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 Ψw) 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 Ψw and soil-root hydraulic continuity and where NaCl controlled soil Ψw. Plant water potentials were measured with a pressure chamber (xylem Ψp) and thermocouple psychrometers (leaf Ψw). Soil Ψw was measured with in situ thermocouple psychrometers. Predawn leaf Ψw and xylem Ψp were significantly more negative than soil Ψw, for many treatments, indicating large predawn soil-plant Ψw 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 Ψp 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 C3 shrubs, and provides a general framework for considering relative contributions of nighttime transpiration and other plant-related mechanisms to predawn disequilibrium.