Comparing flooding tolerance in two loosestrife species

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Brenna N. Braasch, Kali Z. Mattingly and Stephen M. Hovick, Evolution, Ecology and Organismal Biology, The Ohio State University, Columbus, OH

Background/Question/Methods
Many invasive plant species are able to reproduce prolifically under stressful conditions. Purple loosestrife (Lythrum salicaria) is a plant species invasive in North America that is typically found in habitats of wet soil. Ecological factors and genetic diversity have both been important in structuring this species’ invasion. Another ornamental loosestrife species, L. virgatum, is known to hybridize with L. salicaria, but L. virgatum ecology is poorly characterized. This experiment compared flooding tolerance responses in two genotypes each of L. salicaria and L. virgatum collected from sources in their native range. To quantify flooding tolerance, we grew plants under flooded and non-flooded conditions in a greenhouse and compared fitness traits between species. Aboveground vegetative and reproductive biomass were compared following treatment, as well as quantifications of stem aerenchyma formation – a common response to flooding documented for L. salicaria.
Results/Conclusions
Flooding suppressed vegetative biomass compared to the non-flooded condition. Both species responded to flooding stress by reducing vegetative biomass and forming aerenchyma around the stem, but maintained reproductive biomass. That reproductive biomass was not significantly affected by flooding was surprising given that most perennials respond to stress by allocating biomass away from reproduction. Maintenance of high reproductive biomass under stress could be a key to invasiveness in these species.
Comparing the two species, vegetative biomass was significantly higher in L. salicaria, regardless of treatment. Total reproductive biomass was also higher in L. salicaria, though the difference was not statistically significant. The proportion of reproductive biomass for L. virgatum was significantly higher than L. salicaria irrespective of treatment, meaning that per unit aboveground biomass, L. virgatum produces more inflorescences. Qualitative observations suggest that stem aerenchyma production was similar for both species. Quantitative cross-sectional analyses of stem aerenchyma are pending. Lythrum virgatum’s maintenance of high reproductive output under flooding and aerenchyma observation also support identification of this species as a wetland plant with a wide moisture gradient tolerance.
Our documentation of shared habitat between these two species, high reproductive output in L. virgatum, and previous documentation of hybridization between these species suggest that L. virgatum may be an underappreciated part of loosestrife invasions. This is an important omission from our understanding of this otherwise well-studied invader because hybridization has been associated in many cases with increased invasiveness.