The site of action of nitrite on PS II was investigated by measuring the TL profile of nitrite-treated spinach thylakoid membranes. Three bands were observed in control, which were identified as the Q band (7 degrees C), the B band (24 degrees C) and the C band (57 degrees C). In the presence of 20 mmol/L nitrite, the intensity of the Q band decreased, the B band upshifted to 46 degrees C but the C band disappeared. The suppression of the Q band and the upshift of the B band suggested that nitrite caused inhibition at the water oxidizing complex. The effects of nitrite also remained the same in the presence of chloride. In case of ion-sufficient thylakoid membranes, nitrite decreased the Q band peak intensity and caused an upshift in the B band peak temperature. Nitrite showed similar effects in the presence of DCMU. This suggested that the site of action of nitrite is not at the acceptor side but at the donor side of PS II. The inhibition shown by nitrite has been found to be specific for nitrite anion. No other anions such as formate, fluoride or nitrate, were effective.
The reversibility of nitrite‐induced inhibition in relation to energy distribution between the two photosystems was studied in spinach thylakoid membranes. Measurements of electron transfer rate catalyzed by photosystem I (PS I) and photosystem II (PS II), chlorophyll a (Chl a) fluorescence induction kinetics, S2 state multiline spectra, and room temperature electron paramagnetic resonance (EPR) signals indicated that nitrite anions bind PS II in two ways: dissociable (loose) and non‐dissociable (tight). The inhibition caused by the dissociable binding was reversible in washed (nitrite‐treated samples washed with nitrite‐free medium) samples, while the inhibition caused by the non‐dissociable binding was irreversible. At 77 K, an increase in absorption cross section of PS I (as inferred from the excitation spectra of Chl a fluorescence) and a decrease in absorption cross section of PS II in nitrite‐treated sample when compared with sample washed with nitrite‐free medium and control sample suggested that nitrite plays a role in regulating the distribution of absorbed excitation energy between the two photosystems. We propose, for the first time, that the removal of loosely bound nitrite leads to migration of light‐harvesting complex II back to the PS II, and thus the mode of binding of nitrite regulates the extent of migration of antenna molecules between the two photosystems.
The effects of oxalate on PS II and PS I photochemistry were studied. The results suggested that in chloride-deficient thylakoid membranes, oxalate inhibited activity of PS II as well as PS I. To our knowledge, this is the only anion so far known which inhibits both the photosystems. Measurements of fluorescence induction kinetics, YZ* decay, and S2 state multiline EPR signal suggested that oxalate inhibited PS II at the donor side most likely on the oxygen evolving complex. Measurements of re-reduction of P700+ signal in isolated PS I particles in oxalate-treated samples suggested a binding site of oxalate on the donor, as well as the acceptor side of PS I.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.