Magnitude and mechanisms of disequilibrium between predawn plant and soil water potentials

Predawn plant water potential (Uw, measured with leaf psychrometers) and surrogate measurements made with the pressure chamber (termed Upc here) are used to infer comparative ecological performance, based on the expectation that these plant potentials reflect the wettest soil Uw accessed by roots. There is growing evidence, however, that some species exhibit substantial predawn disequilibrium (PDD), defined as plant Uw or Upc at predawn substantially more negative than the Uw of soil accessed by roots.