Effects of foliar application of sodium nitroprusside, kaolin and potassium on yield and some biochemical charectrations of Thymus aenensis Celak under dry farming conditions and complementary irrigation

In producing medicinal plants, it is critical to use plants with a high percentage of active ingredients and consume low amounts of water. Thymus daenensis Celak is a tolerant Thymus species for rainfed cultivation. In addition, using complementary irrigation regimes and applying anti-stress compounds at critical stages of plant growth are suitable programs for sustainable production and optimal water consumption under dry farming conditions. For this purpose, this experiment was to evaluate the effect of different complementary irrigation regimes along with sodium nitroprusside (SNP) as a nitric oxide donor, kaolin, and potassium spraying on yield and some biochemical characteristics of T. daenensis under rainfed conditions. This experiment was designed and implemented as a split plot based on a randomized complete blocks design with three replications in the research field of the Research Institute of Forests and Rangelands-Homand Rangelands Research Station (Damavand) for three years (2017-2019 crop years). The experimental treatments included complementary irrigation as the main factor at three levels: control (no irrigation), one and two complementary irrigation, and spraying of stress-modulating substances as a secondary factor with three compounds of SNP (200 µM), kaolin (5%) and potassium (75 kg/ha from a potassium sulfate source). Cultivation was indirect and through seedlings. To prepare seedlings, the seeds were planted in the cultivation trays in the greenhouse at the end of September. After growing in the greenhouse, the seedlings (8-10 leaves) were transferred to the field in November with rain monitoring. Complementary irrigation and different foliar spraying treatments were done twice in the pre-flowering and 10 % flowering stages. Treatments were applied in the third year of plant growth to increase economic yield. After applying the treatments, the plant's flowering branch was sampled to measure malondialdehyde (MDA). After measuring the morphological characteristics (plant height and canopy diameter) in the full flowering stage, plant shoots were harvested to determine the dry matter yield per square meter and hectare. Percentage and yield of essential oil, total phenol, total flavonoids, and antioxidant capacity were also determined. The results showed that increasing the number of complementary irrigations increased the crown diameter, plant height, and dry matter yield per plant and hectare. Kaolin treatment under two complementary irrigation regimes had the highest height (31.6 cm), and the control treatment under no irrigation had the lowest height (19.3 cm). Also, using kaolin and two complementary irrigations showed the highest dry matter yield per m2 (77.2 grams) and per hectare (771.4 kg). In addition, MDA content was lower in the double complementary irrigation treatment with kaolin foliar spraying (1.22 μmol/g of fresh weight). Foliar spraying of kaolin, potassium, and SNP under rainfed conditions (without irrigation) increased the amount of essential oil (by 1.83, 1.67, and 1.6 percent, respectively) compared to the control treatment (1.4 percent). In contrast, the maximum essential oil yield per hectare was related to the double complementary irrigation regime with potassium foliar spraying (10.6 kg/ha). Foliar spraying with potassium in the regimes of without irrigation (rainfed) and one-time complementary irrigation had the highest content of total phenols and flavonoids, respectively. The highest antioxidant capacity was obtained without irrigation and foliar spraying (control). This experiment showed that complementary irrigation increased dry matter yield per plant and hectare. On the other hand, foliar spraying with anti-stress compounds, especially kaolin, and potassium, along with complementary irrigation, increased essential oil yield per hectare. In general, based on the results of this experiment, it is suggested to use complementary irrigation regimes according to the available water sources. In addition, it is suggested to use kaolin or potassium foliar application under rainfed conditions for T. daenensis.