Root based responses account for Psidium guajava survival at high nickel concentration

The presence of Psidium guajava in polluted environments has been reported in recent studies, suggesting that this species has a high tolerance to the metal stress. The present study aims at a physiological characterization of P. guajava response to high nickel (Ni) concentrations in the root-zone. Three hydroponic experiments were carried out to characterize the effects of toxic Ni concentrations on morphological and physiological parameters of P. guajava, focusing on Ni-induced damages at the root-level and root ion fluxes. With up to 300 ?M NiSO4 in the root-zone, plant growth was similar to that in control plants, whereas at concentrations higher than 1000 ?M NiSO4 there was a progressive decline in plant growth and leaf gas exchange parameters; this occurred despite, at all considered concentrations, plants limited Ni2+ translocation to the shoot, therefore avoiding shoot Ni2+ toxicity symptoms. Maintenance of plant growth with 300 ?M Ni2+ was associated with the ability to retain K+ in the roots meanwhile 1000 and 3000 ?M NiSO4 led to substantial K+ losses. In this study, root responses mirror all plant performances suggesting a direct link between root functionality and Ni2+ tolerance mechanisms and plant survival. Considering that Ni was mainly accumulated in the root system, the potential use of P. guajava for Ni2+ phytoextraction in metal-polluted soils is limited; nevertheless, the observed physiological changes indicate a good Ni2+ tolerance up to 300 ?M NiSO4 suggesting a potential role for the phytostabilization of polluted soils