COMPLEMENTARY CHANGES IN ABSORPTION CROSS-SECTIONS OF PHOTOSYSTEMS-I AND PHOTOSYSTEMS-II DUE TO PHOSPHORYLATION AND MG2+-DEPLETION IN SPINACH THYLAKOIDS

An open question regarding the changes of energy distribution between Photosystems (PS) I and II following protein phosphorylation and thylakoid destacking is whether or not excitation energy redirected from PS II antenna is effectively trapped by PS I reaction centers. In this report, we measured the effects in spinach thylakoids of Mg2+-depletion and of phosphorylation at 5 and 1 mM MgCl2 on: (1) the effective absorption cross-sections (sigma) of both PS II and PS I determined simultaneously from single-turnover flash saturation curves of Chl a fluorescence and of the absorbance change at 820 nm, respectively; (2) the absolute changes in 77 K fluorescence yields emitted by PS II and PS I; and (3) the quenching of room temperature Chl a fluorescence. In all experiments, we observed complementary changes between sigma(PS I) and sigma(PS II), sigma(PS I) consistently increasing at the expense of sigma(PS II). ATP-induced decreases of sigma(PS II) were 6.9% and 11.2% at 5 and 1 mM MgCl2, respectively, whereas sigma(PS I) increased by 12% and 18.6% under these conditions. In absence of Mg2+, sigma(PS II) and sigma(PS I) changed respectively by -26.2% and +38.9% relative to thylakoids resuspended in presence of 5 mM MgCl2. These relative increases of sigma(PS I) are larger than the relative decreases of sigma(PS II) by a factor of 1.5-1.7, probably due to the stoichiometry between PS II and PS I complexes typically found in sun-adapted spinach leaves. Also, we observed that the increases of sigma(PS I) correspond closely to the increases of 77 K fluorescence yields emitted at 735 nm by PS I. However, no clear relationship could be detected between the changes of sigma(PS II) and the quenching of both room temperature and 77 K PS II fluorescence. The reasons for such discrepancy are discussed.