Steady-State Chlorophyll a Fluorescence Transients during Ammonium Assimilation by the N-Limited Green Alga Selenastrum minutum1

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The assimilation of ammonium by the N-limited green alga Selenastrum minutum results in the suppression of photosynthetic electron flow from H2O to CO2 (6, 7, 18). In this study, results are presented which describe the correponding change in steady-state chlorophyll a fluorescence. The addition of ammonium resulted in a transient decline in fluorescence followed by a marked increase. Fluorescence did not return to control levels until the added ammonium had been assimilated. Analysis of the fluorescence transients showed that ammonium assimilation resulted in a rapid increase in nonphotochemical quenching (Qe) peaking 10 to 15 seconds after ammonium addition. Qe then decreased dramatically reaching a minimum value approximately 45 seconds following ammonium addition and returned to the control level only after the added ammonium had been assimilated. There were no effects of ammonium addition on photochemical quenching (Qq) for approximately 10 to 15 seconds at which time both gross O2 evolution (as measured by mass spectrometry) and Qq declined. In the presence of d,l-glyceraldehyde or when cells were held at the CO2 compensation point, the addition of ammonium resulted in a decline in Qe 10 to 15 seconds after addition. The Qe peak and the Qq decline were absent. These results imply that the transient increase in Qe and the subsequent decline in Qq may be attributed to the decline in Calvin cycle activity during ammonium assimilation. The decline in Qe is apparently a direct result of ammonium assimilation. The observation that the Qe peak precedes the Qq decline would be consistent with the decreases in Calvin cycle carbon flow occurring at the kinase reactions prior to glyceraldehyde-3-phosphate dehydrogenase.

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