In addition to membrane translocation, measured in the dark, it was found that pre-illumination of the chloroplasts resulted in an enhancement of sulfate uptake by 25 and of sulfite uptake by 55 as soon as the concentration of the ion in the incubation medium exceeded 2 mmol l-1. This amount which is additionally taken up after pre-illumination is less readily exchanged for other ions. Kinetics of the uptake in relation to pre-illumination time and to light intensity closely parallel those of titration of SH-groups by 5,5′-dithiobis (2-nitrobenzoic acid). As a consequence, 10-6mol l-1DCMU completely inhibits the light triggered increase of uptake of both ions. Uncoupling with 10-6mol l-1CCCP increases the light induced35SO32-binding, but decreases that of35SO42-, demonstrating the need of ATP formation to initiate sulfate reduction. Rates of uptake, measured at different intensities of pre-illumination under nitrogen or in the presence of bicarbonate, suggest that the presence of a carbon skeleton increases the binding rate for both ions. With respect to35SO42-, the data further indicate a rate limiting step (ATP sulfurylase or adenosine 5′-phosphosulfate sulfotransferase) which is activated by light, thus representing a control step to harmonize the rate of CO2fixation and of sulfate incorporation. On the contrary,35SO32-is directly bound in relation to the amount of SH-groups, which in turn are created by the photosynthetic electron transport, resulting in Car-S-SO3-. Since the formation of SH-groups is maximal already at low light intensities, no effective control step for SO32-incorporation is indica
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