Interactive effects of simulated acidic fog and ozone on field-grown alfalfa

Abstract Acid precipitation events, primarily in the form of fogs with pH as low as 2.0, have been reported from both urban and agricultural areas of California. If low pH acidic precipitation increased stomatal conductance, flux of gaseous air pollutants into mesophyll spaces could increase, thereby increasing susceptibility of plants to air pollution injury. The potential for this interaction is acute in California because of high ambient ozone (O 3 ) levels present in areas frequently subjected to fogs. A field experiment was conducted to determine the interactive effects of simulated acidic fog and O 3 on stomatal conductance, photosynthesis, foliar injury, and yield of an established stand of alfalfa ( Medicago sativa L.). Plants were exposed to five levels of O 3 for 12 hr daily for 4 weeks. Ozone was added in proportion to its concentrations in ambient air. Levels ranged from 1 4 of ambient to twice ambient concentrations. Simulated fog solutions of pH 2.0, 3.2, 4.4, and 5.6 acidified with 2:1 HNO 3 :H 2 SO 4 were applied from 0300 to 0500 three times per week for 4 weeks. Treatments were replicated twice. Alfalfa was harvested when plants were at 1 10 bloom stage and crown sprouts exceeded 4 cm in length. Results indicated that both stomatal conductance and photosynthesis decreased with increasing acidity of fog. Foliar injury attributable to acid deposition was observed only in the pH 2.0 fog treatment. Alfalfa yield was significantly reduced by both acid fog and O 3 , but the interaction between the two was not statistically significant. The threshold for significant growth reduction in alfalfa exposed to simulated fog in open-top chambers was pH 2.0 and pH 3.2 for alfalfa in ambient-air plots.