¹⁵ N DILUTION METHOD FOR ESTIMATING THE ABSORPTION OF ATMOSPHERIC NO₂ BY PLANTS

The ¹⁵ N dilution method was used for the quantitative estimation of nitrogen dioxide (NO₂ ) absorbed by Helianthus annuus and Zea mays during a relatively long period. The relationships between the amount of NO₂ absorbed and the concentration of NO₂ and length of exposure were investigated, in order to ascertain the reliability of this method. The total amount of NO₂ -nitrogen absorbed by the plants over two weeks (from two to four weeks after the sowing) increased with increasing concentrations of NO₂ . The rate of absorption of NO₂ per unit leaf area also increased linearly with increasing concentrations of NO₂ from 0 to 1 μl l^-1 . This means that the diffusive resistances of stomata or mesophyll tissues were not changed by the continuous exposure to NO₂ at 1 μl l^-1 for two weeks. The absorption rate per unit leaf area in H. annuus was about three times greater than that in Z. mays. The total amount of NO₂ -nitrogen absorbed by the plants which were continuously exposed to 0.5 μl l ^-1 NO₂ rose not linearly but exponentially as the exposure time increased from zero to three weeks. This might in part have reflected the growth characteristics of the plants, since dry weight and leaf area increased exponentially during the period of exposure to the pollutant. The absorption rate per unit leaf area remained constant during the first two weeks, but thereafter increased significantly, probably due to the uptake of NO₂ -nitrogen through the air-soil-root pathway. These results demonstrated that the values estimated by the¹⁵ N dilution method were reliable, and this method can be recommended for the quantitative determination of NO₂ absorbed by plants during a long period.