Mechanisms of Resistance to Sulfur Dioxide in the Cucurbitaceae

The relative resistance of four cultivars of the Cucurbitaceae (Cucumis sativus L. cv. National Pickling, and inbred line SC 25; Cucurbita pepo L. cv. Prolific Straightneck Squash, and cv. Small Sugar Pumpkin) to SO2 was determined. According to plots of the degree of exposure to SO2 (which depends on the SO2 concentration and the duration of the exposure), there is an 8-fold difference in resistance to this toxic gas among these cultivars. However, if the degree of injury is plotted as a function of the amount of SO2 absorbed, all four cultivars appear similarly sensitive to the gas. We conclude that the principal reason for special and varietal differences in resistance among these cultivars is the relative rate of absorption of the gas. The densities of stomata on the upper and lower surfaces of leaves did not differ sufficiently between cultivars to account for the differences in absorption rates. It remains to be determined whether the differences in rate of SO2 absorption reflect differences in stomatal activity. Resistance of individual leaves changes with position on the plant axis (age of the leaf). There exists a gradient of decreasing resistance from the apex downward. This resistance gradient cannot be accounted for by differences in rates of SO2 absorption. We infer the existence of a biochemically based, developmentally controlled resistance mechanism which functions after SO2 has entered the leaf. Biochemical comparisons of old and young leaves with such differences in resistance should be helpful in determining the biochemistry of SO2 toxicity.