Mechanisms of Resistance to Sulfur Dioxide in the Cucurbitaceae

Abstract

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.