Joint Action of O3 and SO2 in Modifying Plant Gas Exchange

The joint action of O(3) and SO(2) stress on plants was investigated by determining the quantitative relationship between air pollutant fluxes and effects on stomatal conductance. Gas exchange measurements of O(3), SO(2), and H(2)O vapor were made for Pisum sativum L. (garden pea). Plants were grown under controlled environments, and O(3), SO(2), and H(2)O vapor fluxes were evaluated with a whole-plant gas exchange chamber using the mass-balance approach. Maximum O(3) and SO(2) fluxes per unit area (2 sided) into leaves averaged 8 nanomoles per square meter per second with exposure to either O(3) or SO(2) at 0.1 microliters per liter. Internal fluxes of either O(3) or SO(2) were reduced by up to 50% during exposure to combined versus individual pollutants; the greatest reduction occurred with simultaneous versus sequential combinations of the pollutants. Stomatal conductance to H(2)O was substantially altered by the pollutant exposures, with O(3) molecules twice as effective as SO(2) molecules in inducing stomatal closure. Stomatal conductance was related to the integrated dose of pollutants. The regression equations relating integrated dose to stomatal conductance were similar with O(3) alone, O(3) plus added SO(2), and O(3) plus SO(2) simultaneously; i.e. a dose of 100 micromoles per square meter produced a 39 to 45% reduction in conductance over nonexposed plants. With SO(2) alone, or SO(2) plus added O(3), a dose of 100 micromoles per square meter produced a 20 to 25% reduction in conductance. When O(3) was present at the start of the exposure, then stomatal response resembled that for O(3) more than the response for SO(2). This study indicated that stomatal responses with combinations of O(3) and SO(2) are not dependent solely on the integrated dose of pollutants, but suggests that a metabolic synergistic effect exists.