Effect of Potassium Sulphate and Humic acid on Growth, Yield and Essential Oil Content in Hypericum perforatum L.

Introduction: Medicinal and aromatic plants can play an important role in commercial crops, which also represent a safe alternative for chemical pharmaceutical industries. St. John’s Wort (Hypericum perforatum L.) due to its therapeutic efficacy has been used for decades in folk medicine and is considered as a promising medicinal plant with valuable potential as a source of hypericin, essential oils and antioxidants. Studies on agronomic factors such as application of potassium and humic acid as well as nitrogen fertilization on yield, essential oil and antioxidant activity of Hypericum perforatum have not been investigated thoroughly until now. This study was designed to study the effect of using humic acid and potassium sulphate on morphological and phytochemical characteristics of Hypericum perforatum. Materials and Methods: The plant material was prepared from Science and Technology Park in Khorasan-e Razzavi–Mashhad. In spring, the plants were transplanted into the field of Horticulture department, Ferdowsi University of Mashhad, for fertilizing treatments. The experimental layout was factorial in a complete randomized design (CRD), with three replications. Potassium Sulphate (K2SO4) was applied at the rates of 0.0 (K0), 60 (K60) and 100 (K100) kg ha-1. The other treatment were humic acid, which was applied at three rates 0 (H0), 20 (H20) and 40 (H40) L ha-1. Plants were treated at two stages, before flowering by potassium fertilizer and fertigated four times in 15-day intervals with humic acid. Besides the fresh and dry weight, number of flowers, stem height and number of flowering stems were determined at the end of the growing stage. Results and Discussion: The ANOVA indicated that most of measured attributes of Hypericum perforatum plant were significantly affected by both treatments. Data presented revealed that the highest stem (91.6 cm) was recorded at the highest level of potassium sulphate (K100) treatment, while the shortest height (60.4 cm) was recorded at the lowest level of potassium sulphate (K0) treatment. On the other hand, the highest stem (80.33 cm) was achieved with the highest level of humic acid treatment. No significant difference was observed between the highest level of humic acid (H40) and the second level (H20), while both mentioned levels showed a significant difference with control (H0). This means that the control plants possessed the lowest stem height (66.7 cm). The DPPH method for evaluation of antioxidant activity reflects the ability of the present compounds to scavenge hydrophilic free radicals. DPPH inhibition was investigated and the results were evaluated as relative activities against α-tocopherol as positive control. A lower EC50 indicates a higher antioxidant activity. As it is shown, the least EC50 which indicates the highest antioxidant activity belonged to plants treated with potassium sulphate at (100 kg.ha-1). Control plants possessed the least antioxidant activity and showed no significant difference compared with plants treated with 60 kg.ha-1 of this fertilizer. Humic acid had a significant effect on antioxidant activity of H. perforatum. The lowest value of EC50 was related to the highest level of humic acid, while the highest value of EC50 was obtained from control plants. There was also significant difference between control and two other levels (20 and 40 L ha-1) of humic acid. Applied fertilizers at their high levels showed significant effects on EC50 decrease, which means the increment of antioxidant activity of H. perforatum. The high antioxidant activity of this medicinal plant is the reason of flavonoids and phenols existence. It seems that application of these fertilizers have led to increment of the mentioned secondary metabolites and followed by an increase of antioxidant activity in comparison with control plants. Totally, H. perforatum possessed a high antioxidant activity. Based on recorded data of the present study, application of the mentioned fertilizers separately or simultaneously caused the increase of this important medicinal plant fresh and dry weight, and finally the yield. Number of flowers, stem height and flowering stems reached the highest value at the highest level of both fertilizers (K100H40). As potassium plays an important role in the development of reproductive parts of plants, it might be one of the reasons of the above mentioned increase. The highest essential oil content was recorded at high levels of both fertilizers (K100H40). Since the highest essential oil content accumulates in the flowering stems, an increase of these stems height and a higher number of flowers would lead to an increase of essential oil content of H. perforatum. Antioxidant activity of this plant is flavonoids contributed. It seems that application of these fertilizers increased the antioxidant activity by an increase of this pigment content.