Influence of Magnesium Ions on the Action of Photosystems I and II at Different Wavelengths Experiments with Barley Chloroplasts of Different Chlorophyll b Content

Barley leaves grown under a natural light/dark regime have a chlorophyll content of 1300 [ig/g fresh weight and a chlorophyll a /b ratio of 2 .5 -3 . When the plants are grown under cycles of 2 min light/118 min dark, the respective values are 50 and 5 — 9. With chloroplasts with low chlo­ rophyll b content variable fluorescence is depressed by about 30% by MgCl2; with those of high chlorophyll b content a threefold increase is seen instead. Action spectra for variable fluorescence of chloroplasts of high chlorophylll b content show enhancement by Mg2+ around 475 and 650 nm; for the system I-mediated methyl viologen reduction, a depression is seen at these wavelengths. These effects are practically absent in chloroplasts with low chlorophyll b content. The data corro­ borate the hypothesis that a chlorophyll b-containing pigment protein complex is required for re­ gulation of energy transfer to system I and II by magnesium ions.