ABSORPTION OF NITROGEN DIOXIDE BY SUNFLOWER PLANTS GROWN AT VARIOUS LEVELS OF NITRATE

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Abstract
Summary: The effect of varied levels of nitrate on the absorption of atmospheric nitrogen dioxide (NO2) by sunflower (Helianthus annus L.) plants was investigated by using a 15N dilution method. Growth of the plants was greatest at 5 mM nitrate, with less growth at 15 mM nitrate and a considerable reduction at zero nitrate. The total amount of NO2‐nitrogen absorbed by the plant depended on the plant size and total leaf area.Nitrate supply did not influence the rate of absorption of NO2 by the plants when exposed to 0.3 μl l−1 NO2. On the contrary, when the plants were exposed to 2.0 μl l−1 NO2, the absorption rate was significantly lower for the plants grown at zero nitrate compared with those grown at 5 or 15 mM nitrate. The lower rate of NO2 absorption in the nitrate‐deficient plants was not due to the changes in stomatal resistance or stomatal density of the leaves, but could have been due to an increase in mesophyll resistance resulting from accumulation of high levels of nitrite in the leaf tissues. Nitrogen dioxide at 0.3 μl l−1 exerted a nutritional effect on the nitrate‐deficient plants, while NO2 at 2.0 μl l−1 acted as a toxic air pollutant on the plants at all the levels of nitrate supplied.

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