RESUMO
The ultrastructural organization and the photosynthesis reactions of chloroplast membranes were studied in three lethal mutants of Pisum sativum, Chl-1, Chl-19 and Chl-5, all lacking the capacity to evolve oxygen. The rates of 2,6-dichloroindophenol reduction, delayed fluorescence and electron-spin-resonance signal 1 indicate that Chl-1 and Chl-19 have an impaired activity in photosystem II (PS II), while in Chl-5 the electron transport is blocked between PS I and the reactions of CO2 fixation. Ultrathin sectioning demonstrates the presence of giant grana in the chloroplasts of Chl-1 and Chl-19, while the chloroplast structure of the Chl-5 is very similar to that of the wild-type. The grana of the Chl-19 mutant contain large multilamellar regions of tightly packed membranes. When the chloroplast membranes were studied by freeze-fracture, the exoplasmic and protoplasmic fracture faces (EF and PF, respectively) in both stacked and unstacked membranes were found to show large differences in particle concentrations and relative population area (per µm(2)), and also in particle size distribution, between all mutant chloroplast membranes and the wild-type. A close correlation between increasing kmt (ratio of particle concentrations on PF/EF) and PS II activity was observed. The differences in particle concentrations on both fracture faces in different regions of the intact chloroplast membranes of the wild-type are the consequence of a rearrangement of existing membrane components by lateral particle movements since quantitative measurements demonstrate almost complete conservation of intramembrane particles in number and size during the stacking of stroma thylakoid membranes. The results indicating particle movements strongly support the concept that the chloroplast membranes have a highly dynamic structure.
