SUMMARY
The assimilationof ammonium ion in plant cell cytoplasm produces at least one H+ per NH+4; N2 fixation generates 0.1-0.2 H+ per N assimilated; NO-3 assimilation produces almost one OH- per NO-3. H+ or OH- produced in excess of that required to maintain cytoplasmic pH for H+ or OH-, the major process involved is H+ efflux (frequently by active transport) from the cell. IN higher land plants, much of assimilated N occurs as hoot protein; the shoot cells have no direct acess to the H+ and OH- sink of the soil solution.

When ammonium ion is the N source it is assimilated into organic-N in the roots. The shoot is supplied with a mixture of amino-acids, amides and organic acids which an be incorporated (with neutral photosynthate) into cell material without damaging pH changes. Similar considerations apply to symbiotic N2 assimilation in root nodules. IN both cases the excess H+ generated in the root cell cytoplasm is exerted is excreted to the soil solution; there is no mechanism whereby photolithotrophic plant can, in the long term, counter intracellular acidity without resort to active H+ efflux to an extracellular sink.

When nitrate is reduced in roots, the organic compounds involved in N transportged to the shoot are similar to those used when ammonium or N2 is the N source with similar implications for the regulation of shoot pH. The excess OH- generated in the roots is partly excreted to the soil solution, and partly neutralized by the ‘biochemical pH stat’ which produces strong organic acids from essentially neutral precursors.

When nitrate is assimilated solely in shoots, the excess OH- is initially neutralized by the operation of the biochemical pH state. Storage of the inorganic cation-organate in shoot cell vacuoles could lead to turgor and volume regulation problems in these cells. These are avoided when an insoluble salt (calcium oxalate) is the product of the pH stat, or when the cation organate is translocated to the roots where organate breakdown regenerates OH-, whcih is lost to the soil solution.

This mixture of biochemical, and long and short distance transport processes, enables cells remote from a large sink for H+ or OH- to produce protein without unfavourable pH changes. These processes related to pH regulation during N assimilation have important consequences for the carbon and energy economy of the plant.

NITROGEN ASSIMILATION AND TRANSPORT IN VASCULAR LAND PLANTS IN RELATION TO INTRACELLULAR pH REGULATION

INTRODUCTION 169 ENZYMES OF THE NITRATE-REDUCING SYSTEM 171 Reduction of Nitrote to Nitrite 171 Ferredoxin-nitrate reductase •..•.•. •..• 171 NAD(P)H-nitrate reductase ....... ... 172 Reduction of Nitrite to Ammonia 177 Ferredoxin-nitrite reductase ... ........• 177 NAD(P)H-nitrite reductase 179 ENZYME LOCALIZATION AND THE PROVISION OF REDUCTANT 183 REGULATION OF NITRATE REDUCTION 185 Control of the Amount of Active Enzyme 186 Enzyme synthesis •.•.••.•.••.••. ..•....•...•..•..••.••.•. .••.•..•. 187 Enzyme activity 191 Control of Substrate Supply to Nitrate Reductase 193 CONCLUDING REMARKS 196

The Assimilatory Nitrate-Reducing System and its Regulation

Green plants play a unique role among living organisms through their ability to reduce carbon in photosynthesis. While reduced carbon provides the energy source for all life, nitrogen must be viewed as the central element because of its role in substances such as proteins and nucleic acids which form the living material. Proteins serve as enzyme catalysts in metabolic pathways, as structural elements of cytoplasm and membranes and as carriers in transport functions. Nucleic acids provide the means for codification, storage and translation of genetic information. In those and other organic materials, nitrogen appears in a chemically reduced state. Such organic nitrogen constitutes 1.5 to 5 per cent of the dry weight of higher plants, and 80 to 90 per cent of that is in protein. Despite the great abundance of dinitrogen gas in the atmosphere, the element is commonly deficient in agricultural soils in the oxidized (nitrate) and reduced (ammonium) forms which can be absorbed by plants. In aerated soils, bacteria quickly transform ammonium ions to nitrate, and the oxidized form is the principal in one plant nutrition. As a result of nitrogen's critical roles and low supply, the management of nitrogen resources is an extremely important aspect of crop production. It has also been an area for intensive research at all levels from yield behavior to cell physiology. As agronomists, we are concerned particularly with designing strategies to increase the efficiency of nitrogen use in crop systems. This chapter focuses on aspects which are related to the uptake and assimilation of nitrogen into amino acids and proteins, and their subsequent interaction in growth and development. These events are highly dynamic and complex, and are not yet fully understood. Our intent here is to provide a broad review of those subjects for crop physiologists, geneticists and agronomists. We begin with the biochemical and cellular level and integrate towards field performance.

