The photoprotective mechanisms in Secale cereale leaves under Cu and high light stress condition

The influence of excess Cu ions and high light treatment on the function of photosystem II was investigated in order to examine how this heavy metal modifies the photoprotective mechanisms operating at the molecular level in Secale cereale plants. Thus, non-treated plants and those treated with 5 or 50 μM Cu, simultaneously illuminated with 150 μmol m−2 s−1 or 1200 μmol m−2 s−1 light intensity, were studied. To analyze the PSII reaction to the stress conditions, Chl a fluorescence induction was applied. An increase in the value of ΦPSII and Rfd parameters indicated that the photosynthetic apparatus adapted to the high light condition by effective utilization of excitation energy in the light and dark phases of photosynthesis. This phenomenon was accompanied by dissipation of excitation energy within the antenna complexes. The xanthophyll cycle pigments in Secalecereale leaves were separated and quantified by the HPLC technique. The results showed that, under high light irradiance, both 5 and 50 μM Cu induced the process of violaxanthin de-epoxidation and zeaxanthin accumulation. The significant zeaxanthin accumulation was found to be involved in photoprotective energy dissipation as heat, which was supported by correlation between the rate of violaxanthin de-epoxidation and the value of SV parameters. Interestingly, Cu treatment caused violaxanthin isomerization from its trans to 15-, 13- and 9-cis forms in proportional correlation to the metal concentration. This phenomenon was confirmed by a study of Cu-induced violaxanthin isomerization in vitro, which suggests a direct metal–pigment molecule interaction. We also observed that the violaxanthin trans–cis isomerization increased simultaneously with anteraxanthin content. On the basis of these findings, it can be speculated that violaxanthin isomerization is the basic process responsible for the xanthophyll cycle operation.