Water is the primary factor limiting plant growth and distribution in many habitats. Thus, plant water status and water relations characteristics are closely related to plant performance, particularly in arid habitats. Measurements of plant water status just before sunrise, called plant predawn water potential, are used as a standard for comparing plant water availability in different habitats because of the expectation that predawn plant water potential is in equilibrium with soil water potential. The PIs have evidence from field and controlled environment studies that at least two desert shrub species show major departures from the expected soil-plant predawn water potential equilibrium, even when hydraulic continuity (soil-root contact) is not a limiting factor, and the PIs call this phenomenon "predawn disequilibrium." Predawn disequilibrium in the study species provides a focal point for investigating a suite of mechanisms that may account for departure from classical physiological expectations and for study of ecological consequences and adaptive significance of those mechanisms. The study shrubs are Chrysothamnus nauseosus ssp. consimilis (salt-excluding, non-halophytic) and Sarcobatus vermiculatus (salt-accumulating, halophytic) which are similar in life history and rooting characteristics. Mechanisms to be tested, in both greenhouse and field studies, include lack of time for overnight recovery, nighttime transpiration, nocturnal efflux of water from roots, growth-induced reduction in plant water potential, and solute accumulation in intercellular spaces. The PIs will then assess the plant carbon gain consequences of predawn disequilibrium by examining its relationship to photosynthesis, transpiration and water-use-efficiency. This project will contribute to a more accurate understanding of fundamental plant water relations in areas where classical interpretations fail. This improved understanding is essential for making valid ecological interpretations of water relations adaptations of wildland plants.
