Attenuation of Mercury Phytotoxicity with a High Nutritional Level of Nitrate in Alfalfa Plants Grown Hydroponically

Abstract

Mercury (Hg) is one of the most dangerous pollutant heavy metals to the environment, which causes several toxic effects in plants upon accumulation, such as induction of oxidative stress. Nitrate (NO3single bond) is the prevalent form to incorporate nitrogen (N) in higher plants, through its reduction to nitrite (NO2single bond) by the enzyme nitrate reductase (NR). We studied the physiological alterations caused by Hg (0, 6 and 30 µM) in alfalfa plants grown at two different levels of NO3single bond: low, (2 mM; LN), and high (12 mM; HN) for one week using a semi-hydroponic culture system. Several parameters of oxidative stress such as lipid peroxidation, chlorophyll content, biothiol concentration, and ascorbate peroxidase (APX) and glutathione reductase (GR) activities showed that HN plants were less affected by Hg. Nitrate reductase activity and NO3single bond concentration were also altered under Hg stress, with lower impact in plants nourished with high NO3single bond. Our results highlight the importance of the NO3single bond nutritional status to improve tolerance to toxic metals like Hg.