Zea mays (a C4 plant) can withstand high sulfur dioxide concentrations up to 12.0 ppm without any harmful effects. Visible injury symptoms appear only after exposure to sulfur dioxide concentrations beyond 12.0 ppm. The pigments, proteins, amino acids and starch decrease after exposure to higher concentrations of sulfur dioxide. The free sugars, however, increase in response to sulfur dioxide exposure. The activity of PEP - carboxylase is inhibited non -competitively with respect to substrates, PEP and Mg2+ but it is competitive with HCO3-.The high affinity of the enzyme for HCO3- renders the replacement by sulfur dioxide more difficult which results in decreased sensitivity towards sulfur dioxide. NADP - malate dehydrogenase is inhibited uncompetitively with respect to OAA and non - competitively with respect to NADPH. The low inhibitory effect of sulfur dioxide on the enzymes is indicated by high Ki values.In case of NADP - malic enzyme, sulfur dioxide shows a non - competitive inhibition with respect to NADP+ but a competitive one with respect to Mg2+. With respect to malate, the inhibition is partially competitive.