivation of medicinal plants under controlled conditions of aeroponic and hydroponic systems commercially provides a better result in terms of quality improvement, bioactivity and biomass production (5, 4).Mint (Mentha) is one of the most important genus of Lamiaceae family. Different species of the genus have high economic value, due to their active and aromatic substances and are used as raw materials in food, cosmetics, health, beverage and pharmaceutical industries. M. piperita, M. spicata and M. suaveolens are the most common and popular species of the genus for cultivation. M. aquatica is also a perennial plant of the genus that is cultivated in Europe, North and Northwest Africa and Southwest Asia (2). Aeroponic system is more cost effective than other systems. Because of the reduced volume of solution throughput, less water and fewer nutrients are needed in the system at any given time compared to other nutrient delivery systems. However, due to lack of research and sufficient technical information, the use of aeroponic system is not common among farmers and greenhouse owners. In addition, many tips are not yet scientifically known about the cultivation of different plant species in this system (4). Despite the benefits of aeroponics, no research has been previously conducted on the cultivation of different species of mint in this system. Therefore, the present study was conducted with the aim of investigation of growth, yield and morpho-physiological characteristics of five mint species in the aeroponic system.

This study was conducted in greenhouses of faculty of Agriculture and Natural Resources of Lorestan University in 2020. The experiment was performed as a split plot design. Mentha species (including M. aquatica, M. pulegium, M. spicata, M. suaveolens and M. piperita) were considered as the main plot and harvest time (first and second harvests) was used as sub-plot. In each experimental block, one row (with ten plants) of five mint species was planted. In the early stages of growth, plants were fed with half Hoagland nutrient solution and then with complete Hoagland solution. Finally, morphological and physiological traits and some biochemical characteristics of plants were measured. After harvesting the first batch of plants and in order to better evaluation of species in the aeroponics system, all studied traits were measured about two months after the first harvest. Analysis of variance was performed based on the experimental design using SAS software. The comparison of means was also done with the least significant difference (LSD) test at the level of 5%.

The results showed that M. spicata and M. suaveolens had the highest leaf to stem ratio. The highest stem dry weight and plant height were related to M. pulegium. While, the highest leaf area, leaf fresh and dry weight, stem fresh and dry weight, stolon fresh and dry weight, shoot fresh and dry weight, total plant dry weight and relative water content were related to M. aquatic, followed by M. piperita. In addition, M. piperita had shown the highest fresh root weight. The highest amount of carotenoid and chlorophyll, photosynthesis rate and CO2 under the stomata was obtained in M. piperita. In this experiment, most of the yield-attributes traits including leaf area, leaf fresh and dry weight, root fresh weight, shoot fresh weight and total plant dry weight, as well as relative water content and photosynthesis rate were higher in the second harvest than the first one.

All studied species in this experiment were successfully growth in the aeroponic system. Due to high water use efficiency, no need for soil and high crop production, mint cultivation in the aeroponic system can be a healthy and profitable alternative to in soil cultivation approach. According to the results of the present study, M. aquatica has shown higher performance than other species in terms of the yield attributes traits (including leaf number, leaf area, fresh and dry weight of plant, fresh and dry weight of shoots and fresh and dry weight of leaves). Among the other species, the highest shoot dry weight and plant dry weight was obtained by M. piperita. Finally, the high potential of M. aquatica and M. piperita for cultivation in the aeroponic system can be concluded.