Inhibition of gas exchange in bean leaves by NO₂

Abstract

An open gas-flow system was used to examine the effects of the air pollutant NO 2 on gas exchange by primary leaves of bean (Phaseolus vulgaris L.). Apparent photosynthesis and dark respiration were both inhibited by NO 2 concentrations between 1.0 and 7.0 ppm. The degree of inhibition was increased by increasing NO 2 concentration and increasing exposure time. Leaf susceptibility to NO 2 varied during leaf growth. NO 2 was most inhibitory at the ages when maximum rates of apparent photosynthesis or respiration were observed in the NO 2 -free controls (11 or 12, or 8 days after sowing, respectively). The rate of absorption of NO 2 by leaves increased in direct proportion with the NO 2 concentration and declined with increasing exposure time. The NO 2 uptake rate in the dark was about half of its rate during illumination because of greater stomatal resistance to NO 2 absorption in the dark. Transpiration rate was less affected by NO 2 than was photosynthesis or respiration. Accordingly, it is suggested that the principal effects of NO 2 on leaf gas exchange are exerted in the leaf mesophyll and are not on the stomata.