Unveiling Zn hyperaccumulation in Juncus acutus: Implications on the electronic energy fluxes and on oxidative stress with emphasis on non-functional Zn-chlorophylls

Juncus acutus arises as possible hyperaccumulator specie, tolerating exogenous Zn concentrations as high as 60 mM. Zinc concentrations here detected in seedlings germinated in the presence high Zn concentrations, were above the described upper toxic levels for higher plants. Even at the highest Zn concentration, growth inhibition only accounted to approximately 30% of control seedlings biomass, presenting an EC50 value in the range of 10–20 mM of metal. PSII quantum yields showed a marked decline, reflection of changes in the thylakoid structure on the PSII electron donor sites. In fact, the electron transport rate was severely affected by Zn in seedlings exposed to higher Zn concentrations leading to a decrease in their maximum electronic transport rate and consequently presenting lower light saturation and lower photosynthetic efficiencies. Although light absorption capacity was not affected by Zn exposure and uptake, energy trapping flux in the photosynthetic apparatus and transport throughout the electronic chain was severely impaired. This lack of efficiency is related with non-functional Zn-chlorophylls formation. There was a strong linear correlation between exogenous Zn concentration applied and the concentration actually verified in the seedlings tissue with the concentration of both ZnChl a and b. There was also a gradual loss of connectivity between the antennae of the PSII units being this more evident at the higher Zn concentrations and thus impairing the energetic transport. The reduction in light harvesting efficiency, leads inevitably to the accumulation of redox energy inside the cells. To counteract ROS generation, all anti-oxidant enzymatic activities (except catalase) showed a proportional response to exogenous and in vivo Zn concentrations. Not only this plant appears to be highly tolerant to high Zn concentrations, but also it can overcome efficiently the damage produced during this uptake by efficiently dissipating the excessive cellular redox potential accumulated, essentially due to Zn incorporation into the chlorophyll molecule.