Morphological adjustments promote drought avoidance in the wetland plant Acorus americanus

To evaluate drought avoidance and drought tolerance properties in the aquatic macrophyte sweet flag (Acorus americanus), dormant rhizomes were germinated and grown in 15 L microcosms under drought (ca. 10% soil moisture, by weight), moderate water (ca. 20% soil moisture), or flooded conditions (ca. 50% soil moisture). Four weeks after germination, tissue moisture content in plants receiving drought treatments had 29–37% less water in comparison to plants in moderate and flooded conditions. Furthermore, in comparison to well-watered treatments, water potentials (leaf [Ψleaf] and xylem [Ψxylem]) declined by as much as 0.6 MPa in plants with diminished water availabilities. Drought tolerance modifications were not observed among the lower water treatments, including lack of any significant differences in osmotic potentials (at saturated conditions [Ψπsat] and turgor loss point [Ψπtlp]) and bulk elastic modulus (ɛ). Nonetheless, drought avoidance modifications in plants experiencing both drought and moderate water availabilities were observed. Stomatal pores were approximately 4.1 μm shorter and 3.5 μm narrower in drought treated plants compared to flooded individuals. In addition to smaller pores, drought treated plants had approximately 100 fewer stomates mm−2 than flooded plants. Above- and belowground biomass was also lower in drought treated plants in comparison to flooded plants, including a 46 and 73% reduction in below- and aboveground biomass, respectively. Therefore, while many plants use a combination of both drought tolerance and drought avoidance in response to low soil water availabilities, avoidance is the dominant response in Acorus during extended periods of water deprivation.