Latitudinal trends in an invasive plant: genetic differentiation, phenotypic plasticity, and the effects of heavy metals and herbivores on growth, defence and reproductive characteristics

Abstract

Background and aims: Invasive species usually demonstrate remarkable adaptability across diverse environments, successfully inhabiting a wide variety of regions. This adaptability often links to genetic differentiation and phenotypic plasticity, leading to latitudinal trends in phenotypic traits. In this study, we collected seeds of the invasive plant Phytolacca americana from different latitudes and planted them in homogeneous gardens to investigate the latitudinal variation of P. americana phenotypic traits and to evaluate the effects of herbivory and heavy metals on plant growth, defence and reproductive characteristics.

Methods: Phytolacca americana seeds from different latitudes were planted in a homogeneous garden. For the experimental treatment, the seeds were divided into four groups: a heavy metal treatment group and its corresponding control group, and a cover treatment group with its corresponding control group. After the fruits matured, their growth, reproduction and defence indicators were measured.

Key results: Significant latitudinal trends were observed in P. americana's growth and defence characteristics, including changes in branch number, underground biomass, total biomass and leaf tannin content. Compared with previous field surveys on P. americana, our study found that the latitude trends in growth structure and defence traits were consistent, but the latitudinal trend of reproductive structure was different. Moreover, heavy metals and herbivory substantially influenced the plant's growth, reproduction and defence mechanisms, further shaping its latitudinal patterns.

Conclusions: The observed phenotypic variations in P. americana across latitudes can be largely attributed to the synergistic effects of phenotypic plasticity and genetic variation. At a broader geographical scale, adaptations to heavy metal stress and herbivory pressure among different P. americana populations involve distinct trade-offs related to growth, reproduction and defence strategies.

Keywords: Phytolacca americana; Plant invasion; adaptive evolution; heavy metal; herbivore; latitudinal gradient; phenotypic plasticity.

Fig. 1.

Detailed information on the six sampling locations for P. americana that we used in this experiment.

Fig. 2.

Growth traits in the second year of P. americana in relation to latitude. (A, B, C) Number of branches. (D, E, F) Underground biomass. (G, H, I) Total biomass. (J, K, L) Root/shoot ratio. Panels (A, D, G and J) show the relationship of mean values with latitude across treatments. P < 0.05 indicates that the trait has a significant linear relationship with latitude.

Fig. 3.

The results of the principal component analysis (PCA) on the growth variables of P. americana in the second year (A). Higher values of growth architecture indicate a smaller root/shoot ratio, a lower number of branches, a higher plant height and a thicker basal diameter. Relationship between the composite variable of growth architecture and latitude in the second year of P. americana (B,C,D). Fig. 3 (B) Relationship of mean values with latitude across treatments. P < 0.05 indicates a significant linear relationship between this trait and latitude.

Fig. 4.

Relationship between second-year reproductive traits of P. americana and latitude: number of fruits (A, B, C) and reproductive mass ratio (D). (A, D) Relationship of mean values with latitude across treatments. P < 0.05 indicates a significant linear relationship between this trait and latitude.

Fig. 5.

Resistance compound and nutrient contents in the leaves of P. americana in the second year in relation to latitude: tannins (A, B, C) and total saponins (D, E, F). (A, D) Relationship of mean values with latitude across treatments. P < 0.05 indicates a significant linear relationship between this trait and latitude.

Fig. 6.

Heavy metal (Mn) content in the roots and leaves of P. americana in the first (A, B) and second (C, D) year. P < 0.05 indicates a significant linear relationship between Mn content and latitude.