Predawn disequilibrium between plant and soil water potentials in two cold-desert shrubs

Classical water relations theory predicts that predawn plant water potential should be in equilibrium with soil water potential (soil Psi(w)) around roots; and many interpretations of plant water status in natural populations are based on this expectation. We examined this expectation for two salt-tolerant, cold-desert shrub species in glasshouse experiments where frequent watering assured homogeneity in soil Psi(w) and soil-root hydraulic continuity and where NaCl controlled soil Psi(w). Plant water potentials were measured with a pressure chamber (xylem Psi(p)) and thermocouple psychrometers (leaf Psi(w)). Soil Psi(w) was measured with in situ thermocouple psychrometers. Predawn leaf Psi(w) and xylem Psi(p) were significantly more negative than soil Psi(w), for many treatments, indicating large predawn soil-plant Psi(w) disequilibria: up to 1.2 MPa for Chrysothamnus nauseosus (0 and 100 mM NaCl) and 1.8 MPa for Sarcobatus vermiculatus (0, 100, 300, and 600 mM NaCl). Significant nighttime canopy water loss was one mechanism contributing to predawn disequilibrium, assessed by comparison of xylem Psi(p) for bagged (to minimize transpiration) and unbagged canopies, and by gas exchange measurements. However, nighttime transpiration accounted for only part of the predawn disequilibrium. Other mechanisms that could act with nighttime transpiration to generate large predawn disequilibria are described and include a model of how leaf apoplastic solutes could contribute to the phenomenon. This study is among the first to conclusively document such large departures from the expectation of predawn soil-plant equilibrium for C-3 shrubs, and provides a general framework for considering relative contributions of nighttime transpiration and other plant-related mechanisms to predawn disequilibrium.