MAGNITUDE AND MECHANISMS OF DISEQUILIBRIUM BETWEEN PREDAWN PLANT AND SOIL WATER POTENTIALS

Predawn plant water potential (Ψ w , measured with leaf psychrometers) and surrogate measurements made with the pressure chamber (termed Ψ pc here) are used to infer comparative ecological performance, based on the expectation that these plant potentials reflect the wettest soil Ψ w accessed by roots. There is growing evidence, however, that some species exhibit substantial predawn disequilibrium (PDD), defined as plant Ψ w or Ψ pc at predawn substantially more negative than the Ψ w of soil accessed by roots. In the western Great Basin desert, the magnitude of PDD calculated as soil Ψ w minus predawn leaf Ψ 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 Ψ w minus predawn Ψ pc was smaller, up to 0.6 and 2.1 MPa for Chrysothamnus and Sarcobatus , respectively. For both species, mechanisms contributing to PDD included nighttime 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 Ψ w or Ψ pc do not necessarily reflect differences in accessible soil Ψ w and rooting depth, nor does a low predawn plant Ψ w or Ψ pc value necessarily mean that soil Ψ w is also low. Mechanisms contributing to PDD affect relationships between plants and soil resources, as well as the potential for plant–plant interactions. Corresponding Editor: F. C. Meinzer