Why Does the Halophyte Mesembryanthemum crystallinum Better Tolerate Ni Toxicity than Brassica juncea: Implication of Antioxidant Defense Systems

Abstract

The implication of enzymatic and non-enzymatic antioxidative systems in response to Ni was evaluated in the halophyte <i>Mesembryanthemum crystallinum</i> in comparison with the metal tolerant glycophyte species <i>Brassica juncea</i>. Seedlings of both species were hydroponically subjected during 21 days to 0, 25, 50, and 100 µM NiCl₂. Growth parameters showed that the halophyte <i>M. crystallinum</i> was more tolerant to Ni than <i>B. juncea</i>. Malondialdehyde (MDA) content increased to a higher extent in <i>B. juncea</i> than in <i>M. crystallinum</i>. Antioxidant enzymesactivities were differently affected by Ni in both species. Nickel increased shoot superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in <i>B. juncea</i>, whereas these activities were reduced in <i>M. crystallinum</i> when exposed to metal stress. The root SOD, APX and guaiacol peroxidase (GPX) activities increased upon Ni treatments for both species. The content of non-enzymatic antioxidative molecules such as glutathione, non-protein thiols and proline increased in Ni-treated plants, except for GSH content in the shoot of <i>B. juncea.</i> Based on the oxidative balance, our findings confirm the higher tolerance of the halophyte <i>M. crystallinum</i> to Ni-induced oxidative stress comparatively to <i>B. juncea.</i> We suggest that <i>M. crystallinum</i> is able to overcome the produced ROS using the non-enzymatic system, while Ni-induced oxidative stress was more acute in <i>B. juncea</i>, leading this species to mainly use the enzymatic system to protect against reactive oxygen species.