Dithionite‐Induced Fluorescence Quenching does not Reflect Reductive Activation in Spinach Chloroplasts*

Dithionite is found to cause substantial quenching of chlorophyll fluorescence in intact spinach chloroplasts. In a light‐on induction curve, the I‐P fluorescence rise is selectively suppressed, with a half‐maximal effect at 1.7 × 10−5 M dithionite. Quenching analysis by the saturation pulse method reveals that the dithionite‐induced quenching is photochemical, in analogy to the effect of a Hill reagent. The quenching is suppressed by catalase, KCN and by O2 removal before dithionite addition. These observations, in combination with recent findings on H2O2‐induced photochemical quenching (Neubauer and Schreiber, 1989), lead to the conclusion that H2O2 is formed from reduction products of O2 by dithionite and that it is this H2O2 which, via the action of the ascorbate peroxidase, sustains Hill activity. The results are discussed with reference to the concept of reductive activation of the ferredoxin: NADP oxidoreductase put forward by Satoh (Satoh, 1982). It is concluded that such activation is unlikely to play an essential role in spinach chloroplasts. This conclusion is supported by the observation of suppression of the I‐P fluorescence rise by oxidized NADP in dark‐adapted spinach chloroplasts.