Differential Sensitivity of Duckweeds (Lemnaceae) to Sulphite

Abstract

SUMMARYFrond replication and some aspects of carbon assimilation were observed in three species of duckweed (Lemnaceae) grown at two levels of irradiance in the presence and absence of sulphite. The objectives were to examine the relationship between the inhibition of frond replication caused by sulphite, and changes in carbon assimilation, and the role of irradiance in modifying carbon assimilation and sensitivity to sulphite.Under low irradiance (40//mol m^-s~'photosynthetic photon flux density (PPFD)), inhibition of frond replication by sulphite was most pronounced in Lemnagibba, producing a slower frond doubling time and reduced frond fresh weight. Spirodela oligorhiza exhibited a slower doubling time only, whereas neither parameter was aflFected in Lernna vatdiviana. In all species, chlorophyll content increased in response to sulphite enrichment. Soluble protein levels increased when frond replication was significantly inhibited in L. gibba and S. oligorhiza. Photosynthesis and dark respiration rates were reduced in all species by 10 mM sulphite while ribulose bisphosphate carboxylase activity on a soluble protein basis was increased up to 13 %• Reduced carbon assimilation may have contributed to decreased frond replication in L. gibba and S. oligorhiza.In all species, photosynthesis and ribulose bisphosphate carboxylase activity were enhanced under high irradiance (200 //mol m ' •' s'' PPFD). These physiological responses to elevated Hght appear to have contributed to faster rates of frond replication in all three species, and reduced sensitivity of frond replication to sulphite in L. gibba and 5. oligorhiza. It is postulated that the stimulation of carbon assimilation under high irradiance is an important physiological response contributing to the modification by the photoenvironment of the sensitivity of duckweeds to sulphite.