Given that the same changes in plastoquinone redox state initiate both light-state transitions and complementary changes in chloroplast reaction centre gene transcription, it is of interest to ask whether Chloroplast Sensor GDC-0941 Kinase is required for state transitions. Light-induced changes in the rate of run-on chloroplast transcription can be observed in as little as fifteen minutes, suggesting the possibility of synchronous induction of state transitions and the transcription control that leads, on longer time scales, to changes in photosystem stoichiometry. Here we report on state transitions in wild-type Arabidopsis thaliana and in a CSK T-DNA insertion line. State transitions were monitored as one of several components affecting room-temperature chlorophyll fluorescence yield in vivo, and by 77 K fluorescence spectroscopy of isolated chloroplast thylakoids. In addition, direct visualisation of thylakoid protein phosphorylation was carried out by autoradiography of protein gels from samples incubated with ATP. Measurements of the amplitude of chlorophyll fluorescence emission, in dark adapted leaves and in presence of continuous background lights, provide information on photochemical and non-photochemical quenching, with components also arising from high-energy state quenching as well as state 1-state 2 transitions. In dark-adapted leaves, the maximal photochemical yield of PS II, as determined from the Fv/Fm ratio, is essentially the same in the wild-type and the CSK mutant, and has the value of 0.80. Moreover, this value is unchanged by the growth conditions, both in the wild-type and the CSK mutant. This indicates that inactivation of the CSK gene does not affect the maximal photochemical yield of PS II. However, upon the onset of background actinic illumination which excites preferentially PS II, difference can be observed, both in the pre-steady state kinetics manifested in the socalled “Kautsky” effect and the steady-state level of fluorescence emission between the wild-type and the CSK mutants. Particularly, in plants grown under light 1 conditions, the onset of the Kautsky transient, which is an indication of the activation of Calvin circle, is delayed in the CSK mutant with respect to the wild-type, moreover the steady-state fluorescence emission level in the CSK mutant is significantly higher than in the wild-type. This is an indication that, under the same intensity and spectral distribution of actinic illumination, the plastoquinone pool is more reduced in the CSK mutant than in the wild-type control. Superimposition of a light that preferentially excites PS I after 20 minutes causes a drop in fluorescence emission. This is commonly explained in terms of an oxidation of the plastoquinone pool driven by PS I photochemistry which is in part limited by photon absorption when light 2 acts as the actinic source. The nitial effect of light 1 superimposition is accentuated in the CSK mutant, which might indicate that the high reduction level of PQ pool observed under light 2 actinic illumination results only from PS I limitation of the linear electron transport chain of thylakoids.