Xylem embolism

McCully ME. Root xylem embolisms and refilling. Relation to water potentials of soil, roots, and leaves, and osmotic potentials of root xylem sap. Plant Physiol 1999 119 1001-1008. 

Sperry, J, S, Nichols, K, L., Sullivan, J, E, M., and Eastlack, S. E. (1994). Xylem embolism in ring-porous, diffuse-porous, and coniferous trees of Northern Utah and interior Alaska. F.cology 75, 1736- 1752. 

For application of Poiseuille s law to a complex tissue such as the xylem, care must be taken to ensure that particular vessel elements or tracheids are conducting (e.g., not blocked by embolisms), the actual radii must be determined (note the r4 dependence iiiEq. 9.11a), and corrections may be necessary for lumen shape, tracheid taper, and cell wall characteristics including pits (Calkin et al., 1986 Schulte et al., 1989a). For instance, if the lumen is elliptical with major and minor axes of a and b, respectively, then r4 in Equation 9.11a should be replaced by a3b3/ (8a2 + 8b2). 

As rubber plantations are currently expanding to new areas under drier climatic conditions than traditional ones (north-eastern Thailand, Mato-Grosso in Brazil, north-western India), the performance of rubber trees under water stress has become a major issue. Two main traits are to be considered performance (the ability to transfer water and grow under water stress) and resistance (the ability to avoid damage and survive under water stress). Performance mainly relies on the conductivity of the hydraulic system from root to leaf, whereas resistance mainly relies on susceptibility to cavitation, leading to embolism, within xylem vessels. Stomatal closure occurs to avoid severe cavitation. Hence, differences in susceptibility to cavitation among rubber clones are likely to induce differences in resistance to severe water stress. 

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