Ultrastructural observations on leaf tissues of Triticum aestivum exposed to sulphure dioxide.

Two wheat cultivars, Mec, sensitive and Chiarano, more tolerant, were used to evaluate the ultrastructural response to long-term, non marking exposures to low SO2 concentration (30, 60, 90 ppb). Significant chloroplast alterations, although discontinuously distribuited, occurr in Mec at all SO2 levels tested; end granal membranes swell and burst, suggesting SO2-induced acidification of the thylacoid lumen. The more tolerant Chiarano displays similar modifications only as a consequence of a 90 ppb SO2 exposure. The number but not the fine structure of affected chloroplasts varies significatively in relation to SO2 levels and leaf age. Such alterations problably modify the balance between PSII/PSI efficiency