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-product ion 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 to bacco chloroplasts by mass spectrometry. In the wild type tobacco thes e three reactions are driven by three populations of photosynthetic un its 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, b ut 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 Va n Gorkom [Biochim. Biophys. Acta (1981) 635:111] on a strong differenc e in the relative amounts of PSIIalpha and PSIIbeta in the mutant comp ared with the wild type (the ratio about 1:3 and 3:1, respectively) it is concluded that mainly the stroma-exposed PSIIbeta units are compet ent to N-2-evolution.