Inhibition of Apparent Photosynthesis by Air Pollutants

The reversible effects of hydrogen fluoride, sulfur dioxide, and chlorine exposures on net carbon dioxide absorption rates (apparent photosynthesis) of alfalfa (Medicago sativa L.) and barley (Hordeum vulgare L.) plants were studied. Pollutant exposures required (i) to reversibly depress CO₂ uptake rates and (ii) to cause cellular necrosis in the leaves were appraised. Plant responses to these air pollutants were compared with responses observed from previous equivalent studies with ozone, nitric oxide, and nitrogen dioxide. The experimental data show that CO₂ uptake could be reversibly suppressed by exposure dosages of these pollutants which did not cause cellular destruction in the leaves. However, except for the nitrogen oxides, some necrosis resulted from treatments which depressed CO₂ uptake rates more than 25–60% (depending upon the pollutant) by the end of 2-hour fumigation trials. The six air pollutants can be ranked in the following order according to the relative amounts that plant CO₂ uptake rates were depressed by the end of 2 hours of pollutant exposure: HF > 0₃ ≥ Cl₂ > SO₂ > NO₂ > NO. Carbon monoxide was also tested but did not measurably reduce plant CO₂ uptake when applied in concentrations ranging up to 80 ppm. The phytotoxicants ranked in essentially the reverse order when compared on the basis of the rapidity that CO₂ uptake was suppressed as a function of exposure time. Nitric oxide treatments caused very rapid reductions in the plant CO₂ uptake rates during the first hour of exposure. By this time, plant uptake rates had attained new (depressed) steady-state equilibrium levels which were then maintained over the remainder of the expo,sure periods. Hydrogen fluoride treatment induced much more gradual reductions in plant CO₂ uptake rates throughout the test periods. Plant responses to the other pollutants, SO₂, NO₂, Cl₂, and O₃, were intermediate between those caused by NO and HF. The experimental plants were fumigated after steady-state CO₂ exchange rates were established and the percent reduction in the CO₂ uptake (along with the rates of recovery) were determined. The investigations were conducted in two identical internalrecirculating environmental chambers (control and treatment chambers) designed to precisely control and continuously monitor light, temperature, wind parameters, relative humidity, and gas exchange rates.