Effects of sulfite on phosphoenolpyruvate carboxylase and nicotinamide adenine dinucleotide phosphate-dependent malate dehydrogenase in epidermal peels in two cultivars of pea

We have identified a differential response of stomatal conductance to sulfur dioxide in two cultivars of pea ( Pisum sativum L. cvs P715 and Nugget). The response to sulfite exposure of PEPC activities present in epidermal peels obtained from the two cultivars was qualitatively in agreement with the results obtained for stomatal conductance. With epidermal tissue isolated from the more sensitive cultivar, we have investigated the effect of light and sulfite on guard cell phosphoenolpyruvate carboxylase (E.E. 4.1.1.31.) and NADP-dependent malate dehydrogenase (E.C. 1.1.1.82), two enzymes of the malate biosynthetic pathway. No difference was found between the substrate-saturated activity of phosphoenolpyruvate carboxylase in epidermal tissue incubated in the light or in the dark under the same conditions. Substratesaturated NADP-dependent malate dehydrogenase activity increased nearly 3-fold during a 60 min incubation in the light. Incubations of epidermal tissue in the light in the presence of sulfite resulted in a decrease in the activity of both enzymes. Our results suggest that the inhibition of these two enzymes of the malate biosynthetic pathway may be one cause of sulfur dioxide-mediated stomatal closure.