Bromus tectorum responses to intergenerational drought: Implications for evolvability and selection under climate warming

Abstract

AbstractBromus tectorum poses a substantial threat to sagebrush steppe in the North American intermountain west. Responses of B. tectorum to increasing aridity across the western USA may influence the species’ ability to invade new habitats and further dominate colonised regions. We investigated the phenotypic plasticity of stomatal density, root and shoot length, and root:shoot length ratio across replicates of 23 genotypes subjected to two generations of high and low watering regimes. Shoot lengths greatly increased, root lengths increased and root:shoot ratios decreased with higher levels of progeny (F1) watering. We did not detect explicit transgenerational parental (P) effects on the reaction norms of any trait. Stomatal density did not vary consistently with F1 watering, suggesting a diversified bet‐hedging strategy. As a consequence, heritability for stomatal density did not differ significantly from zero, indicating that evolvability was extremely low. Highest overall potentials for phenotypic selection (the ratio of among‐to‐within genotype trait variances) occurred for shoot length, followed by root length, stomatal density and root:shoot length ratio. Potentials for phenotypic selection were variable among traits and watering treatments. Highest potentials for stomatal density occurred in well‐watered progeny of water‐starved parent plants (i.e. plants from the combined low P H2O, high F1 H2O regime), whereas the highest potentials for phenotypic selection for shoot length occurred for the combined high P H2O, high F1 H2O treatment. We conclude that B. tectorum expresses complex phenotypic trade‐offs with successive exposure to drought, and that phenotypic responses for some traits may allow the species to expand its fundamental niche in response to climate change.