Correlation between the "low"-salt-induced increase in the F730/F685 fluorescence emission ratio at 77 K in isolated chloroplasts, and the organization of chlorophyll in photosystem I pigment-protein complexes of thylakoids.

Isolated pea or spinach chloroplasts suspended in "high"-salt phosphate buffer exhibit a low F730/F685 fluorescence emission ratio at 77 K; in contrast, removal of cations by incubation in "low"-salt Tricine buffer induces a drastic increase in the F730/F685 ratio. Parallel to the F730/F685 ratio increase, a gradual organization of chlorophyll (Chl) in the pigment-protein complexes of the Photosystem I, chlorophyll-protein complex Ia, and light-harvesting complex I (LHC-I), is observed. The kinetics of the two processes are closely correlated, all changes being completed within 5-10 min from Tricine addition. On the other hand, the inability of low-salt Tricine to induce any changes in the F730/F685 ratio in bean plastids, isolated and suspended in high-salt phosphate buffer, correlates with the lack of extensive changes in the organization of the Photosystem I complexes, and more specifically of LHC-I. The latter is attributed to the higher stability of complexes in bean, arising from stronger association of thylakoids in grana stacks in this species; this is probably due to higher levels of residual divalent cations present in the isolated thylakoids of bean compared to pea (or spinach). The results suggest that the F730/F685 ratio changes, observed in chloroplasts by manipulation of their ionic environment, reflect modulation of Chl organization within the pigment-protein complexes of the photosynthetic units.