EVIDENCE FOR PREDOMINANT PARTICIPATION OF PHOTOSYSTEM II-BETA-UNITS IN NITROGEN FORMATION BY HYDROXYLAMINE-TREATED TOBACCO CHLOROPLASTS

Relative optical cross sections for flash-induced O-2- and N-2-production with water and 1 mM NH2OH as electron donors to PSII, respectively, as well as that for PSI-mediated O-2-uptake have been measured in tobacco chloroplasts by mass spectrometry. In the wild type tobacco these three reactions are driven by three populations of photosynthetic units different with respect to the antenna size. The antenna size of O-2-evolving units is twice as large compared to the N-2-evolving one, but both of these have the same spectral characteristics in the far red region. In contrast to the wild type, the antenna sizes of O-2- and N-2-evolving units in the chlorophyll b-deficient tobacco mutant Su/su var. Aurea are the same as the sizes of the N-2-evolving units in the wild type chloroplasts. Taking into account the data of Thielen and Van Gorkom [Biochim. Biophys. Acta (1981) 635:111] on a strong difference in the relative amounts of PSIIalpha and PSIIbeta in the mutant compared with the wild type (the ratio about 1:3 and 3:1, respectively) it is concluded that mainly the stroma-exposed PSIIbeta units are competent to N-2-evolution.