Articles
POLLUTANTS GENERATED IN GREENHOUSES DURING CO2 ENRICHMENT
The heat of combustion causes atmospheric O2 and N2 to combine to produce nitrogen monoxide (NO) which is then slowly oxidized to NO2. The ratio of NO to NO2 in greenhouses is variable but NO is usually predominant.
The concentration of NOx (i.e.
NO + NO2) in a greenhouse atmosphere is very high compared with that in most situations out-of-doors, and may exceed that adjacent to very busy urban roads.
Calculations show that the NOx emitted from some burners could make a significant contribution to the nitrogen budget of the crop if the NOx entered the plants in a utilizable form.
In practice the contribution to plant nutrition seems to be very limited and clear symptoms of injury sometimes occur.
Even in the absence of visible injury, there may be growth inhibitions of which the grower is unaware.
The presence of NOx in the greenhouse atmosphere does therefore reduce the benefits of CO2-enrichment in many circumstances.
Biochemical studies have shown that metabolic responses to NOx differ between species and between cultivars.
The flux of NOx to the leaves is thought to increase the amounts of substrate for nitrite and nitrate reductases, and increases in the contents of amines during fumigation suggest that pollutant-derived NO2 and NO3 ions are being metabolised.
The responses of the two enzymes to fumigation do, however, exhibit some peculiarities which are not easily explained and which suggest that detoxification mechanisms will be difficult to evaluate.
Although the main emphasis of this review is on the effects of NOx pollution it is noted that other forms of pollution can also be very important, especially the unburnt hydrocarbons from burners that are incorrectly operated or poorly maintained.