Interaction of nickel with oxidative and antioxidative molecules in Cichorioideae species

The uptake of nickel (Ni) by Asteraceae/Cichorioideae species Cichorium intybus, Leontodon hispidus and Hieracium aurantiacum exposed to Ni (0.3 or 30 μM) over 14 days and subsequent changes of metabolites were compared in order to identify their phytoaccumulation potential. Hieracium contained the most Ni (194 and 1558 μg Ni/g DW at 30 μM Ni in shoots and roots) but had unchanged amount of antioxidants (vitamin C and thiols) in the shoots and an elevated amount in the roots, which may be the reason for the absence of visible damage. On the contrary, Leontodon reacted by a decrease in antioxidants to an excess of Ni, which can be related to enhanced oxidative stress (an increase in ROS and a decrease in nitric oxide detected by fluorescence microscopy). All roots were anatomically in the secondary state and Ni-induced cell wall thickening (i.e. lignin/suberin deposition) was most visible in Hieracium roots, which also contained 2-times more Ni than the other species. Among essential elements, mainly Fe accumulation was affected by Ni excess. The content of soluble phenols increased while organic acids (malic and citric) decreased sometimes extensively (up to 90%) in individual species. PCA analyses showed that especially ascorbic acid, thiols and phenols affect the separation in the shoots especially with regard to applied concentration of Ni, while these metabolites in the roots clearly separated the species (Cichorium from the others). The data show the highest tolerance to Ni in Hieracium, but the highest phytoaccumulation of Ni was found in Cichorium (626 μg Ni/plant or 122 μg Ni/shoot at a dose of 30 μM Ni).