Physiological site of ozone effects on CO sub 2 assimilation in Glycine max

The physiological site of action of ozone exposure on CO{sub 2} assimilation was examined in Glycine max (soybean) via diagnostic analyses of foliar gas exchange. Seedlings were exposed to either 0 or 0.5 {mu}L L{sup {minus}1} O{sub 3} for 4h. Ozone caused a 36% reduction in CO{sub 2} assimilation (A), a 57% reduction in stomatal conductance (g{sub s}), and a 14% reduction in internal CO{sub 2} partial pressure (C{sub i}). Although the drop in C{sub i} implies a stomatal limitation to A, the declines in A and O{sub 3} preceded the change in C{sub i}. The reduction in C{sub i} explained only 36% of the reduction in A O{sub 3}-treated plants, and the stomatal limitation was no greater in O{sub 3}-treated leaves than in controls. CO{sub 2} and light response curves indicated reductions due to O{sub 3} in CO{sub 2} assimilation even under CO{sub 2}- and light-saturated conditions, implying a direct effect on the mesophyll tissue's capacity to assimilate CO{sub 2}. We conclude that although O{sub 3} has marked effects on both A and g{sub s}, the decline in A is principally a consequence of the pollutant's effect on the intrinsic photosynthetic capacity of the mesophyll.