Calcium and magnesium effect on divalent cation deficient Hydrilla verticillata thylakoid electron transport activity.

The role of divalent cations like magnesium (Mg2+) and calcium (Ca2+) was irrvestigated on energy distribution process of Hydril/a verticil/ata thylakoids. Effect of these cations was tested on relative quantum yield of photosystem (PS) II catalyzed electron transport activity, room and liquid nitrogen temperature fluorescence emission properties and thylakoid light scattering characteristics. The electron transport activity was found to be stimulated in the presence of these cations in a light intensity independent manner. The concentration of cation required for maximum stimulation was nearly 10-12 mM. Comparatively, Ca2+ was more effective than Mg2+. Cation induced stimulation in electron transport activity was not accompanied by increase in chlorophyll a fluorescence intensity either at room (25°C) or liquid nitrogen (77°K) temperatures. Furthermore, 540 nm absorption and 90° light scattering properties of thylakoids remained insensitive towards divalent cations. These facts together suggest that divalent cations in Hydril/a thylakoids are not effective in supporting the excitation distribution between the interacting photosystem complexes.

Effect of calcium ion on Hydrilla verticillata thylakoid membrane O2 evolution.

Calcium ion-dependent reactivation of O2 evolution activity has been investigated in Hydrilla verticillata thylakoid preparations. Washing the thylakoids in calcium-free buffer or calcium-free buffer containing 1.5 M NaCl or 1.5 M NaCl plus 20% methanol, reversibly inhibited O2 evolution activity. The activity was restored on addition of calcium as calcium chloride and partially by strontium chloride. Immobilization of thylakoids with glutaraldehyde (GA) arrested the loss in O2 evolution activity caused by calcium-free high salt washing. However, calcium sensitivity was discernible in GA immobilized thylakoids subjected to calcium-free high salt washing. Since glutaraldehyde checks the loss of extrinsic thylakoid polypeptides due to washing, it is assumed that the calcium ion has regulatory functions in the photosynthetic electron transport, besides its interaction with thylakoid proteins.

Characterization of aliphatic amine 1,4-diazobicyclo (2,2,2) octane-induced stimulation of beet spinach thylakoid electron transport.

Electron transport activity of beet spinach thylakoids was enhanced in the presence of aliphatic amine, DABCO (1,4-diazobicyclo (2,2,2) octane), a hydrophilic proton trapping agent. The extent of stimulation was pH-dependent and similar to the effect of the uncoupler ammonium chloride on electron transport. The stimulation of whole-chain (H2O-->MV) electron transport activity was observed only at high (rate-saturating) light intensity. The light-induced proton uptake coupled to electron transport of thylakoid was also arrested by DABCO, suggesting that DABCO uncouples thylakoid phosphorylation by proton trapping.