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



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.