Evaluation of Influence of Selenium Foliar Application on Growth and Physiological Characteristics of Basil (Ocimum basilicum L.) in Arsenic-Contaminated Soil

Basil (Ocimum basilicum), as an annual plant, belongs to the family Lamiaceae, native to Asia (Iran, India, Pakistan, Thailand, etc.) and grows in warm and semi-warm regions. Arsenic (As) is a highly dangerous heavy metal for plants and human. This quasi-metal is widely distributed in the earth's crust, and also is the twentieth most abundant element in the earth's crust. Selenium (Se) is a non-metallic chemical element and also has similarities to arsenic. Global interest in the biological effects of selenium on the environment and the food chain is growing because selenium is essential as a micronutrient for many organisms, including humans and other animals (although it is toxic in high concentrations). Given that arsenic is one of the most important factors limiting crop production in the agricultural sector, it seems necessary to conduct researches in this field to eliminate or reduce the toxic effects of these element on agricultural plants.           

In this study, the effect of selenium on increasing the tolerance rate of basil plant against arsenic heavy metal stress was investigated as a factorial experiment based on completely randomized design under greenhouse condition. The first factor included arsenic-contaminated soil (4576 mg kg-1) and non-contaminated soil, and the second factor included different concentrations of selenium treatment (0, 5 and 10 mg L-1 selenium sodium). Foliar application of selenium was conducted at two stages. The first stage of foliar application was carried out at 4-leaf stage and the second stage of foliar application was done two weeks after the first stage. Leaf number, leaf area, stem height, fresh and dry weight of leaf and stem, chlorophyll and carotenoid content, free proline content, phenol, flavonoid and anthocyanin content, electrolyte leakage and antioxidant enzymes activity were evaluated at the end of the experiment.

The results showed that arsenic stress negatively affected the morphological traits such as leaf number and area, fresh and dry weight of leaf, fresh and dry weight of stem and plant height, but selenium treatment alleviated the toxicity of arsenic and increased the values of these traits, so that treated plants had higher values of morphological characteristics comparing to control plants under arsenic heavy metal stress. The highest values of morphological traits were observed in plants treated with the highest concentration of selenium (10 mg L-1). In terms of physiological traits, arsenic heavy metal stress reduced the values of some of these traits, so that the lowest amounts of chlorophyll a, chlorophyll b and total chlorophyll, anthocyanin, phenol, and flavonoids were recorded in plants grown in arsenic-contaminated soil without foliar application of selenium. On the other hand, the lowest activity rates of guaiacol peroxidase, catalase and ascorbate peroxidase were observed in plants grown in non-contaminated soil and without selenium foliar application. Also, the both arsenic stress and selenium treatment increased proline accumulation and soluble sugars content of treated plants comparing to control plants. Heavy metal stress increases the production and accumulation of reactive oxygen species (ROS) and damages the membrane structure resulting in more electrolyte leakage. Selenium application led to increasing plant resistance rate against arsenic stress, mainly by enhancing cells antioxidant capacity and osmotic potential.

Considering basil is one of the most important aromatic vegetables which has fresh consumption, crispness and wateriness of its leaves and young shoots is one of the most important commercially traits which directly affect crop marketability. Considering arsenic stress has a negative effect on leaf morphological traits (number, area, fresh weight and dry weight) and stem characteristics (fresh weight, dry weight and height), it is recommended to use selenium foliar application as an efficient technique to overcome adverse effects of arsenic stress on basil production in regions with contaminated soils.