Sensitivity of the photosynthetic apparatus to UV-A radiation: role of light-harvesting complex II-photosystem II supercomplex organization.

In this work we study the effect of UV-A radiation on the function of the photosynthetic apparatus in thylakoid membranes with different organization of the light-harvesting complex II-photosystem II (LHCII-PSII) supercomplex. Leaves and isolated thylakoid membranes from a number of previously characterized pea species with different LHCII size and organization were subjected to UV-A treatment. A relationship was found between the molecular organization of the LHCII (ratio of the oligomeric to monomeric forms of LHCII) and UV-A-induced changes both in the energy transfer from PSII to PSI and between the chlorophyll-protein complexes within the LHCII-PSII supercomplex. Dependence on the organization of the LHCII was also found with regard to the degree of inhibition of the photosynthetic oxygen evolution. The susceptibility of energy transfer and oxygen evolution to UV-A radiation decreased with increasing LHCII oligomerization when the UV-A treatment was performed on isolated thylakoid membranes, in contrast to the effect observed in thylakoid membranes isolated from pre-irradiated pea leaves. The data suggest that UV-A radiation leads mainly to damage of the PSIIalpha centers. Comparison of membranes with different organization of their LHCII-PSII supercomplex shows that the oligomeric forms of LHCII play a key role for sensitivity to UV-A radiation of the photosynthetic apparatus.