Time-dependent quenching of chlorophyll a fluorescence from (pigment) system II by (pigment) system I of photosynthesis in Chlorella.

The quenching of System Ii fluorescence bv System I light was discovered in 196o (ref. I) and was later investigated in detail 2-~. DUYSENS AND SWEEI~S a suggested that System II reduces a compound "Q" , and System I oxidizes this reduced Q back to its oxidized form. The Q in the oxidized, but not in the reduced, form is a quencher of fluorescence. We have measured the time dependence of this quenching of System II fluorescence by System I light in Chlorella p3,re~zoidosa. In the first few seconds of the fluorescence transient, however, we observe a stimulation, and only thereafter a quenching. The quenching effect attains a maximum value after 3o 12o sec of System II illumination and then declines to a low level. A comparison of this data with the time-course of System II fluorescence suggests that additional assumptions are required to explain our data. Chlorella pyret~oidosa (Emerson strain 3) was grown for 4 days in inorganic medium a and resuspended in NaHCO3 K2CO~ buffer, pH 9.2. Several cultures and at least twenty samples have been tested for the results presented here. Measurements were made with the instruments described earlier G,7. Fluorescence was excited by intense blue light (2 peak, 48() nm; half-maximum band width, IOO nm; absorbed quanta, approx. 4-1o 1~ sec.cm2), and was measured at 685 nm (half-band width, 5 nm). Quenching was caused by 7IO nm light (half-band width, 13 nm; absorbed quanta, approx. 5" IO1~ sec .cm2). 546 nm light of similar band width and intensity caused no change in fluorescence yield. Filters were used to transmit fluorescence and to prevent nearly all the exciting light from entering the measuring monochromator. With 71o nm light alone, the light leak was very low, less than 2 °o of blue-excited fluorescence; it was measured and subtracted from the blue -~71o nm excited fluorescence. The experimental procedure was a series of 5 min dark 5 rain (blue) light cycles. Blue light preferentially excited System II. Typical fluorescence transients resulting from the dark-light cycles have been published previously 7,s. The Svstem I I fluorescence yield (observed at 685 nm) undergoes a slow induction labelled S, M and T (see Figs. I and 2). Interaction with System I was studied bv adding 71o nm light at different times during the induction of fluorescence mentioned above. The 7Io-nm light caused no fluorescence quenching in the first 5-1o sec of the (blue) light period (Figs. i and 2). Instead, the fluorescence yield increased slightly