Copper Uptake and Its Effects on Two Riparian Plant Species, the Native Urtica dioica, and the Invasive Fallopia japonica

Abstract

Copper accumulating in stream sediments can be transported to adjacent riparian habitats by flooding. Although being an essential element for plants, copper is toxic at high concentrations and restricts, among other things, plant growth. Besides copper, invasive plants, such as Fallopia japonica, which are known to be tolerant toward heavy metals, modify riparian habitats. If the tolerance of F. japonica is higher compared to native plants, this could accelerate invasion under high heavy metal stress. Therefore, we aimed to compare the effect of copper on two common riparian plants, the invasive F. japonica and the native Urtica dioica. We performed a pot experiment with a gradient from 0 to 2430 mg kg^-1 of soil copper. We hypothesized that (i) negative effects on plant growth increase with increasing soil copper concentrations with F. japonica being less affected and (ii) accumulating higher amounts of copper in plant tissues compared to U. dioica. In support of our first hypothesis, growth (height, leaf number) and biomass (above- and belowground) of F. japonica were impacted at the 810 mg kg^-1 treatment, while the growth of U. dioica was already impacted at 270 mg kg^-1. Due to 100% mortality of plants, the 2430 mg kg^-1 treatment was omitted from the analysis. In contrast, chlorophyll content slightly increased with increasing copper treatment for both species. While U. dioica accumulated more copper in total, the copper uptake by F. japonica increased more strongly after exposure compared to the control. In the 810 mg kg^-1 treatment, copper concentrations in F. japonica were up to 2238% higher than in the control but only up to 634% higher in U. dioica. Our results indicate that F. japonica might be able to more efficiently detoxify internal copper concentrations controlling heavy metal effects compared to the native species. This could give F. japonica a competitive advantage particularly in polluted areas, facilitating its invasion success.

Keywords: bioaccumulation; contamination; copper; copper sulphate; heavy metals; inorganic pollution; plant growth.

Figure 1

Plant growth parameters of Fallopia japonica and Urtica dioica recorded bi-weekly across the growing season (n = 10 time points). Shown are boxplots with median values represented by the black line. Each color represents one copper treatment from control to 810 mg kg⁻¹. Within each treatment, the time in the season is shown from left to right. (a) Plant height of Fallopia japonica, (b) plant height of Urtica dioica, (c) leaf number of Fallopia japonica, (d) leaf number of Urtica dioica, (e) chlorophyll content of Fallopia japonica, (f) chlorophyll content of Urtica dioica.

Figure 2

Biomass of above- and belowground plant material of Fallopia japonica and Urtica dioica collected at the end of the growing season in each copper treatment (n = 10 replicates). Shown are boxplots with median values represented by the black line. Each color corresponds to one copper treatment from control to 810 mg kg⁻¹ copper. (a) Aboveground biomass of Fallopia japonica, (b) aboveground biomass of Urtica dioica, (c) belowground biomass of Fallopia japonica, (d) belowground biomass of Urtica dioica.

Figure 3

Plant copper content in mg g⁻¹ for each copper treatment (0 mg kg⁻¹ in green, 90 mg kg⁻¹ in blue, 270 mg kg⁻¹ in yellow and 810 mg kg⁻¹ in orange, n = 5) in Fallopia japonica and Urtica dioica. The y-axis is scaled to the same values for aboveground, root, and rhizome samples, respectively, but differs between the three sample types. Shown are boxplots with median values represented by the black line. (a) Aboveground copper content in Fallopia japonica, (b) aboveground copper content in Urtica dioica, (c) root copper content in Fallopia japonica, (d) root copper content in Urtica dioica, (e) rhizome copper content in Fallopia japonica, (f) rhizome copper content in Urtica dioica.

Figure 4

Experimental setup of the experiment. General view of the pots arranged in a randomized block design and sample pictures of Fallopia japonica (left) and Urtica dioica (right) growing in the pots in spring 2021.