Differences in drought adaptation between subspecies of sagebrush (Artemisia tridentata)

Three subspecies of Artemisia tridentata occupy distinct habitats in the Great Basin of North America: ssp. wyomingensis in low, arid elevations; sap. vaseyana in high, mesic elevations; and ssp. tridentata in intermediate zones. We evaluated differences in the drought experienced and drought tolerance among the subspecies. Drought tolerance was measured by two traits: the xylem pressure (Psi(x)) causing xylem cavitation and Psi(x) causing loss of leaf turgor (Psi(tlp)). As expected from habitat, ssp. wyomingensis experienced a more severe summer drought than ssp. vaseyana (minimum Psi(x) = -7.5 MPa and -3.8 MPa, respectively). Despite the large difference in drought exposure, the subspecies exhihibed similar drought responses, including a reduction in transpiration (E) below 0.5 mmol.s(-1).m(-2), a shedding of 60-65% of foliage, and >95% decline in soil-to-leaf hydraulic conductance. The similarity in the drought response was consistent with pronounced differences in Psi(x), causing 50% loss in xylem conductivity by cavitation (Psi(50)). The Psi(50) was -4.9 MPa in ssp. wyomingensis vs. -3.0 MPa in ssp. vaseyana; ssp. tridentata was in termediate (Psi(50) = -3.9 MPa). Differences in cavitation resistance were preserved in a common garden, suggesting that they arose by genetic differentiation. A water transport model indicated that the greater cavitation resistance in ssp. wyomingensis was a necessary adaptation for its more arid habitat, and that the similar restrictions of E among subspecies were required to avoid hydraulic failure. The Psi(tlp) was also lower in ssp. wyomingensis than in ssp. vaseyana. Although Psi(tlp) decreased in both subspecies during drought, the adjustment was not sufficient to maintain turgor. Turgor loss may have been adaptive in minimizing shoot growth and stomatal conductance under hydraulically limiting circumstances.