Photoprotective and antioxidant responses of Populus clones I-214 and Eridano submitted to elevated Zn concentrations

Rooted cuttings of two poplar clones (Populus x canadiensis Mönch.- clone I-214 and P. deltoides x maximowiczii - clone Eridano) were treated for three weeks in hydroponic culture with high levels of Zn (1mM and 5mM). In plants, excessive Zn accumulation can contribute to the formation of Reactive Oxygen Species (ROS) whose accumulation can directly affect growth and photosynthetic activity. Here we characterized photoprotective and antioxidant responses in leaves to determine the ability of both clones to cope with stress conditions induced by Zn treatments. Chloroplast pigments neoxanthin, lutein, violaxanthin (V), antheraxanthin (A), zeaxanthin (Z), chlorophylls a, and b, ? -carotene, ?-tocopherol and ( ?+ ?) tocopherol were analyzed by HPLC while ascorbate (ASA), glutathione (GSH) and phenolic content were determined by spectrophotometric techniques. Furthermore, gas-exchange and chlorophyll fluorescence were characterized with portable systems (Li-COR 6400 and Mini-Pam Walz, respectively). Increasing [Zn] markedly declined photosynthesis, stomatal conductance and growth especially in I-214, which showed signs of very severe damage (i.e. wilted and necrotic leaves). The elevated ASA, ?- and (?+ ?) tocopherol content and de-epoxidation index (A+Z/VAZ), which is related with the dissipation of excess irradiance energy as heat, shown by Eridano clone under increasing [Zn], maintained the PSII functionality as observed from the higher values of maximum PSII quantum. Eridano showed a higher tolerance to Zn with a greater capability to accumulate Zn in roots. Lower Zn accumulation in Eridano leaves was associated to the observed higher photo - and antioxidant protection and improved gas-exchange rates and growth with respect to I-214.