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

SUMMARY Atmospheric pollution by the oxides of nitrogen, NO and NO 2 , 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 SO 2 are also present with NO or NO 2 then free radical‐induced injury, similar to that caused by O 3 alone, also occurs. CONTENTS Summary 395 I. Introduction 396 II. Uptake and cycling of oxides of nitrogen 396 III. Biochemical responses to NO and NO 2 405 IV. Physiological responses to NO and NO 2 410 V. Combinations of NO and NO 2 with other pollutants 416 VI. Recapitulation and beyond 418 Acknowledgements 420 References 420