A study on the lateral distribution of the plastoquinone pool with respect to photosystem II in stacked and unstacked spinach chloroplasts.

The quenching of Photosystem II (PS II) chlorophyll fluorescence by oxidised plastoquinone has been used in an attempt to determine their relative distribution in the partition zone and stroma-exposed thylakoid membranes. Thus, the PS II-plastoquinone interaction was determined in stacked (2.5 mM MgCl2) and largely unstacked (0.25 mM MgCl2) membranes. A method to correct for spillover or other quenching changes at the different MgCl2 concentrations, which would compete with the plastoquinone-induced quenching, was devised utilising the quinone dibromothymoquinone. This compound is demonstrated to behave as an ideal (theoretically) PS II quencher at both high and low MgCl2 concentrations, which indicates that it distributes itself homogeneously between partition zone and stroma-exposed membrane regions. In passing from the stacked to the unstacked configuration, the PS II-plastoquinone interaction decreases less than the PS II-dibromothymoquinone interaction. This is interpreted to mean that plastoquinone is present in both the partition zone and stroma-exposed membranes, with somewhat higher concentrations in the stroma-exposed membranes. Thus, plastoquinone is well placed to transport reducing equivalents from the partition zones to the stroma-exposed membranes

Grana formation in chloroplasts may promote energy transfer between photosystem II units.

Here we have utilised the observation [2,13] that lowering the medium pH to 5.4 brings about the formation of grana which are morphologically indistinguishable from those induced by metal cations at neutral pH values. These grana seem also to be biochemically similar to those produced by metal cations as indicated by the chlorophylls/b ratios of digitonin fractions [ 141. We now demonstrate that incubation of chloroplasts at pH 5.4 strongly favors energy transfer between photosystem II units, and suggest that this may be due to grana formation