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

Abstract

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