Unraveling zinc's impact: Variable effects across concentrations in Phaseolus coccineus thylakoids

This study analyzed an effect of nine different zinc (Zn) concentrations on the structure and function of pigment-protein complexes in the thylakoids of Phaseolus coccineus exposed to Zn concentrations ranging from 25 to 1000 μM. Zn exposure induced non-linear concentration-dependent changes in Lhcb1 and Lhcb2 phosphorylation, accompanied by alterations in 77 K fluorescence emission and fluorescence lifetime parameters, indicating reorganization of the LHCII antenna and changes in the excitation energy distribution within the photosynthetic apparatus. Fluorescence lifetime analyses showed that 100 μM Zn²⁺ is associated with changes in the relative amplitudes of decay components, including a decrease in the 70 ps component and an increase in the 1.6 ns and 0.28 ns components, suggesting modifications in energy transfer dynamics and possible partial uncoupling between antenna complexes. However, decay-associated lifetimes are not uniquely diagnostic, and alternative interpretations cannot be excluded. The highest Zn²⁺ concentration (1000 μM) was associated with pronounced alterations in pigment-protein organization in the thylakoid membranes, supported by Fourier-transform infrared analysis showing the disappearance of the band around 1590 cm^-1. Changes in carotenoid and chlorophyll content appear to play an important role in the functioning of the photosynthetic apparatus of P. coccineus under heavy metal stress. Overall, Zn treatment induced coordinated, multi-level reorganization of the thylakoid membrane, involving changes in LHCII phosphorylation, oligomeric state, excitation energy distribution, and fluorescence lifetime characteristics. Together, these datasets reveal a dynamic and non-linear response of the photosynthetic apparatus to increasing Zn concentrations, reflecting both regulatory acclimation and stress-induced structural remodeling.