Lack of Evidence for the Drought Tolerance Hypothesis of Hyperaccumulation in Astragalus Species

Premise of research.Despite a lack of experimental support, enhanced drought tolerance is one of the recurring hypotheses suggested to explain trace element hyperaccumulation. Trace elements may have osmolytic and/or antioxidative properties that help hyperaccumulating plants reduce water loss or its resultant damage. Selenium in particular seems like a promising candidate because of its known antioxidative and drought protectant properties in nonaccumulators, but it has not been tested in hyperaccumulators.Methodology.Here we investigate the drought tolerance hypothesis in a controlled greenhouse setting using a full-factorial design with seedlings of a selenium hyperaccumulator in the genus Astragalus and a nonaccumulating congener.Pivotal results.While selenium increased plant survivorship and longevity and drought reduced them, we found no evidence that selenium improved the drought tolerance of either species.Conclusions.Drought tolerance appears to not be a mechanism that would explain the evolution of hyperaccumulation in Astragalus, although it may still operate in some as-yet untested circumstances.