Vessel length and conductivity of Ulmus branches: ontogenetic changes and relation to resistance to Dutch elm disease

Changes in age of the hydraulic architecture of Ulmus minor and U. minor × U. pumila juvenile wood were studied and related to tolerance to Dutch elm disease (DED). The xylem vessel dimensions and the conductivity to air of 2- to 7-year-old branches were analyzed and quantified. No obvious differences in vessel length distribution and conductivity were found to explain differences in DED tolerance among the U. minor clones, or, at the taxon level, the higher DED tolerance of U. minor × U. pumila. Among the U. minor clones, the more susceptible one had wider vessels and a higher maximum vessel diameter than the more tolerant clone. Relations between vessel lengths, vessel diameters and branch sizes were highly significant, and varied between taxa. The diameter and length of vessels increased with age, and average values stabilized 1–2 years earlier for U. minor than for U. minor × pumila. Mean maximum vessel length was significantly higher in U. minor and increased more with age and maximum vessel diameter than in U. minor × pumila. With each 0.2 m increase in height up the stem, conductivities for U. minor and U. minor × U. pumila decreased by 59 and 50 %, respectively, probably due to shortening of the vessels. The implications of xylem structure for the means of pathogen movement and resistance to DED are discussed.     

Aus dem Küstenlande

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Hochsommerflora der Umgebung von Görz

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Botanische Forschungsreisen Bericht über einen Ausflug nach dem südlichen Istrien

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Seasonal changes in wood formation of Ulmus pumila and U. minor and its relation with Dutch elm disease

Elms containing narrow and scattered vessels have been reported to be more resistant to Ophiostoma novo-ulmi (Dutch elm disease pathogen) than elms with large and contiguous vessels. However, recent measurements in Ulmus pumila and U. minor showed a contrary trend. The pin method was applied to 4-yr-old branches of eight clones planted in Madrid. During 2002, radial growth increments and vessel diameters were measured monthly, and beetle trapping was undertaken weekly. U. minor formed larger vessels at the beginning of the season, coinciding with a peak of captured beetles, but, up to June 15, vessels were larger for U. pumila. The number of vessels per group, the transversal area per vessel group, and the mean theoretical hydraulic conductances were significantly higher for U. minor on most dates. Researchers should take into consideration the seasonal changes in vessel size. The results highlight that seasonal variation of vessel diameters and hydraulic parameters, in combination with beetle abundance, are the main factors that could explain the different susceptibility of both elm species to O. novo-ulmi.     

Ausflug nach Rovigno

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