The effect Mg2+ on the reduction of NADP by an artificial electron donor.

The effect of Mg2+ on the light-dependent reduction of NADP by an artificial electron donor has been investigated with isolated broken chloroplasts. In both short and long wavelength actinic light, addition of the salt under most conditions stimulates the yield and the saturation rate. The magnitude of the stimulation is a function of pH. The data indicate that previous interpretations of the Mg2+ effect in this system invoking changes in the spill-over rate between the photosystems is no longer tenable. It is suggested that addition of the salt causes activation of inactive reaction centers.

Light-dependent interactions of phenazine methosulfate with 3-(3,4-dichlorophenyl)-1,1-dimethylurea-poisoned chloroplasts.

The chlorophyll fluorescence of isolated chloroplasts in the presence of phenazine methosulfate (PMS) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) can be quenched in a light-depentdent reaction. This phenomenan has been studied and the following observations were made: 1. Quenching occurs under non-phosphorylating conditions and is stimulated by Mg2+ ions. 2. Under the same conditions, a light-dependent, Mg2+ stimulated transient decrease of absorption at change and binding of PMS. Others, such as methylamine, ammonia, gramicidin and nigericin do not. It is suggested that fluorescence quenching, transient absorption change and binding of PMS are casually related. The concept, postulated by others, that a high-energy state of the chloroplast membrane is involved in the fluorescence lowering is questioned.

Light-dependent development of photosynthetic competence in Scenedesmus mutant No. 8.

The heterotrophically grown, P-700-free mutant No. 8 of Scenedesmus obliquus is unable to carry out photosynthesis. Yet, chloroplast particles isolated from the alga reduced ferricyanide. They also reduced methyl viologen in the presence of the artificial donor reduced 2,6-dichlorophenol indophenol with a low yield but an appreciable saturation rate. NADP reduction or P-700 turn-over could not be detected. When grown mixotrophically, the mutant showed increasing P-700 activity with a concomitant increase in the rate of photosynthesis. Both activities were lost again when the algae were returned to darkness. Isolated chloroplast particles showed a good P-700 turn-over and reasonable rates of NADP reduction. The data suggest that the mutation occurred at a site preceding the formation of the pigment. The results on the photochemical activities are discussed in the light of reports concerning the involvement of P-700 in linear electron transport.

Antagonistic relationships between electron transport and P700 in chloroplasts and intact algae.

The effects of divalent salts and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) upon nicotinamide adenine dinucleotide phosphate (NADP) reduction and P700 in isolated chloroplasts are described and compared with the influence of DCMU on oxygen evolution and P700 in intact cells. Most experiments were carried out with a steady-state relaxation spectrometer. A kinetic mechanism for the estimation of P700 fluxes is proposed and experimentally tested. Good agreement between theory and experiment was found. Concurrent measurements of P700 and NADP reduction revealed two antagonisms: addition of divalent cations caused an increase in the yield of NADP reduction and a decrease in the yield of P700. Conversely, in the presence of Mg2+ low concentrations of DCMU decreased the yield of NADP reduction and increased the yield of P700. Aging of the chloroplasts at 30 degrees C exerted a similar effect. With far-red actinic light, Mg2+ stimulated the yield of NADP reduction without affecting the flux. Also, in the absence of Mg2+, DCMU inhibited both reactions although P700 required a higher herbicide concentration for fractional inhibition than NADP reduction. In the presence of Mg2+, chloroplasts resembled intact algae in which a high rate of oxygen evolution was accompanied by little P700 turn-over. Titration with DCMU decreased the rate of photosynthesis and increased P700 flux. On the whole, the data suggest that P700 relaxing in 20 msec is not directly involved in linear electron transport.