A comparison of the absorption changes near 325 nm and chlorophyll a fluorescence characteristics of the Photosystem II acceptors Qa and Q400

Heterogeneous photosystem 2 activity in isolated spinach chloroplasts

A study was made of the fluorescence induction curves from gently-broken spinach chloroplasts inhibited with DCMU. It was found that there were four kinetically different phases associated with such curves of which only the fastest did not appear to follow exponential kinetics. A comparison of the effects of various concentrations of DCMU on the rate of oxygen evolution and on the fluorescence induction curve did not support the hypothesis that any of the kinetic phases was simply an artefact caused by incomplete inhibition of electron transport. It was also found that 5 min of dark incubation did not maximally oxidize the electron acceptors to photosystem 2 since some acceptors were only oxidized following far-red illumination, suggesting a heterogeneity among these acceptors with respect to their re-oxidation properties. Investigation of the effect of the Q400 oxidation state on the fluorescence induction curve revealed that it only influenced the slowest kinetic phase and that Q400 did not seem to be associated with the other phases.

Measurement of stoichiometry of photosystem II to photosystem I reaction centers

A wide range of values for the photosystem II to photosystem I stoichiometry have been reported. It is likely that some of this variation is due to measurement artifacts, which are discussed. Careful measurements of photosystem II reactions by absorption change at 325 nm, and flash yields of oxygen evolution, of protons from oxidation of water and of reduction of dichloroindophenol give equivalent results. Stoichiometries other than 1:1 are routinely found, and they vary with growth conditions as well as plant type. Two atrazine binding sites are found for every photosystem II reaction center that is active in oxygen evolution.

Reconciliation of the absorption change at 325 nm and other flash-yield determinations of concentrations of active photosystem II centers

The concentration of photosystem II was determined in thylakoids of dwarf peas by the use of the following methods: absorption change at 325 nm; atrazine binding; and flash yields of oxygen evolution (Emerson-Arnold method), of protons from oxidation of water, and of reduction of DCIP. For the first time all of the flash-yield measurements have been done on the same sample and give equivalent values for the concentration of photosystem II. Agreement of the absorption change measurement at 325 nm with the other measurements was accomplished by the introduction of important improvements to the methods of Melis and co-workers [Proc. Natl. Acad. Sci. USA (1980) 77, 4712-4716]. The atrazine-binding method gave photosystem II values that were twice as large as any of the other photosystem II measurements. Possible reasons are discussed for this discrepancy in terms of the secondary acceptor (Q400) of Ikegami and Katoh [Plant Cell Physiol. (1973) 14, 829-836]. The concentration of photosystem I was measured by absorption change at 705 nm. From the concentration values of photosystem II and I the system II/I stoichiometry was calculated.