Summary• We hypothesized that poplar can be useful for the extraction of zinc (Zn) from contaminated soils and water. However, the physiological response and the base of tolerance of poplar plants to this heavy metal are little known.• Plants of Populus deltoides × P . nigra ( P. × euramericana ) I-214 clone were grown in glasshouse and treated with three different Zn concentrations (1, 100 and 1000 µ M ). Growth analysis, Zn concentration in leaves, stem and roots, and photosynthetic parameters were measured at different times of exposure during the annual growth cycle.• Significant reductions in foliage and total dry mass, and impairment of gas exchange properties occurred at applications of 100 µ M and 1000 µ M Zn. Modifications of leaf area, chlorophyll b concentration, diameter at the stem base, and Zn concentrations of old leaves, stem and roots were also shown after Zn treatment.• We concluded that poplar plants have the potential to be used for plantations in Zn-contaminated soils, in the range of tested Zn concentrations.

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Activated oxygen species such as superoxide radicals, singlet oxygen, hydrogen peroxide and hydroxyl radicals can be produced in plants exposed to low, non‐freezing, non‐injurious temperatures. To prevent or alleviate oxidative injury, plants have evolved several mechanisms which include scavenging by natural antioxidants and enzymatic antioxidant systems such as superoxide dismutases, catalase and peroxidases. Although overproduction of hydrogen peroxide and increased tolerance to oxidative stress can be induced in wheat by low‐temperature treatments, data concerning changes in the enzymatic antioxidant systems are almost absent. With the aim to provide this information, antioxidant enzyme (superoxide dismutases, catalase and peroxidases) activities were analysed in leaves and roots of Triticum aestivum cvs Brasilia (frost resistant in field) and Eridano (less frost resistant in field) seedlings grown at day/night temperatures of 24/22°C (control treatment) and 12/5°C (low‐temperature treatment). Our data showed that superoxide dismutase activities were unaffected by low‐temperature treatment both in leaves and roots. Catalase activity in leaves and roots was decreased in 12/5°C‐grown seedlings, but Brasilia maintained higher catalase activity than Eridano. Differences were also observed in guaiacol peroxidase activities between control and acclimated seedlings: Higher guaiacol peroxidase activities were found in the leaves of 12/5°C‐grown seedlings while in roots these activities were lower. Moreover, Brasilia guaiacol peroxidase activities were higher than Eridano. Superoxide dismutase and peroxidase zymogram analyses showed that synthesis of new isoforms was not induced by low‐temperature treatment. Changes in the activities of antioxidant enzymes induced by cold acclimation support the hypothesis that a frost‐resistant wheat cultivar, in comparison with a less frost‐resistant one, maintains a better defence against activated oxygen species during low‐temperature treatment.

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