Flash Pattern of Oxygen Evolution in Greening Etioplasts of Oat

Etioplasts isolated from oat leaves that have received two 5 min illum ination periods, spaced apart by a 90 min dark interval, are able to evolve oxygen. If this oxygen evolution is measured as consequence o f short saturating light flashes a pattern is obtained which in agreement with the Kok-Joliot model shows a damped oscillation with a periodicity o f four. As usual for most other systems the maximum flash yield is observed under the third flash. In continuous light such slightly greened etioplasts show a small oxygen gush. From our studies it appears that the second 5 min illumination period is obligatory for oxygen evolution. Om ission o f this period leads to a condition in which under flashes strictly no oxygen is evolved. In continuous light under these conditions no oxygen gush but rather an uptake is observed after onset o f illumination. It appears that in greening etioplasts the flash pattern shows some peculiarities when compared to the normal green conditions. Thus, a flash sequence o f a greening etioplast always shows a substantial positive amperometric signal under the first flash. The half time o f the state which would belong to this signal is 3 to 4 min in a greening etioplast, isolated from leaves that have received 2 h light. In the course o f greening this half tim e decreases continuously and reaches a normal and constant value o f approx. 1 0 s in chloroplasts from green oat or etiochloroplasts that are prepared from leaves that have received 15 h o f light. The result is discussed in terms o f metastable S3 which seems to be a property o f the developing oxygen evolving system in greening oat leaves. In the greening system as we use it is shown for the first time that the photoenzyme protochlorophyllide chlorophyllide reductase is DC M U sensitive.