Ascent of sap

The ascent of sap in the xylem tissue of plants is the upward movement of water from the root to the crown. Xylem is a complex tissue consisting of living and non-living cells. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. Both of these cell types have thick, lignified secondary cell walls and are dead at maturity. Although several mechanisms have been proposed to explain the phenomenon, the cohesion-tension mechanism^[1] has the most evidence and support. Although cohesion-tension has received criticism, for example due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism.^{[2] [3]}

The more recently proposed compensating pressure (CP) theory favors a version of vital theory proposed by Jagdish Chandra Bose. However, experimental evidence has not supported it ^[4]

An alternative theory points out that liquids are at the walls submitted to intermolecular forces inferring density gradients making heterogeneous liquid layers. The crude sap motion takes the disjoining pressure gradient into account and the sap flow dramatically increases such that the watering of nanolayers may be analogous to a microscopic flow. Application to microtubes of xylem avoids the problem of cavitation and enables to understand why the ascent of sap is possible for giant trees. [5]

References

1. ^ "On the Ascent of Sap" by Henry H. Dixon and J. Joly. Philosophical Transactions of the Royal Society of London. B, Vol. 186, pp. 563–576. (1895)
2. ^ Xylem Structure and the Ascent of Sap, 2nd ed. 2002. by Melvin T. Tyree and Martin H. Zimmermann (ISBN 3-540-43354-6) Springer-Verlag
3. ^ "The Cohesion-Tension Theory" by Angeles G, Bond B, Boyer JS, Brodribb T, Brooks JR, Burns MJ, Cavender-Bares J, Clearwater M, Cochard H, Comstock J, Davis SD, Domec J-C, Donovan L, Ewers F, Gartner B, Hacke U, Hinckley T, Holbrook NM, Jones HG, Kavanagh K, Law B, López-Portillo J, Lovisolo C, Martin T, Martínez-Vilalta J, Mayr S, Meinzer FC, Melcher P, Mencuccini M, Mulkey S, Nardini A, Neufeld HS, Passioura J, Pockman WT, Pratt RB, Rambal S, Richter H, Sack L, Salleo S, Schubert A, Schulte P, Sparks JP, Sperry J, Teskey R, Tyree M. New Phytologist, Vol. 163:3, pp. 451–452. (2004)
4. ^ Stiller, Volker and John S Sperry. Canny's compensating pressure theory fails a test American Journal of Botany. 1999;86:1082–1086.

5.^ H. Gouin. Liquid-solid interaction at nanoscale and its application in vegetal biology. Colloids and Surfaces A. 2011, Vol. 383, pp. 17–22. arXiv:1106.1275 and references herein.