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SummaryThe effect of long‐term (30 days) NaCl treatments (100 mM and 200 mM) on the activity of some antioxidant enzymes, level of antioxidant metabolites, water relations and chloroplast ultrastructure, was studied in potato (Solanum tuberosum cv. Désirée) leaves. Salt stress negatively affected relative water content, leaf stomatal conductance and transpiration rate. In treated plants, proline was enhanced, but there was a significant decrease in ascorbate and proteins. Total superoxide dismutase activity was increased. The isozyme patterns detected in native gels from salt‐irrigated plants were not changed although all the isoforms appeared more heavily stained due to higher activity. In contrast, at both levels of NaCl, catalase activity decreased and ascorbate peroxidase activity showed no significant change in comparison with an untreated control. At ultrastructural level, only thylakoid swelling and a decrease in the amount of grana stacking was observed in treated plants. The overall behaviour of the antioxidant enzymes suggests an increase of cellular H2O2 that would contribute to the oxidative stress of potato plants, but which may be alleviated somewhat by the enhanced levels of proline.

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Given their sessile nature, plants continuously face unfavorable conditions throughout their life cycle, including water scarcity, extreme temperatures and soil pollution. Among all, metal(loid)s are one of the main classes of contaminants worldwide, posing a serious threat to plant growth and development. When in excess, metals which include both essential and non‐essential elements, quickly become phytotoxic, inducing the occurrence of oxidative stress. In this way, in order to ensure food production and safety, attempts to enhance plant tolerance to metal(loid)s are urgently needed. Nitric oxide (NO) is recognized as a signaling molecule, highly involved in multiple physiological events, like the response of plants to abiotic stress. Thus, substantial efforts have been made to assess NO potential in alleviating metal‐induced oxidative stress in plants. In this review, an updated overview of NO‐mediated protection against metal toxicity is provided. After carefully reviewing NO biosynthetic pathways, focus was given to the interaction between NO and the redox homeostasis followed by photosynthetic performance of plants under metal excess.