Document Type : Original Article

Authors

• Somaye Asadi ¹
• Mohammad Moghaddam ¹
• Abdollah Ghasemi Pirbaloti ²
• Amir Fotovat ³

¹ Department of Horticulture Science, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran

² Research Center for Medicinal Plants, Shahr-e-Qods Branch, Islamic Azad University, Shahr-e-Qods, Tehran, Iran

³ Department of Soil Science, Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran

Abstract

Introduction
The presence of heavy metals in the environment of plants is a stressful agent that causes physiological changes and can reduce plant growth potential and, in a more severe condition, cause it to disappear. Lead is one of the toxic metals for humans, as well as unnecessary for plants. Due to the solubility of this element in water, it is easily absorbed by the root system of the plant. In this way, the growth and metabolism of the plants are affected by the increase of these metals in the environment. This metal disrupts plant growth, root growth, seed germination, seedling development, transpiration, and cell division. Lead toxicity reduces the growth and yield of the product, jaundice of young leaves, decreases the absorption of certain essential elements, such as iron and reduces photosynthesis.
The use of plant growth regulators to reduce the absorption and accumulation of heavy metals in plants can be a suitable method for reducing the related damages. Jasmonates play a regulative role in the development and response to environmental stresses and reduce the damage caused by these stresses in the plant.
Basil (Ocimum basilicum L.) belongs to the family Lamiaceae and is one of the medicinal plants and valuable herbs that are cultivated throughout the world. This plant is widely used in medicine and industry. The purpose of this study was to investigate the effect of lead toxicity on growth characteristics and its absorption in aerial parts and roots and the effect of methyl iasmonate as a growth regulator on these characteristics.

Material and methods
This research was carried out in greenhouse of Faculty of Agriculture, Ferdowsi University of Mashhad during spring and summer of 2015, as a pot factorial experiment in a completely randomized design with three replications. The treatments consisted of three levels of lead nitrate (Pb(NO3)2) (0 (control), 200 and 400 mg/kg soil) and three levels of methyl jasmonate (zero (control), 0.5 and 1 mM ) which was applied as spraying. The evaluated traits included plant height, inflorescence length, number of branches, number of nodes, internode length, leaf length and width, stem diameter, fresh and dry weight of aerial parts and root fresh and dry weight, as well as lead accumulation in aerial parts and roots were measured at flowering stage.

Results and discussion
The results of analysis of variance showed that the interaction of treatments on the length of flowering, number of branches, fresh and dry weight of aerial part and root fresh weight at 5% probability level, and on root dry weight and lead absorption in aerial part and root of the plant was significant at 1% probability level. Whereas the interaction of treatments on plant height, leaf length and width, stem diameter, number of nodes and internode length was not significant. In plants treated with lead, number of branches per plant, inflorescence length, fresh and dry weight of aerial part and root showed a significant decrease compared to control plants and application of 0.5 mM methyl jasmonate in lead stress conditions improved these traits. Growth of plants in the medium containing lead was caused accumulation this element in the aerial part and root, so that the lead concentration in the aerial part was higher than the root, and methyl jasmonate in both concentrations reduced the accumulation of lead in the root and aerial parts of basil. Khudsar et al. (2000) reported that the interaction of heavy metals with sulfidryl groups and the inactivation of plant proteins prevents growth of aerial part and root. Disruption of hormone activity, such as auxin, in heavy metals treatment can also reduce plant growth (Potters et al., 2007). Lead contributes to water stress and thus reduces leaf area, photosynthesis, plant dry weight and plant height, and the number of nodes (Azmat et al., 2006). Regarding the role of methyl jasmonate in reducing lead accumulation, it can be said that this growth regulator activates as a signal molecule the expression of the genes involved in plant defense mechanisms, including the biosynthesis pathway of phytolatin, which connections to toxic ions and convert them to non-toxic ions. Based on the results of this study, it seems that the basil is susceptible to the storage of heavy metals such as lead in its leaves, which can be reduced by using appropriate concentrations of methyl jasmonate.

Keywords

• Dry weight of aerial parts
• Fresh weight of aerial parts
• Growth regulator
• Hazardous index
• Lead toxicity