RELATIONS BETWEEN VULNERABILITY TO XYLEM EMBOLISM AND XYLEM CONDUIT DIMENSIONS IN YOUNG TREES OF QUERCUS-CERRIS

Measurements of xylem conduit length and width and the distribution of xylem conduit ends were made in internodes (I), nodes (N) and twig ju nctions (J) of 1-, 2- and 3-year-old twigs of plants of Quercus cerris L. Parallel measurements were also made of the loss of hydraulic cond uctivity of twigs subjected to pressure differentials across conduit p it membranes, equalling the leaf water potential at the turgor loss po int, The loss of theoretical hydraulic conductivity was calculated as the ratio of Sigma r(4) (where r is the conduit radius) of the non-con ducting conduits to that of all the conduits in the outermost wood rin g of I, N and J. Stem zones such as 1-year-old nodes and junctions wer e localized with narrower and shorter xylem conduits and with higher p ercentages of conduit ends than internodes. Such 'constricted zones' w ere less vulnerable to embolism than internodes. Latewood conduits wer e consistently narrower, shorter and less vulnerable to embolism than earlywood ones, A positive relation therefore existed between conduit diameter and length and vulnerability to embolism, The overall vulnera bility to embolism of Q. cerris plants is discussed in terms of xylem conduit width and length and of the distribution of conduit ends.