Common milkweed (Asclepias syriaca) and showy milkweed (Asclepias speciosa) are widely distributed in the United States with common milkweed found in the east (including Eastern South Dakota) and showy found in the west (including Western South Dakota). South Dakota experiences a strong east to west moisture gradient, which correlates with the milkweed species’ distribution, and the two species hybridize in the center of the state. We aimed to determine if water-related traits are linked to the distribution of the species and hybrids by growing them under various water levels, ranging from high moisture to drought conditions in a greenhouse. In addition, we collected leaf samples for both species and hybrids from a common garden in Sioux Falls, SD, as well as along a longitudinal (moisture) transect across the state of South Dakota.
Results/Conclusions
Asclepias speciose was found to have a lower stomatal density but greater stomatal length (174 stomata/mm2 (SE=15) and 0.22 mm (SE=0.0004), respectively) compared with A. syriaca plants (235 stomata/mm2 (SE= 12) and 0.19 mm (SE=0.0006), respectively) grown in a common garden. A. speciosa is also known to have greater leaf hair density which is consistent with the lower precipitation levels and a need to conserve water. The hybrids had intermediate stomatal traits compared with the two species. Midday water potential measures under drought conditions in a greenhouse were not significantly different for the two species (average for both species was -1.55 MPa, SE=0.02); however, hybrids had a marginally significant (p<0.08) lower midday water potential average (-1.9 MPa, SE=0.02), suggesting a potential mechanism for reinforcing barriers between these two species.
Leaf collections every 30-40 miles along a longitudinal (east to west) gradient across the state of South Dakota also showed linear increases in stomatal length (0.21 to 0.24 mm) and decreases in stomatal density (275 to 183 stomata/mm2). Despite the changes in stomatal characteristics, there were no significant differences between A. speciosa, A. syriaca, or hybrids in their carbon isotope ratios (d13C) from populations across the state of South Dakota (d13C averaged -27.7 (SE=0.09) across both species and hybrids). This work contributes to a greater understanding of what limits the range of these two species and how moisture level and hybridization has an effect on the species’ distributions.