Increased ploidy of Butomus umbellatus in introduced populations is not associated with higher phenotypic plasticity to N and P

Abstract

Separate introductions or post-introduction evolution may lead to multiple invader genotypes or cytotypes that differ in growth rates, biomass or chemical profile responses (phenotype) to a range of environments. If the invader has high trait plasticity to a range of resource levels, then sediment N or P enrichment may enhance invasiveness. However, the ways in which ploidy, plasticity, and available N or P interact are unknown for most species despite the potential to explain spread and impacts by invaders with multiple introduced lineages. We conducted a common garden experiment with four triploid and six diploid populations of Butomus umbellatus, collected from across its invasive range in the USA. Plants were grown under different N or P nutrient levels (4, 40, 200, 400 mg L^-1 N; 0.4, 4, 40 mg L^-1 P) and we measured reaction norms for biomass, clonal reproduction and tissue chemistry. Contrary to our expectation, triploid B. umbellatus plants were less plastic to variation in N or P than diploid B. umbellatus in most measured traits. Diploid plants produced 172 % more reproductive biomass and 57 % more total biomass across levels of N, and 158 % more reproductive biomass and 33 % more total biomass across P than triploid plants. Triploid plants had lower shoot:root ratios and produced 30 % and 150 % more root biomass than diploid plants in response to increases in N and P, respectively. Tissue chemistry differed between cytotypes but plasticity was similar; N was 8 % higher and C:N ratio was 30 % lower in triploid than diploid plants across levels of N and plant parts, and N was 22 % higher and C:N ratio 27 % lower across levels of P and plant parts. Our results highlight differences in nutrient response between cytotypes of a widespread invader, and we call for additional field studies to better understand the interaction of nutrients and ploidy during invasion.

Keywords: Biomass allocation; flowering rush (Butomus umbellatus); nutrient enrichment; phenotypic plasticity; plant invasion; polyploidy.

Figure 1.

(A) Map of locations where B. umbellatus plants were collected for this work. Genotypes and ploidy are indicated by the symbols: White triangle = triploid G1, black circle = diploid G3, black square = diploid G4, black triangle = diploid G5. Also shown are (B) a close-up of a B. umbellatus inflorescence, and (C) a triploid B. umbellatus infestation in Oconto Falls, WI, USA.

Figure 2.

Reaction norms for B. umbellatus cytotypes (A, C, E) and populations (B, D, F). Reported are the means ± SE for growth and reproductive responses to four levels of nitrogen. Within cytotypes, we used four triploid populations of a single genotype and six diploid populations of three different genotypes (four G4, one G3 and one G5 population). DW = dry weight.

Figure 3.

Reaction norms for B. umbellatus cytotypes (A, C, E) and populations (B, D, F). Reported are the means ± SE for growth and reproductive responses to three levels of phosphorus. Within cytotypes, we used four triploid populations of a single genotype and six diploid populations of three different genotypes (four G4, one G3 and one G5 population). DW = dry weight.

Figure 4.

Mean ± SE tissue chemistry PCs for flowering rush diploid or triploid plant parts, grown in increasing levels of nitrogen (A, B) or phosphorus (C, D). The individual variable loadings for the PCs are shown in adjacent panels.

Figure 5.

Mean ± 95 % CI for phenotypic plasticity (Hedge’s g) of introduced B. umbellatus cytotypes in response to N or P enrichment. TTL = total biomass, RE = reproductive biomass, S:R = the ratio of shoot (leaf) to root biomass.

Figure 6.

Plasticity (Hedge’s g) in tissue chemistry PCs of introduced B. umbellatus cytotypes in response to P or N enrichment. Points are mean values and error bars are 95 % confidence intervals.