Is polyploidy always an advantage? Plant responses to nutrients in two invasive Ludwigia congeners

Background/Question/Methods

Understanding the drivers of plant invasiveness is critically important to identifying and managing potential invaders. Current theories suggest that polyploidy provides an advantage to the establishment and spread of some invasive plant species. Our objective was to compare growth responses to variation in nutrient availability in two invasive aquatic plant species varying in ploidy level, Ludwigia peploides subsp. montevidensis (diploid) and Ludwigia hexapetala (decaploid). In the field, perennial L. hexapetala produces denser mats and taller stands, compared to its diploid congener, suggesting a polyploid growth advantage. Since dispersal of hydrochorous ramets is the primary mode of spread, we focused our study on the establishment of ramet fragments. In a four-week outdoor mesocosm experiment, field-collected fragments of the two congeners were grown under identical hydrological regimes with two soil nutrient levels (high versus low) to determine how ploidy and resource availability influence growth and morphology at the establishment stage of an invasion. Data on morphological characteristics were recorded at harvest. Biomass data was determined after samples were dried.

Results/Conclusions

Under high nutrient conditions, L. peploides ssp. montevidensis produced 1.8-fold more biomass that L. hexapetala (P <0.0001), but L. hexapetala produced main shoots that were 1.2-longer than L. peploides ssp. montevidensis (P <0.0001). L. peploides ssp. montevidensis produced ≈1.7-fold more leaves than L. hexapetala (P<0.0001) under high nutrient availability. The two species varied greatly in their architecture, with L. peploides ssp. montevidensis producing significantly fewer primary branches (P <0.0001) but significantly more secondary branches (P <0.0001) compared with L. hexapetala in response to increased nutrient availability. The biomass allocation data and the morphological differences between the two congeners did not support the initial expectation that L. hexapetala would out perform L. peploides ssp. montevidensis during the initial colonization stage of invasion. These results challenge the idea that being polyploid is always an advantage and raise questions regarding conditions under which polyploidy is not advantageous. Identifying the conditions that cause these invasive congeners to thrive can inform conservation efforts as well as identify locations at risk of invasion.