Atmospheric pollution by the oxides of nitrogen, NO and NO2 , can cause reductions in growth but rarely visible injury. This review considers their uptake into foliage, as well as their subsequent metabolism and physiology, and attempts to explain why these gases are often phytotoxic. The combined stresses of resisting cellular acidification, enhanced levels of nitrite (and ammonia), and the direct interference of the free radical ('N=O) with critical enzymes, reaction centres and regulatory mechanisms are thought to be the main reasons why oxides of nitrogen, especially NO, inhibit growth. If other air pollutants such as SO2 are also present with NO or NO2 then free radical-induced injury, similar to that caused by O3 alone, also occurs. CONTENTS Summary 395 I. Introduction 396 II. Uptake and cycling of oxides of nitrogen 396 III. Biochemical responses to NO and NO2 405 IV. Physiological responses to NO and NO2 410 V. Combinations of NO and NO2 with other pollutants 416 VI. Recapitulation and beyond 418 Acknowledgements 420 References 420.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1990.tb00467.x

Tansley Review No. 24 Why are atmospheric oxides of nitrogen usually phytotoxic and not alternative fertilizers

Role and regulation of L-glutamate dehydrogenase activity in higher plants

Regulation of nitrate reductase activity in higher plants

Publisher Summary This chapter describes nitrate and nitrite reduction. Nitrate is the predominant form of nitrogen available to most cultivated plants grown under normal field conditions. Although ammoniacal fertilizers are used almost exclusively, the ammonia derived from these fertilizers is oxidized to nitrate by soil organisms. Nitrification is extremely rapid when the soil is well aerated, moist, and above 7°–10°C. Certain bacteria can utilize nitrate nitrogen as the sole nitrogen source for the synthesis of all nitrogen containing compounds of the cell. This nitrate assimilation can occur under both aerobic and anaerobic conditions. Nitrate reduction by partially purified enzyme preparations is frequently stimulated by phosphate. Nitrate reduction, mediated by either reduced pyridine nucleotides or reduced viologen dyes, is inhibited by cyanide and azide; however, pyridine-nucleotide-mediated reduction of cytochrome c is insensitive to these inhibitors. A reactivation of an inactive nitrate reductase apoenzyme extracted from molybdenum-deficient plants can be achieved by the addition of acid-treated nitrate reductase or by addition of phosphate buffer washes of nitrate reductase absorbed on adenosine monophosphate -Sepharose.

Nitrate and Nitrite Reduction

The timing of the auxin response was followed in oat and corn coleoptile tissue by a sensitive optical method in which the elongation of about a dozen coleoptile segments was recorded automatically. The response possesses a latent period of about 10 min at 23°C, which is extended by low concentrations of KCN or by reducing the temperature, but is not extended by pretreatments with actinomycin D, puromycin, or cycloheximide at concentrations that partially inhibit the elongation response. Analysis of the data indicates that auxin probably does not act on the elongation of these tissues by promoting the synthesis of informational RNA or of enzymatic protein. Not excluded is the possibility that auxin acts at the translational level to induce synthesis of a structural protein, such as cell wall protein or membrane protein. While the data do not provide direct support for this hypothesis, the speed with which cycloheximide inhibits elongation suggests that continual protein synthesis may be important in the mechanism of cell wall expansion.

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Timing of the auxin response in coleoptiles and its implications regarding auxin action.

1. Possible mechanisms regulating the activities of three enzymes involved in nitrate assimilation, nitrate reductase, nitrite reductase and glutamate dehydrogenase, were studied in radish cotyledons. 2. Nitrate-reductase and nitrite-reductase activities are low in nitrogen-deficient cotyledons, and are induced by their substrates. 3. Glutamate dehydrogenase is present regardless of the nitrogen status, and the enzyme can be increased only slightly by long-term growth on ammonia. 4. Although nitrate is the best inducer of nitrate reductase, lower levels of induction are also obtained with nitrite and ammonia. The experiments did not distinguish between direct or indirect induction by these two molecules. 5. Nitrite reductase is induced by nitrite and only indirectly by nitrate. 6. The induction of both nitrate reductase and nitrite reductase is prevented by the inhibitors actinomycin D, puromycin and cycloheximide, indicating a requirement for the synthesis of RNA and protein. 7. The decay of nitrate reductase, determined after inhibition of protein synthesis, is slower than the synthesis of the enzyme. Nitrite reductase is much more stable than nitrate reductase. 8. The synthesis of nitrate reductase is not repressed by ammonia, but is repressed by growth on a nitrite medium. 9. There is no inhibition of nitrate reductase, nitrite reductase or glutamate dehydrogenase by the normal end products of assimilation, but cyanate is a fairly specific inhibitor of nitrate reductase.

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J. Ingle

Papers

The regulation of activity of the enzymes involved in the assimilation of nitrate by higher plants.