Linking root traits to copper exclusion mechanisms in Silene paradoxa L. (Caryophyllaceae)

Background and Aims Copper is one of the most important pollutants in mine-contaminated soils. This study tests the response in a sensitive population vs a tolerant one of the model species Silene paradoxa in order to understand the general mechanisms of tolerance at the micromorphological and ultrastructural level. Methods Two populations of Silene paradoxa were grown in hydroponics and exposed to different CuSO4 treatments. The roots were investigated with light, fluorescence and transmission electron microscope. Callose and lignin were spectrophotometrically determined. Results The tolerant population constitutively possessed a higher amount of mucilage and was able to reduce the length of the zone between the apex and the first lignified tracheids. Callose production decreased. It did not show remarkable copper-induced ultrastructural modifications, apart from the presence of precipitates in the tangential walls. The sensitive population showed huge nucleoli with a spongy periphery in the central cylinder together with the presence of electrondense granules in the mitochondria. Plastids were rarely observed and generally very electrondense and elongated. Conclusions In the copper tolerant population of S. paradoxa some of the root traits concurring to generate metal-excluding roots were suggested to be mucilage and lignin production and the reduction of the subapical root zone.