Fraction Of Open Reaction Centers Research Articles

Fluorescence Induction From Photosystem II: Analytical Equations for the Yields of Photochemistry and Fluorescence Derived From Analysis of a Model Including Exciton-Radical Pair Equilibrium and Restricted Energy Transfer Between Photosynthetic Units

The theoretical relationships between the fluorescence and photochemical yields of photosystem II (PSII) and the fraction of open reaction centres are examined in a model based on the following assumptions: (a) a homogeneous, infinite PSII domain; (b) exciton-radical pair equilibrium; and (c) different rates of exciton transfer between 'core' and 'peripheral' antenna beds. Simple analytical relations are examined for the yields and their time-courses in induction experiments. Variation of the inter-unit transfer rate allows continuous transition from the case of 'separated units' to the pure 'lake' model. Widely used relations for estimating the fraction of closed reaction centres from the complementary area of the fluorescence, or the photochemical yield from fluorescence levels are derived. An experimental induction curve is analysed, considering its composition of 'α' and 'β' centres. The sigmoidicity of the induction kinetics is characterised by a single parameter J (corresponding to Joliots' 'p'), that is shown to depend on both the connectivity of the photosynthetic units and on reaction centre parameters. On the other hand, the relation between J and the extreme fluorescence levels (or the deviation from the linear Stern-Volmer dependence of 1 /Φf, on the fraction of open traps) is only controlled by antenna connectivity. Experimental data are more consistent with a model of 'connected units' for PSIIα than with the pure 'lake' model.

Photosynthetic electron transport in tobacco leaves infected with tobacco mosaic virus

Tobacco (Nicotiana tabacum L. cv. Samsun) plants inoculated with different strains of tobacco mosaic virus (TMV) inducing mosaic symptoms of widely varying severity were studied with in vivo chlorophyll fluorescence. This method was used to deduce photosynthetic electron transport efficiency in relation to symptom expression. The quantum yields of photosystem II (PS II) electron transport rate were significantly diminished in virus strains inducing loss of chlorophyll. The reduction in young mosaic‐diseased leaves appeared to be due in part to a reduction in the fraction of open reaction centers, whereas in older leaves exhibiting less pronounced symptoms the reduction was mainly caused by a reduced efficiency of capture of excitation energy of open PS II reaction centers. Upon infection with any of the five virus strains PS II seemed to be irreversibly damaged in the inoculated leaves and the ones directly above, indicative of a possible increased susceptibility to photoinhibition in these leaves (Somersalo and Krause 1989) even when no symptoms were apparent. Symptom expression did not appear to be related to the influence of the virus on PS II activity, because the severest effects occurred in the inoculated leaves, which either remained symptomless or developed slight yellowing only. This study demonstrates the usefulness or modulated chlorophyll fluorescence measurements for the investigation of plant‐virus interactions. It is particularly important when visual symptoms are absent.