[Investigations on the effect of blue light on the photosynthetic O2 exchange]

1. In cells of Chlorella fusca darkened for 12 and more hours and preilluminated with red light, irradiation with blue light of low intensity (5×10(-10) einstein cm(-2) sec(-1), 465 nm) for 2 minutes results in a substantial enhancement (up to 80%) of the "gross O2 evolution" (difference between the rate of O2 exchange in dark and light) in a subsequent red light period (5×10(-10) einstein cm(-2) sec(-1), 670 nm). 2. By analysing the kinetics of O2 exchange after the light has been switched on and off it is possible to distinguish between true photosynthetic O2 evolution and a photosynthetic inhibition of respiratory O2 uptake. Only the latter component is strongly enhanced by blue light, whereas O2 evolution is almost unchanged. This is also shown when both components are separated by using various inhibitors. 3. It was shown in different ways that the inhibitory effect of photosynthesis on the respiratory O2 uptake is strongly dependent on the respiratory activity. This inhibition is most pronounced at medium activity, but nearly zero at very low respiratory activity. Therefore it is concluded that blue light works primarily only by enhancing the respiratory activity, which declined to very low values during the preceeding long lasting dark period. 4. This interpretation is strongly supported by the finding that any acceleration of the respiratory dark turnover by uncouplers or by glucose at low concentrations produces quite the same effect on the photosynthetic O2 exchange as blue light does.