Effect of amino acid foliar application on morphophysiological characteristics and thyme essential oil under different irrigation regimes

In order to study foliar application of amino acids on morphophysiological properties and thyme essential oil under different irrigation regimes, an experiment was conducted as a split plot based on a randomized complete block design with three replications in the Research Farm of Tarbiat Modares University Faculty of Agriculture in the year 2020 crop season. Experimental factors were irrigation regimes at three levels including normal irrigation, moderate water stress, severe water stress (withholding irrigation until  25%, 45% and 65% of soil available water depletion in the root zone respectively and then irrigation up to field capacity from plant establishment to harvest time), were randomized to the main plot units and spraying at 6 kinds (aspartic acid, tyrosine, proline, commercial amino acids, distilled water and without spraying) were located in subplot units. The results showed that with reduction of irrigation to the level of severe water stress, plant height, fresh and dry weight of leaves, leaf area, total chlorophyll, percentage of essential oil decreased, and glycol peroxidase and proline showed a significant increase compared to optimal irrigation. Catalase activity also increased under moderate water stress and decreased under severe water stress. Foliar application of amino acids had a positive additive effect on the studied traits. The highest essential oil yield (about 11.20 kg/ha) was assigned to plants under commercial and proline amino acid foliar application under full irrigation conditions. Based on the general results of proline foliar application, increasing the concentration of osmolites such as proline under water stress conditions was able to help the plant withstand drought stress by helping to maintain osmotic pressure and compensate for part of the reduction in growth and dry weight. Therefore, consumption of this type of amino acid under stress conditions is recommended to reduce the effects of drought stress. Under non-stress conditions, foliar application of this type of amino acid had the greatest effect on plant growth and yield.