Low light grown duckweed plants are more protected against the toxicity induced by Zn and Cd

Duckweed (Lemna minor L.) plants acclimated to three light regimes: 50 low light (LL), 300 medium light (ML) and 500 high light (HL) μmol m–2 s–1 photosynthetic photon flux density (PPFD) increased photoprotective potential according to irradiance status as shown by the two- to fourfold higher contents of xanthophyll cycle pigments (VAZ), total glutathione (GLU) and α-tocopherol. LL, ML and HL plants were then exposed to Cd or Zn for 9 d and illuminated at 300 μmol m–2 s–1. Both metals inhibited plant growth and caused the accumulation of phytochelatins, but phytochelatin synthesis did not result in glutathione depletion in the Zn treatments. An antioxidative response was observed with a large increase in total ascorbate (ASC), tocopherol and VAZ pigments. These mechanisms seemed to be able to partly alleviate the adverse effects of Zn but not of Cd. The damage caused by Cd led to the complete bleaching of chlorophylls (Chl) at the end of the experiment in HL and ML plants. The pattern of antioxidative response was different when comparing Cd and Zn treatments, with a much larger accumulation of ASC in Cd-treated plants, and GLU on Zn-treated plants. Paradoxically, LL plants that initially had a lower level of photoprotection, were much less affected by Cd or Zn. Thus, these plants showed very small or no reductions in photochemical efficiency, chlorophyll content and a lower ASC accumulation. Although we could not explain this paradoxical effect, we can conclude that other reasons related to the previously light grown conditions could be more determinant to explain heavy metal induced damage than the basal level of antioxidant content in Lemna plants.