Root Growth and Zn Uptake by Two Ecotypes of Deschampsia caespitosa as Affected by High Zn Concentrations

Abstract

Summary Inhibition of root elongation by high Zn concentrations was quite different in two ecotypes of Deschampsia caespitosa. A 5 to 10 times higher Zn supply was necessary to induce the same inhibition in the Zn tolerant as compared to the non-tolerant ecotype. After 10 h exposure to 100 μM Zn, which severely depressed root growth in the non-tolerant but not in the tolerant ecotype, the Zn concentrations in the roots were similar in both ecotypes. In short term uptake (10 h) and desorption studies (30 min) using 65Zn, up to 50% of the Zn(65Zn) taken up could be desorbed by unlabelled Zn within 30 min at 4°C. Zn bound in the cell walls was estimated to account for up to 30% of the total Zn content of the roots. Binding of Zn in the cell walls could not explain the differences in Zn tolerance of the two ecotypes. At the same degree of inhibition of root-elongation rate in both ecotypes, in the tolerant ecotype the 1 mm apical root sections had approximately a 3 times greater intracellular Zn content than the same sections of the non-tolerant ecotype. This suggests that the responsible factor for Zn tolerance is not exclusion from the cells but rather a higher internal tolerance of the root tip meristems. Since large vacuoles could not be observed in the 1 mm apical root sections, compartmentation of Zn in the vacuoles could only be of minor importance. Accordingly we suggest that specific binding of Zn in the cytoplasm is a more likely mechanism for the detoxification of Zn in the apical meristems.