Zinc Toxic Effects, Distribution and Translocation in the Hyperaccumulator Arabidopsis halleri and the Non-Accumulator Arabidopsis lyrata

Abstract In this work, zinc (Zn) distribution and translocation, as well as its effects on growth and transpiration rate were compared in the non-accumulator Arabidopsis lyrata ssp. petraea (L.) O’Kane & Al-Shehbaz and the Zn hyperaccumulator Arabidopsis halleri (L.) O’Kane & Al-Shehbaz. Two-month-old plants, grown on half-strength Hoagland’s nutrient solution (at 2 µM Zn), were exposed to 80, 160, and (only A. halleri) 3000, 4000, 5000, or 6000 µM ZnSO4 for 1 week. The plants grown at 2 µM Zn served as control. Root and shoot fresh and dry weights, water contents in them and transpiration rate were assessed. The Zn content in roots and shoots was determined using flame atomic absorption spectrophotometry. The Zn translocation factor was calculated as shoot-to-root Zn concentration ratio. Zinc distribution over the root and shoot tissues was examined histochemically using a Zn-sensitive fluorescent indicator Zinpyr-1. At 80 and 160 µM, Zn stimulated root growth in A. halleri, whereas in A. lyrata and in the hyperaccumulator at higher Zn levels, root biomass decreased compared to control. A decrease in the accumulation of shoot biomass in A. halleri was observed at significantly higher Zn concentrations in the medium and its content in shoots compared to A. lyrata, and in both species it was accompanied by a decrease in water content. The Zn translocation factor was much higher in A. halleri compared to A. lyrata, which is partially related to the absence of Zn effect on transpiration rate in A. halleri even at 6000 µM Zn in the medium. Zinc was detected in all root and shoot tissues. In the leaves of A. halleri, it accumulated in large water-storage cells and stomatal guard cells of leaf epidermis, whereas in the leaves of A. lyrata, its accumulation was observed in the mesophyll and subsidiary cells of stomatal complex. It is concluded that transpiration, continuing at a high rate even under excess Zn, promotes efficient Zn translocation into the shoots of the hyperaccumulator and Zn detoxification in leaf epidermis, where the transpiration stream is terminated.