Evidence for Predominant Participation of Photosystem IIβ-Units in Nitrogen Formation by Hydroxylamine-Treated Tobacco Chloroplasts

摘要

Relative optical cross sections for flash-induced O2- and N2-production with water and 1 mMNH2OH as electron donors to PSII, respectively, as well as that for PSI-mediated O2-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 O2-evolving units is twice as large compared to the N2-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 O2- and N2-evolving units in the chlorophyllb-deficient tobacco mutant Su/su var. Aurea are the same as the sizes of the N2-evolying 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 PSIIβand PSIIβin the mutant compared with the wild type (the ratio about 1 : 3 and 3 : 1 , respectively) it is concluded that mainly the stroma-exposed PSIIβunits are competent to N2-evoluti