Abstract

The amphibious water plant Crassula helmsii is an invasive copper (Cu)-tolerant neophyte in Europe. It now turned out to accumulate Cu up to more than 9,000 ppm in its shoots at 10 microm (=0.6 ppm) Cu(2+) in the nutrient solution, indicating that it is a Cu hyperaccumulator. We investigated uptake, binding environment, and toxicity of Cu in this plant under emerged and submerged conditions. Extended x-ray absorption fine structure measurements on frozen-hydrated samples revealed that Cu was bound almost exclusively by oxygen ligands, likely organic acids, and not any sulfur ligands. Despite significant differences in photosynthesis biochemistry and biophysics between emerged and submerged plants, no differences in Cu ligands were found. While measurements of tissue pH confirmed the diurnal acid cycle typical for Crassulacean acid metabolism, Delta(13)C measurements showed values typical for regular C3 photosynthesis. Cu-induced inhibition of photosynthesis mainly affected the photosystem II (PSII) reaction center, but with some unusual features. Most obviously, the degree of light saturation of electron transport increased during Cu stress, while maximal dark-adapted PSII quantum yield did not change and light-adapted quantum yield of PSII photochemistry decreased particularly in the first 50 s after onset of actinic irradiance. This combination of changes, which were strongest in submerged cultures, shows a decreasing number of functional reaction centers relative to the antenna in a system with high antenna connectivity. Nonphotochemical quenching, in contrast, was modified by Cu mainly in emerged cultures. Pigment concentrations in stressed plants strongly decreased, but no changes in their ratios occurred, indicating that cells either survived intact or died and bleached quickly.

Highlights

  • The amphibious water plant Crassula helmsii is an invasive copper (Cu)-tolerant neophyte in Europe

  • We investigated physiological mechanisms of Cu-induced inhibition of photosynthesis, Crassulacean acid metabolism induction, and Cu accumulation and complexation in C. helmsii

  • The maximum quantum yield of charge separation in the photosystem II (PSII) reaction center (RC), as measured by Fv/Fm = (Fm 2 F0)/Fm, reflected the changes of F0 and Fm as mentioned above

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Summary

Introduction

The amphibious water plant Crassula helmsii is an invasive copper (Cu)-tolerant neophyte in Europe. The basic fluorescence yield (F0), in contrast, decreased during this time if plants from submerged stock culture were grown in the emerged state during the experiment (treatments ECt and ECu in the left column of Fig. 4A).

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