Low photosynthetic activity is linked to changes in the organization of photosystem II in the fruit of Arum italicum

The low photosynthetic activity of fleshy green fruits is currently attributed to their special anatomy rather than to a down-regulation of photosystem II (PSII). However, it is unclear whether the organization of PSII, which is highly conserved in leaves, is also shared by non-foliar structures, such as fleshy fruits. To obtain new information on this aspect, the photosynthetic activity and the organization of PSII were investigated in the berry of Arum italicum Miller during maturation (ivory to green) and early ripening (green to yellow). The berry developed an “internal CO2 recycling” photosynthesis; gross photosynthesis at the green stage was 25% of the leaf lamina. SDS-PAGE, BN-PAGE and 77 K spectrofluorimetry showed that the thylakoid membrane accumulated a very high amount of free LHCII trimers and only few PSII and PSI complexes. The pattern of PSII forms was similar to that of the lamina (monomers, dimers, LHCII-PSII supercomplexes), but increase in CP43-less PSII cores and low F695/F680 fluorescence ratio at room temperature indicated that PSII was less stable than in the leaf lamina. Beside effective PSII photoprotection, we propose that LHCII serves as a temporary storage of chlorophylls to provide a visual signal that fruit is not mature for seed dispersal. We conclude that the low photosynthetic activity of A. italicum berry depends on the scantiness of reaction centres and the reduced functionality of PSII.