Clonal integration and heavy-metal stress: responses of plants with contrasting evolutionary backgrounds

Physiological integration between ramets can ameliorate the growth and survival of clonal plants in spatially-heterogeneous environments, as ramets from favourable patches can provide support to those found in stressful patches. However, the advantage conferred by clonal integration might also depend on the evolutionary history of plants with regards to the presented stress. Here, we compared the benefit of clonal integration in response to the distribution of a heavy metal as a stress factor, and asked if this benefit would differ between ecotypes that have either undergone selection to tolerate heavy metals or not. In a greenhouse experiment, we grew pairs of connected and severed ramets of the metal hyperaccumulator Arabidopsis halleri, which originated from populations of either metalliferous or non-metalliferous soils. The ramets were grown in paired pots, which were contaminated with cadmium (Cd) either heterogeneously (100 or 0 ppm Cd per pot) or homogenously (50 ppm Cd per each pot). A. halleri ecotypes that originated from non-metalliferous soils performed better when ramets were connected and the distribution of Cd was heterogeneous. However, clonal integration had no effect on the performance of genotypes from metalliferous soils, regardless of the distribution of Cd. These results support the hypothesis that clonal integration is beneficial in stressful environments as long as the stress is patchily distributed, and particularly for plants that did not undergo selection to withstand it.