title: Effect of Leaf Water Potential on Internal Humidity and CO2Dissolution: Reverse Transpiration and Improved Water Use Efficiency under Negative Pressure.
abstract: The pull of water from the soil to the leaves causes water in the transpiration stream to be under negative pressure decreasing the water potential below zero. The osmotic concentration also contributes to the decrease in leaf water potential but with much lesser extent. Thus, the surface tension force is approximately balanced by a force induced by negative water potential resulting in concavely curved water-air interfaces in leaves. The lowered water potential causes a reduction in the equilibrium water vapor pressure in internal (sub-stomatal/intercellular) cavities in relation to that over water with the potential of zero, i.e., over the flat surface. The curved surface causes a reduction also in the equilibrium vapor pressure of dissolved CO2, thus enhancing its physical solubility to water. Although the water vapor reduction is acknowledged by plant physiologists its consequences for water vapor exchange at low water potential values have received very little attention. Consequences of the enhanced CO2solubility to a leaf water-carbon budget have not been considered at all before this study. We use theoretical calculations and modeling to show how the reduction in the vapor pressures affects transpiration and carbon assimilation rates. Our results indicate that the reduction in vapor pressures of water and CO2could enhance plant water use efficiency up to about 10% at a leaf water potential of -2 MPa, and much more when water potential decreases further. The low water potential allows for a direct stomatal water vapor uptake from the ambient air even at sub-100% relative humidity values. This alone could explain the observed rates of foliar water uptake by e.g., the coastal redwood in the fog belt region of coastal California provided the stomata are sufficiently open. The omission of the reduction in the water vapor pressure causes a bias in the estimates of the stomatal conductance and leaf internal CO2concentration based on leaf gas exchange measurements. Manufactures of leaf gas exchange measurement systems should incorporate leaf water potentials in measurement set-ups.
authors.affiliation: Department of Physics, University of HelsinkiHelsinki, Finland; Department of Forest Sciences, University of HelsinkiHelsinki, Finland; Viikki Plant Science Centre, University of HelsinkiHelsinki, Finland.
authors.forename: Timo
authors.initials: T
authors.lastname: Vesala
authors.affiliation: Earth and Environmental Sciences Division, Los Alamos National Laboratory Los Alamos, NM, USA.
authors.forename: Sanna
authors.initials: S
authors.lastname: Sevanto
authors.affiliation: Department of Physics, University of Helsinki Helsinki, Finland.
authors.forename: Tiia
authors.initials: T
authors.lastname: Grönholm
authors.affiliation: Department of Physics, University of Helsinki Helsinki, Finland.
authors.forename: Yann
authors.initials: Y
authors.lastname: Salmon
authors.affiliation: Department of Forest Sciences, University of Helsinki Helsinki, Finland.
authors.forename: Eero
authors.initials: E
authors.lastname: Nikinmaa
authors.affiliation: Department of Forest Sciences, University of Helsinki Helsinki, Finland.
authors.forename: Pertti
authors.initials: P
authors.lastname: Hari
authors.affiliation: Department of Forest Sciences, University of Helsinki Helsinki, Finland.
authors.forename: Teemu
authors.initials: T
authors.lastname: Hölttä
keywords: CO2 assimilation; Kelvin effect; carbon uptake; redwood; water potential; water uptake; water use efficiency