Nitrogen and plant production

Factors Affecting Mineral Nutrient Acquisition by Plants

(I) Inorganic components . . . . . . . (a) Nitrate . . . . . . . . . (b) Cation-anionuptake . . . . . . (2) Organic compounds . . . . . . . (a) Amino acids and other organic nitrogenous compounds (b) Organic acids . . . . . . . . (3) Effects on flowering . . . . . . . (4) Ammonium plus nitrate as a nitrogen source . . . ( 5 ) Metabolic changes characteristic of ammonium toxicity . VII . Summary . . . . . . . . . . VI . Physiological effects of nitrogen sources . . . .

Ammonium and nitrate nutrition of plants

Nitrate Uptake by Wheat as Influenced by Ammonium and Other Cations1

Nitrate Uptake and Reduction as Affected by Calcium and Potassium1

Ammonium markedly inhibited nitrate absorption by nitrogenstarved wheat seedlings but did not decrease the proportion of absorbed nitrate that was reduced. Seedlings high in nitrate (absorbed prior to the experimental periods) reduced similar amounts of this nitrate regardless of whether or not ammonium was present and being absorbed during the period of measurement. Ammonium or products of ammonium assimilation did not interfere with the induction, stability, or activity of nitrate reductase. Consequently, the hypothesis that ammonium depresses nitrate uptake indirectly by inhibiting nitrate reduction is rejected, and it is suggested that the ammonium effect is directly on the nitrateuptake process.

The influence of ammonium on nitrate reduction in wheat seedlings.

Intact wheat seedlings cultured in high nitrate solutions (high-NO
 3
 -
 cultures) reduced NO
 3
 -
 when placed for 24 hr in dilute CaSO4 solutions although they leaked NO
 3
 -
 back to solution during this period. Most of the reduction seemed to occur in shoots since in parallel experiments detached shoots reduced nearly as much of the previously absorbed NO
 3
 -
 as intact cultures. Detached roots leaked greater quantities of NO
 3
 -
 back to solution than did intact cultures, and failed to reduce any of their previously absorbed NO
 3
 -
 during the experimental period. Seedlings of identical age cultured without a nitrogen source and rich in carbohydrate reserves (low-N cultures) rapidly absorbed NO
 3
 -
 from dilute Ca(NO3)2 solutions and reduced 80% of that absorbed. Detached low-N roots also absorbed NO
 3
 -
 and reduced 40% of that absorbed. Total NO
 3
 -
 reduction by intact low-N cultures over the 24-hr period was comparable to that of high-NO
 3
 -
 cultures in spite of the fact that in vitro nitrate reductase activity of the former did not reach the levels found initially or at the end of the period in the latter, and that the total NO
 3
 -
 absorbed by low-N cultures was less than that initially present in high-NO
 3
 -
 cultures.

P. L. Minotti

Papers

Nitrate Uptake by Wheat as Influenced by Ammonium and Other Cations1

Nitrate Uptake and Reduction as Affected by Calcium and Potassium1

The influence of ammonium on nitrate reduction in wheat seedlings.

Nitrate reduction in the roots and shoots of wheat seedlings.

Some Consequences of Nitrogen Nutrition on Uptake and Transport of Strontium and CESIUM1