Xylem efficiency vs. safety: Acclimation to drought of seedling root anatomy for six Patagonian shrub species

With the aim to seek evidences on the possible trade off between hydraulic efficiency and hydraulic safety, from both an evolutionary and an acclimation point of view, we compared root xylem anatomy and derived functional properties for seedlings of six Patagonian shrub species grown in a greenhouse under two levels of water availability (control and drought). Root central cylinder area, vessel diameter (b) and double-wall thickness (t) were measured; from these data, the sum of vessel radii to the fourth power and wall strength (t/b)²] were calculated as indicators of hydraulic efficiency and safety respectively. Across species, we observed only a weak negative correlation between hydraulic efficiency and hydraulic safety. Within species, Lycium chilense, the species with the most mesic leaves of the group, showed significant acclimation to drought for both functional attributes, lowering efficiency and increasing safety by developing a higher proportion of small and more resistant xylem vessels.