Abstract— In 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) poisoned chloroplasts of algae and‘ higher plants the area over the fluorescence induction curve increases with biphasic first order kinetics (Melis and Homann, 1975). Two possibilities are considered to explain the biphasic nature of the area growth. The first is a sequential double reduction of the primary electron acceptor in system II while the second envisages a heterogeneity of its photochemical centers. The kinetic properties of the area growth after firing a single saturating flash proved to be incompatible with the predictions of the “sequential double reduction” model. This conclusion was corroborated by results obtained from a kinetic analysis of the area restoration process in the dark, and an analysis of the partially restored areas. Assuming an existence of a heterogeneous pool of photochemical centers, the growth of the area over the fluorescence curve could be further analyzed to yield two components, a fast a‐component, and a relatively slow β‐component. The kinetic characteristics of these components, and the effect of a short saturating flash on their respective size, led to the conclusion that one type of photochemical center had a faster recombination rate of the photochemically separated charges and was less efficient in trapping excitation energy.

Abstract— The size of the area over the fluorescence rise curve of chloroplasts is a measure of the total number of quanta utilized in photosystem II during the fluorescence induction, while the growth of the area reflects the progress of photochemical events. In the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), the growth kinetics of the area are affected by the reoxidation of the primary acceptor Q‐ with stored oxidizing charges on the donor side of system II. At low light intensities, a slow component of this back reaction may limit the steady state fluorescence emission. At higher intensities, however, the fluorescence rise is limited solely by photochemical events, although fast thermochemical reactions like the immediate recombination of photochemically separated charges may affect the efficiency of the photochemistry. A kinetic analysis of the area growth at moderate light intensities revealed that it occurred in two first order phases which were described by the rate constants kα and kβ. The biphasic nature suggested a sequential two‐electron reduction of the primary acceptor Q, or the presence of two different types of photochemical centers in system II. The rate constants were light intensity dependent. They also were affected by changes in pH, by an addition of NH2OH, or by a preillumination with short flashes prior to addition of DCMU. It is suggested that the pH of the medium, the presence of NH2OH, and the flash induced state Sn of the water splitting enzyme, control the values of kα and kβ by changing the rate constants of electron carrier interactions in the reaction center complex, with a resulting modification of the frequency of back reactions between the primary donor and the primary acceptor.

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Sumlmary. Photosynthetic reactions of whole leaves and isolated chloroplasts from various mutants of Nicotiana tabacum have been correlated to the lamellar structure seen in electron micrographs of the chloroplasts. In this way it could be established that a fuilly,active photosystem I can be associated with single unfolded thylakoids. The complete photosynthetic electron transport svstem including the oxygen evolving apparatus of photosystem II, on the other hand, apDears to require a close packing of at least 2 thylakoids. The unusual high capacity for photosynthesis observed earlier for leaves of certain aurea mutants is reflected by a correspondingly high activity of the isolated chloroplasts in the Hill reaction. These chiloroplasts contain extended areas where 2 thylakoids touch 'by forming simple lamellar overlappings instead of the familiar stacks of lamellar discs.It is not surprising that the evolution of the photosynthetic apparatus from its simplest form in the green photosynthetic bacteria to the elajborate system of the green -plants was accompanied by the evolution of an ever more complicated pi-gomented structure. The relationship, however, between the photosvnthetic processes and the membrane systems oll which they occur is still very little understood. A possible approach to this problem was opened by recent publications of Schmid and Gaff ron (30,31,33