A test for hydrotropic behavior by roots of two coastal dune shrubs

Root hydrotropism could be a means by which plants forage for limited and patchy distributions of soil water. While root hydrotropism has been induced in distinctly artificial conditions, it is unclear if it operates in natural settings. Here, we tested for this possibility in seedlings of two species of dune shrubs. Growth of individual roots in sand-filled observation chambers was monitored in response to moisture-rich patches and resultant soil water gradients. Chambers were designed so that roots could intercept the moisture gradients but not the moisture-rich patches simply through gravitropism. While up to 12% of the Eriogonum parvifolium roots grew into the moisture-rich patches, comparable root growth was observed in the control. None of the Artemisia californica roots grew into the patches. Thus, in a reasonable simulation of field conditions, we found no compelling evidence for hydrotropic root behavior in seedlings of these two dune shrubs. Our results leave the ecological significance of root hydrotropism in question.