What are the driving forces for water lifting in the xylem conduit?

Abstract

After Renner had shown convincingly in 1925 that the transpirational water loss generates tensions larger than 0.1 MPa (i.e. negative pressures) in the xylem of cut leafy twigs the Cohesion Theory proposed by Böhm, Askenasy, Dixon and Joly at the end of the 19th century was immediately accepted by plant physiologists. Introduction of the pressure chamber technique by Scholander et al. in 1965 enforced the general belief that tension is the only driving force for water lifting although substantial criticism regarding the technique and/or the Cohesion Theory was published by several authors. As typical for scientific disciplines, the advent of minimal- and non-invasive techniques in the last decade as well as the development of a new, reliable method for xylem sap sampling have challenged this view. Today, xylem pressure gradients, potentials, ion concentrations and volume flows as well as cell turgor pressure gradients can be monitored online in intact transpiring higher plants, and within a given physiological context by using the pressure probe technique and high-resolution NMR imaging techniques, respectively. Application of the pressure probe technique to transpiring plants has shown that negative absolute pressures (down to - 0.6 MPa) and pressure gradients can exist temporarily in the xylem conduit, but that the magnitude and (occasionally) direction of gradients contrasts frequently the belief that tension is the only driving force. This seems to be particularly the case for plants faced with problems of height, drought, freezing and salinity as well as with cavitation of the tensile water. Reviewing the current data base shows that other forces come into operation when exclusively tension fails to lift water against gravity due to environmental conditions. Possible candidates are longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels as well as gel- and gas bubble-supported interfacial gradients. The multiforce theory overcomes the problem of the Cohesion Theory that life on earth depends on water being in a highly metastable state.

A non-invasive plant-based probe for continuous monitoring of water stress in real time: a new tool for irrigation scheduling and deeper insight into drought and salinity stress physiology

“…This could be verified by measurements using the minimal-invasive microcapillary cell turgor and xylem pressure probes * Note that pressure probes mounted in a non-contact mode close to the leaf surface showed no P p changes indicating that they resulted from effects of temperature on stomata and in turn, on turgor pressure. (Zimmermann et al 2002(Zimmermann et al , 2004. As indicated in Figure 4 the ZIM-probes provide the same information as the highly sophisticated microcapillary probes, but with the important difference that measurements can be performed in the field .…”

Mucilage and polysaccharides in the halophyte plant species Kosteletzkya virginica: Localization and composition in relation to salt stress

“…following addition of 20 mM NaCl, which shows the importance of ions in the formation of gels (cited in Zimmermann et al, 2002a).…”

Phytotoxicity of multi-walled carbon nanotubes on red spinach (Amaranthus tricolor L) and the role of ascorbic acid as an antioxidant

“…8c). Stomata closure prevents water loss [60], leading to decreased CO 2 concentration inside the leaf. Lee et al [61] demonstrated that pathogen-infected guard [62].…”