Temperature-dependence and Genetic Variation in Resource Acquisition Strategies in a Model Freshwater Plant

ABSTRACT Understanding how competition varies with environmental stress is critical to anticipating species and community responses to rapid environmental change. While the stress-gradient hypothesis predicts the strength of competition to decrease with increasing stress, our understanding of how competition varies with stress is limited by a lack of mechanistic understanding of how resource-use traits underlying competitive dynamics respond to stress. Here, we use duckweeds in the Lemna species complex to measure how phenotypic and genetic variation in R* (a resource acquisition trait representing the minimum resource requirement for positive population growth) varies with high-temperature stress to better understand how stress alters competitive ability for essential resources. We found that heat stress increased the R* of Lemna plants for nitrogen acquisition. Because lower R* values predict dominance in competitive dynamics where resources are limiting, this indicates that under stressful, high temperatures, plants could experience increased sensitivity to competition due to the higher resources required to sustain positive population growth rates. We found minimal genetic variation in R* across 11 local genotypes within the Lemna species complex, indicating that selection on resource acquisition strategies for essential resources such as nitrogen may be constrained in nature. The expression of genetic variation in R* for nitrogen was further reduced under heat stress, suggesting that the response to selection for R* could be particularly constrained under high-temperature stress. Contrary to predictions drawn from the gleaner-opportunist trade-off, we did not find evidence for a trade-off in resource acquisition strategies under benign conditions or high-temperature stress. Plants with lower R* (i.e., higher growth rates under lower nitrogen levels) were not constrained to have lower growth rates under higher nitrogen levels, possibly because the chosen genotypes have not diverged across resource acquisition strategies or because Lemna spp. has escaped this constraint. Importantly, our work outlines that high-temperature stress could increase sensitivity to competition through increased requirement for resources while reducing the evolutionary potential for Lemna species to respond to selection for resource traits. This study acts as a key step to understanding the mechanistic traits behind competitive dynamics in resource-limited and stressful environments.