Bicarbonate utilization and pH polarity. The response of photosynthetic electron transport to carbon limitation in Potamogeton lucens leaves

By the process of pH polarity, several submersed angiosperms can use bicarbonate as carbon source for photosynthesis. Under conditions of relatively high light intensity and low CO2 availability, the pH of the apoplast and unstirred layer becomes acid at one side of the leaf and alkaline at the other. In the acid region, bicarbonate is converted into CO2, which diffuses into the leaf where it is fixed. Previous experiments on the light-dependent reduction of extracellular electron acceptors led to the hypothesis of redox regulation. Under conditions of high light and low CO2, excess reducing power in the chloroplast was supposed to be shuttled to the cytoplasm where it can upregulate the plasma membrane proton pump, leading to activation of polarity. Chlorophyll a fluorescence is an indicator for photosynthetic electron transport, the energization of thylakoids, and the reoxidation of chloroplast NADPH. It was used therefore to test redox regulation in vivo in Potamogeton lucens L. leaves. The fluoresence parameter, qP, an indicator for photochemical quenching and NADPH reoxidation, appeared to be rather insensitive to the inorganic carbon concentration and to the presence or absence of polarity. In contrast, qN, an indicator for non-photochemical quenching related to thylakoid energization, photoinhibition, and state transitions, increased under conditions of low CO2 - high light and polarity. Taken together the data show polarity to be an effective mechanism to make bicarbonate accessible as carbon source and seem to agree with the idea of redox regulation of pH polarity.Key words: bicarbonate utilization, chlorophyll a fluoresence, pH polarity, redox regulation, Potamogeton lucens, submerged aquatic macrophyte.

The oxidation of exogenously added organic anions by platelets facilitates maintenance of pH during their storage for transfusion at 22 degrees C

Previous studies of platelet metabolism during the storage of platelet concentrates (PCs) at 22 degrees C for transfusion have shown high rates of both oxygen consumption and aerobic glycolysis with little oxidation of pyruvate produced by glycolysis. During storage in plasma, free fatty acids are major oxidative fuels. Glutamine is also present as a potential fuel but relatively little is metabolized beyond glutamate. In synthetic media, acetate is oxidized and provides a poorly understood buffering function. In the current work, acetate and pyruvate (6 to 25 mmol/L, sometimes with their 14-C-labeled counterparts) were added to PCs stored in plasma. They were both vigorously metabolized, predominantly to CO2 (0.51 +/- 0.08 and 0.31 +/- 0.06 mmol/d/10(12) platelets for acetate and pyruvate, respectively). The metabolism of these exogenous substrates was associated with significantly increased oxygen consumption and decreased glucose consumption, fatty acid oxidation, and bicarbonate utilization for buffering. In a more limited number of studies, similar findings were observed with addition of beta-hydroxy-butyrate. Superior maintenance of pH in the presence of these additives could be attributed to the fact that the metabolism of an organic anion requires that a proton be brought with the anion into the metabolic pathway, thus providing an alkalinizing effect. Pyruvate (but not acetate) also stimulated the metabolism of glutamate. Studies with 14-C-labeled glucose suggested significant activity of the hexose-monophosphate shunt during PC storage and confirmed that little, if any, pyruvate derived from glucose was fully oxidized. Taken together, the results provide a relatively complete picture of the pathways of energy metabolism used by platelets during PC storage and suggest a strategy by which organic anions such as acetate can be used to improve the results of such storage.