Acid‐base regulation during nitrate assimilation in Hydrodictyon africanum

Abstract

The acid-base balance during NO3 assimilation in Hydrodictyon africanum has been investigated during growth from (1) an analysis of the elemental composition of the cells, (2) the alkalinity of the ash and (3) the net H"^ changes in the medium during growth. These investigations agree in showing that some 0.25 excess organic negative charges are generated per N assimilation from NO3 as N-source and CO2 as C-source; the excess OH" (0.75 0H~ per NOJ assimilated) appears in the medium. Approximately half of the excess organic negative charge is attributable to cell wall uronates; the remainder is intracellular. All of the excess 0H~ appearing in the medium must have crossed the plasmalemma (as net dovmhill H"^ influx or 0H~ efflux).Previous work has shown that the value of XJJQQ is more negative than 1//^^ during NO3 assimilation, suggesting that the active electrogenic H"*" extrusion pump is still operative despite the net downhill H"*^ infiux. The interpretation of this in terms of H'^-NOs symport which causes the entry of more H"^ than is consumed in NO3 metabolism, with extrusion of the excess H"*" via the active, electrogenic H"^ pump, was tested by measuring shortterm H"^ infiux upon addition of NOJ. A net H"^ infiux occurs before NO3 assimilation (as indicated by additional O2 evolution in the light) has commenced, suggesting a mechanistic relation of H"^ and NO3 infiuxes. This is consistent with the interpretation suggested above. Determinations of cytoplasmic pH showed no significant effect of NO3 assimilation, suggesting that cytoplasmic pH changes sufficient to change the 'pH-regulating' H"f luxes are smaller than the errors in the determination of cytoplasmic pH.

Active ion uptake and maintenance of cation-anion balance: A critical examination of their role in regulating rhizosphere pH

“…How- ever, when NO3-fed plants were deprived of NO 3 at 15 or 17 days old, there was a rapid release of H + ions from roots. When NO 3 was withheld from half of the root system of NO 3-fed plants, the roots deprived of NO 3 (where excess cation uptake was occurring) immediately started to produce H + ions even though the NO3-fed half of the root system (where excess anion uptake was occurring) continued to produce OH-ions and supply the whole plant with N O 3 • Using giant celled alga (Hydrodictyon africanum) as a study plant, Raven and Jayasuriya (1977) and Raven and Michelis (1979) provided strong evidence for a model of N O 3 uptake in which NO 3 influx was mechanistically coupled 1:1 with H + influx. Research showed that an electrogenic H + pump was still operating despite the net H + influx.…”

Uptake and Reduction of Nitrate: Algae and Fungi

“…In some cases the regulation of nitrate uptake can be separated from that of nitrate reduction, leading to a rather complicated pattern of induction/repression and activation/inactivation processes. In many cases the direct linkage between uptake and reduction and the complicated response to external factors has made investigation very difficult, and has not allowed us to apply the biophysical equations as used for uptake studies with chloride or potassium ions (WALKER 1976, RAVEN 1976.…”

H+extrusion and organic‐acid synthesis in N2‐fixing symbioses involving vascular plants

“…an excess of 1-33H* for each N assimilated. Consideration of S and P (assuming a N : S : P ratio of 120:7:2) reduces the excess to about 1-22H' per N. The organate whose generation results in the excess H* at the functional pH values in the plant comprises proteins and polyuronates, but predominantly low A/,, solutes such as organic-and amino-acid anions (see Raven & De Michelis, 1979, 1980. Maintenance of intracellular acid-base balance requires that the excess H* shall be removed from the cells and, ultimately, the plant, by H+ efflux (Raven, 1985(Raven, a, b, 1986.…”