Etiolated oat mesophyll protoplasts and etioplasts released from etiolated protoplasts were able to perform protochlorophyliide (PChlide) reduction but showed a high amount of inactive P633/628» compared with intact leaves. In špite of this, the anabolic reduction charge (ARC, NAPDH+H+/NADP+ + NADPF1+H+) was maintained at 0.66 - 0.77 up to 30 min of irradiation with weak "white light" (10 W m‘2). This indicated a high buffering capacity of the isolated systém to provide NADPF1+H+, only at the expense of intemal reserves. A change in the redox statě of the NADP+/NADPH+H^ couple during protochlorophyliide reduction was not observed in etioplasts. The resulting redox potential was in the range of -330 mV optimal for protochlorophyliide reduction.