干旱区生态环境知识资源中心

Predawn plant water potential (Psi(w), measured with leaf psychrometers) and surrogate measurements made with the pressure chamber (termed Psi(pc) here) are used to infer comparative ecological performance, based on the expectation that these plant potentials reflect the wettest soil Psi(w) accessed by roots. There is growing evidence, however, that some species exhibit substantial predawn disequilibrium (PDD), defined as plant Psi(w) or Psi(pc) at predawn substantially more negative than the Psi(w) of soil accessed by roots. In the western Great Basin desert, the magnitude of PDD calculated as soil Psi(w) minus predawn leaf Psi(w) was as large as 1.4 and 2.7 MPa for two codominant shrub species, Chrysothamnus nauseosus and Sarcobatus vermiculatus, respectively. The magnitude of PDD calculated as soil Psi(w) minus predawn Psi(pc) was smaller, up to 0.6 and 2.1 MPa for Chrysothamnus and Sarcobatus, respectively. For both species, mechanisms contributing to PDD included night-time transpiration and putative leaf apoplastic solutes, but not hydraulic conductance limitations. Hydraulic lift also occurred in both species and likely contributed to PDD for Sarcobatus. Finding large magnitude PDD in field populations emphasizes that species differences in predawn plant Psi(w) or Psi(pc), do not necessarily reflect differences in accessible soil 41, and rooting depth, nor does a low predawn plant T, or Psi(pc) value necessarily mean that soil Psi(w), is also low. Mechanisms contributing to PDD affect relationships between plants and soil resources, as well as the potential for plant plant interactions.