Arsenate and arsenite: the toxic effects on photosynthesis and growth of lettuce plants

Arsenate (AsV) and arsenite (AsIII) contamination can promote several disturbances in plant metabolism, besides affecting directly human and animal health due to the insertion of this metalloid in the food chain. Therefore, the arsenic (As) uptake and accumulation, the changes in gas exchange and in chlorophyll a fluorescence parameters as well as the chloroplastic pigments content were measured. The As accumulation in leaves and roots increased with the increase of AsV and AsIII concentration, except at the highest AsIII concentration, probably because of AsIII extrusion mechanism. Although the highest As concentration has been found in roots, significant amount was transported to the leaves, especially when plants were exposed to AsIII. The As accumulation decreased the relative growth rate (RGR) of leaves and roots. However, at 6.6 μmol L−1 AsV, an increase in leaves RGR was observed, possibly related to the changes in phosphate (PV) nutrition caused by As. AsV and AsIII interfered negatively in the photosynthetic process, except at 6.6 μmol L−1 AsV. The observed reduction seemed to be associated to the interference in the photochemical and biochemical steps of photosynthesis; however, chlorophyll a fluorescence results indicate that the photosynthetic apparatus and chloroplastic pigments were not damaged. So, lettuce plants demonstrated to be able to accumulate As and also to protect the photosynthetic apparatus against the harmful effects of this metalloid, probably through the activation of tolerance mechanisms.