Gaseous ammonia and oxides of nitrogen can lead to both a stimulation and a reduction in plant growth. Using examples of effects at both the cellular and whole plant level, discuss the relative merits/disadvantages of life in a polluted environment.
Major sources for ammonia (NH3) emissions are connected with areas of intensive agriculture and high rates of fertilizer manufacture and application. Fertiliser is often applied as ammonium or urea.
During combustion, the primary oxide of N formed is nitrogen monoxide or nitric oxide (NO), only a little of which comes from N in the fuel. The majority of the NO is generated from the direct combination of atmospheric O and N within the flames (Palmer & Seery,1973). NO is a free radical and readily reacts with other molecules to lose or gain an electron. Hence, the effects of NO are generally localised to an area close where it was generated.
Oxidation of NO by O2 yields nitrogen dioxide but this reaction occurs very slowly at low concentrations of NO. Oxidation of NO by ozone, however, occurs rapidly even at very low concentrations (Willix, 1976)*. This, therefore, is regarded as the main mechanism of generation of atmospheric NO2. Other pollutants such as hydrocarbons and SO2 can also react with NO2 but the relative importance of these reactions is dependent upon other environmental conditions (Willix, 1976)*.
The effects of ammonia and oxides of nitrogen:
Three major types of deposition of ammonia and oxides of nitrogen can occur: as a gas, in aqueous form as the ammonium ion or as an aerosol water droplet. Short term exposures to NO2 do not usually cause depressions of growth but, at high concentrations, visible leaf injury may occur. In the short term (less than 24 hours) and at very low levels of oxides...