[Accumulation, subcellular distribution, and chemical forms of zinc in three tree species]

In order to explore the mechanism underlying zinc (Zn) accumulation and tolerance in woody garden species, the effects of different Zn concentrations (0, 250, 500, 1000, 2000 mg·kg^-1) on leaf, branch, root biomass and leaf ultrastructure of Koelreuteria paniculata, Ailanthus altissima, and Ginkgo biloba were studied in a pot pollution simulation experiment. The concentration of Zn in plant organs, the subcellular distribution and chemical forms of Zn in leaves and roots were further analyzed. The results showed that all the three species could survive under diffe-rent Zn concentrations, but the biomass of leaves, stems and roots decreased compared with the control. Excessive Zn could lead to cell deformation, cell wall rupture and organelle disintegration of leaves in K. paniculata and A. altissima, while the cells in leaves of G. biloba could maintain normal morphology, indicating that G. biloba had a better tolerance to Zn than K. paniculata and A. altissima. With the increases of Zn concentration, Zn concentration in the organs of the three species showed an increasing trend, and the Zn concentration in K. paniculata and A. altissima was significantly higher than that in G. biloba, indicating that the Zn accumulation ability of K. paniculata and A. altissima was stronger than that of G. biloba. Zn was mainly distributed in the cell walls of leaves and roots, accounting for 26.9%-71.8% and 28.1%-82.6%, respectively. Under the treatment with the highest Zn concentration (2000 mg·kg^-1), Zn concentration in the soluble components (mainly vacuoles) could be higher than that in the cell walls. In addition, Zn mainly existed in NaCl-, HAc- and HCl-extracted forms in leaves, accounting for 57.4%-82.7%, and Zn mainly existed in NaCl- and HAc-extracted forms in roots, accounting for 42.8%-67.2%, all of which were forms with relatively low activity. Therefore, cell wall retention, vacuoles segregation and accumulating Zn in less active forms might be important mechanisms underlying Zn accumulation and tolerance in the three trees.