DIFFERENTIAL SENSITIVITY OF DUCKWEEDS (LEMNACEAE) TO SULPHITE

S ummary The thiol contents and emissions of hydrogen sulphide (H 2 S) of duckweeds (Lemnaceae) differentially sensitive to sulphite enrichment were studied, at two levels of irradiance. The objectives were to examine the relationship of selected parameters of sulphite metabolism to sulphite sensitivity, and the role of irradiance in modifying the metabolic effects of sulphite. Under low irradiance, thiol contents were increased 30 to 40% by sulphite in all three duckweeds examined. Hydrogen sulphide was emitted by all three species, but emission rates in Lemna valdiviana , which is sulphite‐tolerant, were up to four times higher than in the other species. Under high irradiance, sulphite increased the thiol contents by an average of 40 % in L. valdiviana and Spirodela oligorhiza , but only by 20% in Lemna gibba . The greater light enhancement of thiol content exhibited by L. valdiviana and S. oligorhiza may be indicative of larger or more numerous sulphur sinks. The emission rates of H 2 S were also enhanced under high irradiance, and L. gibba exhibited a 17% increase relative to its rate in low irradiance. In comparison, L. valdiviana and S. oligorhiza exhibited 55 % and 60% increases, respectively. The ability to form elevated internal levels of thiols, and to emit H 2 S were found to be important in relation to sulphite tolerance in duckweeds. Enhancement of both processes under high irradiance may contribute to increased tolerance of sulphite in L. gibba and S. oligorhiza . It is hypothesized that thiol production and emission of H 2 S are important sulphite detoxification processes in duckweeds, and that enhancement of sulphite detoxification is fundamental to the modification by the photoenvironment of the sensitivity of duckweeds to sulphite.