Helianthus porteri, a Granite Outcrop Endemic, Does Not Have More Drought Resistant Traits Than Congeners

Abstract Helianthus porteri, a sunflower endemic to drought prone granite outcrops, has been hypothesized to be drought resistant. We compared H. porteri to three Helianthus species (H. annuus, H. agrestis, H. carnosus) from less drought prone habitats for traits associated with drought avoidance and tolerance in greenhouse experiments. Under well-watered conditions, H. porteri had high specific root length suggestive of high capacity for water uptake, but intermediate root mass ratio (RMR) and shallow rooting inconsistent with enhanced capacity for drought avoidance. In response to mild drought, none of the species exhibited osmotic adjustment, and H. porteri had no change in RMR and no greater capacity to increase water-use efficiency, again, inconsistent with greater drought avoidance. In response to cessation of watering, H. porteri wilted at a leaf water potential similar to a wet habitat species, inconsistent with greater drought tolerance. Overall, under the conditions assessed, we found no evidence that H. porteri possesses traits that confer a unique ability to avoid or tolerate drought as compared to congeners.