Energy distribution between photosystems (PSI & PSII) under prolonged and continuous white light irradiance was assessed by monitoring the progress of their fluorescence emission (F-PSI/F-PSII) at 77 K. Our observations indicate F-PSI/F-PSII to oscillate with the progress of irradiance treatments at all intensities tested (100, 200, 500, and 800 mu E m(-2) S-1). The amplitude of the oscillation increased with the progress, whereas the periodicity of the oscillation increased with the intensity of the incident irradiance. Spectral analysis indicated fluctuation of F-PSI to be the major determinant of the observed oscillation. The first rise and fall of F-PSI/F-PSII overlapped with phosphorylation and dephosphorylation of LHCII, but oscillation of F-PSI/F-PSII continued for several cycles without any further phosphorylation of LHCII. Moreover, in presence of DCMU where linear electron flow (LEF) is suppressed and LHCII phosphorylation is completely abolished, the oscillation of F-PSI/F-PSII was not abolished. These data indicated that LHCII phosphorylation was not essential for the observed oscillation of energy distribution between the photosystems. In contrast, in the presence of inhibitors of cyclic electron flow (CEF) like Antimycin A (AA) and rotenone, the oscillation of F-PSI/F-PSII was either abolished or severely dampened. Additionally, the oscillation was also abolished in presence of uncouplers like NH4Cl and nigericin that cancels the trans-thylakoid a dagger pH. Thus, trans-thylakoid a dagger pH, generated through CEF, appear to be an important determinant of oscillation of F-PSI/F-PSII in isolated thylakoids. The phenomenon of oscillation could be associated with a CEF mediated chromatic adaptation of PSI in presence of excess irradiance.