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Comparative Water Relations of Carex geyeri and Calamagrostis rubescens

Two rhizomatous codominant understory species, an overwintering dryland sedge, Carex geyeri, and a deciduous grass, Calamagrostis rubescens, were studied in the field for diurnal and seasonal trends in water relations. Diurnal trends in leaf xylem potential (ψl) showed that C. geyeri had minimum daily values of 0.40-0.71 MPa more negative than C. rubescens. Rhizome xylem potential (ψr) was more negative for C. rubescens, but, in both species, ψr was much less negative (1.8-2.3 MPa) than ψl. During the growing season, osmotic potentials (π's) of both species were similar when measured with the freeze-and-thaw dewpoint (DP) technique, but π at incipient plasmolysis, using the pressure/volume (P/V) technique, was 0.25-0.77 MPa more negative for C. geyeri. Using DP values, I found estimates of minimum daily turgor were consistently negative (< -0.30 MPa) but that, using P/V values, they were seldom more negative than -0.10 MPa. Apparently differences in bound water content make the DP technique inappropriate for comparative studies of these two species.