Kinetics of the flash-induced electrochromic absorbance change in the presence of background illumination. Turnover rate of the electron transport. II. Higher plant leaves

The flash-induced electrochromic absorbance change (A₅₁₅) was measured in leaves of higher plants in the absence and presence of continuous monochromatic background illumination of different intensities and wavelengths. The variation of the amplitude of A₅₁₅ in background light was used to estimate the steady-state turnover time of the electron transport. In red light we obtained about 5 msec which was accounted for by the turnover of the linear electron transport. With far red background illumination or in the presence of the photosystem 2 inhibitor, DCMU, the steady-state turnover time tentatively assigned to photosystem 1 cyclic electron transport was much larger (100 msec).Increasing the intensity of background illumination with far red light gradually diminished the slow rise of A₅₁₅ in parallel with suppression of the initial rise generated by photosystem 1. At high intensities of the red light, however, while A₅₁₅ was attenuated, the slow rise was not eliminated and its proportion relative to the initial rise did not vary appreciably.