Evidence for Predominant Participation of Photosystem II{beta}-Units in Nitrogen Formation by Hydroxylamine-Treated Tobacco Chloroplasts

Relative optical cross sections for flash-induced O₂- and N₂-productionwith water and 1 mM NH₂OH as electron donors to PSII, respectively,as well as that for PSI-mediated O²-uptake have been measuredin tobacco chloroplasts by mass spectrometry. In the wild typetobacco these three reactions are driven by three populationsof photosynthetic units different with respect to the antennasize. The antenna size of O₂-evolving units is twice as largecompared to the N₂-evolving one, but both of these have thesame spectral characteristics in the far red region. In contrastto the wild type, the antenna sizes of O₂- and N₂-evolving unitsin the chlorophyll b-deficient tobacco mutant Su/su var. Aureaare the same as the sizes of the N₂-evolying units in the wildtype chloroplasts. Taking into account the data of Thielen andVan Gorkom Biochim. Biophys. Acta (1981) 635:111] on a strongdifference in the relative amounts of PSII_ß and PSII_ßin the mutant compared with the wild type (the ratio about 1: 3 and 3 : 1 , respectively) it is concluded that mainly thestroma-exposed PSII_ß units are competent to N₂-evolution. (Received November 11, 1993; Accepted April 25, 1994)