Light intensity saturation properties of O2 yields in a sequence of flashes in Chlorella

As a function of the light intensity of flash n in a sequence, the O2 yields Yn, Yn+1 and Yn+2 have been measured: n = 1, 2, 3 and 6 in the examples given. It is shown that: (1) No double hit exists in the first saturating flash in Chlorella. (2) the flash saturation curve of the O2 yield Yn+1 as a function of the intensity of flash n exhibits a small sigmoidal shape at weak light. (3) If Yn+1 is detected at different times after the flash n of variable intensity, a well developed lag distinguishes the saturation curve of the O2 yield measured a long time after flash n (200 ms) with respect to that measured at shorter time (300 microseconds). Nevertheless, a large amount of double hits with the transitions S1 leads to S3 cannot occur in each flash, because it would lead to a periodicity of three rather than four in the O2 yield pattern. The saturation curve of the transition S2* leads to S3 is different from the other S-state saturation curves which are close to an exponential function; even with a short flash (0.3 microseconds), this curve shows a small lag at low light intensity, and its saturation intensity is higher than that of the other transitions. The low quantum yield of the transition S2* leads to S3 at low flash light intensity is explained by a product, T, partially inhibiting the formation of S3; at higher intensity, the quantity of formed S2* being larger than that of available T, only a part of S2* is inhibited and the quantum yield is higher than at low intensity.