FOLIAR ENTRY AND INCORPORATION OF ATMOSPHERIC NITROGEN DIOXIDE INTO BARLEY PLANTS OF DIFFERENT NITROGEN STATUS.

The uptake of atmospheric NO through leaf surfaces and the influence on uptake of combined nitrogen by the roots were investigated in hydroponically grown barley plants exposed to 0.3 μI 1 nitrogen dioxide (NO ). The response to NO was dependent on the nitrogen status of the plant at the time of exposure. Only with low nitrate supplies to the roots did exposure of leaves to atmospheric NO cause significant increases in the nitrogen (N) content of barley. Levels of both nitrate and reduced forms of N increased in plants in response to the pollutant and were directly due to uptake of NO by leaves, since the root environment was completely sealed from atmospheric sources of NO . Moreover, these increases were not due to the stimulation of nitrate uptake by the roots from the nutrient solution, although levels of activity of both nitrate and nitrite reductases were affected by exposure to NO . Atmosphere NO did, however, affect the ability of a plant to respond to a change in the amount of nitrate supplied to the roots, and the use of both N-labelled NO and nitrate indicated that the cycling of nitrogen within the plant was also influenced by exposure to atmospheric NO .