Photosynthetic and growth responses of Populus clones Eridano and I-214 submitted to elevated Zn concentrations

The effect of two toxic levels of Zn were analysed in two poplar clones (Populus deltoides x maximowiczii -- clone Eridano, and Populus x canadiensis euramericana Mönch. -- clone I-214), both of which are commonly used in plantations in Italy. We examined their capacity to accumulate and tolerate toxic levels of Zn and the effect of this accumulation on plant growth and development, leaf morphology, gas-exchange and nutrient composition. Rooted cuttings were treated for three weeks in hydroponic culture with 0 (control), 1 mM and 5 mM zinc chloride (ZnCl2). Both clones showed Zn toxicity effects in their physiological responses, especially at 5 mM [Zn], but Eridano displayed high tolerance at 1 mM [Zn]. Growth and biomass production declined mainly in I-214, with a severe reduction of root growth. Biomass parameters and tolerance index (Ti) indicated a higher capacity of Eridano to tolerate toxic levels of Zn. Leaf morphological changes were observed, and appeared earlier in I-214 at 1 mM [Zn], with a decline in leaf area and thickness, and an increase in leaf density. Gas-exchange parameters pointed to a drastic decrease in net photosynthesis (A), stomatal conductance (gs) and transpiration rate (E) at 5 mM [Zn], although Eridano showed good tolerance at 1 mM [Zn]. With increasing Zn in the nutrient solution, Zn concentration increased in all plant structures, accumulating mainly in roots, especially in Eridano. The heavy metal accumulation was similar at 1 mM and 5 mM [Zn] for each clone, and the capacity of both clones to take up Zn from the medium was reduced. The concentration of Zn accumulated in young leaves accounted for the decline observed in biomass, photosynthesis, Fe and chlorophyll content. In hydroponic culture with highly toxic Zn levels, Eridano showed a higher capacity of accumulation, translocation to aerial parts, and tolerance than I-214, thus appearing as a very promising clone for the phytoremediation of Zn-polluted substrates.