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Effects of nitrogen dioxide on gas exchange in phaseolus vulgaris leaves Srivastava, Hari Shanker

Abstract

The present investigation was undertaken to survey the general features of physiological responses of plants to NO2, and to understand the mechanism of inhibition of gas exchange by NO2. To achieve these objectives, the effects of NO2, on photosynthesis, respiration and transpiration and the rate of NO2, uptake by primary bean (Phaseolus vulgaris. L. cv. 'Pure Gold wax'), leaves were examined in various environmental conditions using an open gas flow system. Apparent photosynthesis, respiration and the evolution of CO2, into CO2- free air, were all inhibited by NO2, concentrations between 1.0 and 7.0 ppm. The degree of inhibition was increased by increasing NO2, concentration and exposure time. A 2 and 5 h exposure to 3.0 ppm NO2, inhibited the gas exchange of bean leaves at all plant ages and in all the environmental conditions examined. Photosynthesis was most inhibited in leaves of intermediate ages, at optimum temperatures, at high light intensities, at relative humidities between 45 and 80% and in leaves of plants grown without any external source of nitrogen. The inhibition was rather less affected by changing C02 or 02 concentration. Maximum inhibition of respiration was observed in the youngest leaves, at high temperatures and in the leaves of nitrogen deficient plants. In most cases, the maximum inhibition of C02 exchange coincided with the maximum control rate in the absence of NO2. The inhibition of transpiration by NO2, was generally small and in a few cases either there was no effect of NO2, on transpiration or it was slightly increased by NO2. This indicated that the primary effects of NO2, were within the leaf mesophyll and not on the stomata. The uptake of N02 was also modified by plant age and environmental conditions. The rate of NO2 uptake increased with increasing concentrations of N02 and decreased with increasing exposure time. It was highest in the leaves of intermediate ages, in the light, at higher temperatures, at low and O2 concentrations, and in nitrogen deficient plants. In most cases, the maximum rate of N02 uptake was correlated with the maximum inhibition of gas exchange, but in several cases, it was not. Although stomatal resistance influenced the rate of N02 uptake to some extent, mesophyll resistance to NO2 was mainly responsible for the regulation of its absorption. In addition to Phaseolus vulgaris L.,10 other angiosperm species were examined. All species absorbed substantial amounts of N02 from an atmosphere of 3.0 ppm NO2, and all experienced a concomitant inhibition of photosynthesis. The rates of N02 uptake and the degree of inhibition varied according to species. The average rate of N02 uptake after a 2 h exposure to 3.0 ppm N02 -2 -1 was 0.391 mg N02 dm h and the average inhibition of photosynthesis with the same dose of N02 was 14.3%. An estimation of N02 uptake on a worldwide basis indicated that a concentration of 0.1 ppm N02 in the world's atmosphere could provide as much as 11% of the total nitrogen requirement of the terrestrial plants. Furthermore, the experiments reveal that the effect of N02 on plant metabolism is not restricted to a particular pathway or process; rather it is generalized. It appears that N02 may inhibit gas exchange by disrupting the structure of cell organelles and/or by interfering with the activities of enzymes.

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