Alterations in photosynthetic pigments, antioxidant machinery, essential oil constituents and growth of menthol mint (Mentha arvensis L.) upon nickel exposure

Abstract

The present study was conducted in view of the alarming increase in the concentration of various heavy metals like nickel, copper, lead, cadmium, and arsenic etc., in the soils globally. Little attention has been paid towards the deleterious effect of these heavy metals particularly nickel (Ni) on the performance of pharmaceutically exigent essential oil-bearing crops like menthol mint (Mentha arvensis L.). Menthol mint, a dominant essential oil-producing aromatic plant, is widely cultivated in Uttar Pradesh (India) and has wide applications in pharmaceutical industry. Nickel, an essential micronutrient and a main component of the plant enzyme urease, has vital role in plants and is required in very low amounts for normal growth and functioning of plants. In this study, Ni was applied in the form of NiCl2 with various concentrations of 20, 40, 60, 80, and 100 mg kg−1 Ni of soil and one set was kept as control. Interestingly, Ni at 20 mg kg−1 of soil significantly enhanced all the parameters studied, including constituents of essential oil, particularly menthone and menthyl acetate content. However, a significant reduction was observed in most of the parameters studied with increasing concentrations of Ni. The growth (plant height, fresh and dry weights and herbage yield), biochemical (photosynthetic pigments, chlorophyll fluorescence and carbonic anhydrase activity), and quality parameters (content of essential oil & its active constituents) exhibited a significant reduction over 20 mg kg−1 of soil. However, the activities of enzymatic antioxidants, such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX), and the level of proline (PRO) were enhanced at the higher concentrations of Ni. Thus, the present study confirms that Ni applied at 20 mg kg−1 of soil proved beneficial dose, whereas 100 mg kg−1 of soil produced toxic response for the overall growth and development of the mint crop.