Fragment viability, regenerative capacity and protoplast isolation of invasive Australian swamp stonecrop (Crassula helmsii)

Australian swamp stonecrop (Crassula helmsii) is an amphibious plant native to Australasia and highly invasive in Europe. Managing the spread and impacts of this invader is challenging due to its ability to regenerate from small vegetative plant fragments. In several infested water systems in the Netherlands, the dispersal of this species is currently being prevented using water filters with a mesh size as small as 1 mm² in their outflows. However, it remains unclear whether these filters are truly capable of preventing the spread of small regenerative fragments. To identify potential shortcomings in the management of C. helmsii dispersal, we investigated the regeneration of detached vegetative fragments. A laboratory experiment was conducted to study the regenerative capacity of C. helmsii fragments of varying sizes (0.5–2 mm). We examined the growth performance (stem count, cumulative stem length) of fragments originating from nodes, leaves, shoots, roots, and meristem tissue cuts. All fragments originating from nodal tissue were able to successfully regenerate. Isolated meristem tissue of 0.5 mm was able to regenerate but exhibited malformed growth. Shoot tips demonstrated the best growth performance. While we successfully isolated protoplasts from C. helmsii node tissue, our cultures were compromised, and regeneration could not be assessed. Our research indicates regenerative potential from node fragments as small as a cluster of apical meristem cells. These findings suggest shortcomings in current measures for dispersal prevention of C. helmsii, as they do not prevent the dispersal of plant fragments with a length <2 mm. We recommend re-evaluating the effectiveness of various types of dispersal barriers currently applied in vulnerable natural areas to prevent the spread and subsequent regeneration of small vegetative fragments of C. helmsii.