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Mycorrhization and Stress Tolerance of Ecotypes of Beech (Fagus sylvatica L.): "Conventwald" - Project


In a case study (Conventwald, near Freiburg, Germany) the effect of drought stress on different ecotypes of 7-year-old beech trees was investigated with regard to type and degree of mycorrhization and stress tolerance. The ecotypes originated from different areas of Baden-Württemberg and represented different types of stand and climate. Conditions of drought stress were created during two subsequent vegetation periods (1997, 1998) by covering one plot with a roof for the duration of approx. 3 months (mid of July until mid of October). Based on morphology and sequence analysis of DNA (ITS and NL primers) we identified 8 different types of ectomycorrhiza in 1997 (Byssocorticium atrovirens, Cenococcum geophilum, Fagirhiza spinulosa, Laccaria amethystina, Lactarius subdulcis, Russula ochroleuca, Sphaerozone ostiolatum, Xerocomus chrysenteron). Of these, three (B. atrovirens, L. subdulcis, X. chrysenteron) had an abundance which was sufficient for a comparative analysis. Out of this group, B. atrovirens and L. subdulcis were comparably distributed within both uncovered and covered plots. In contrast, X. chrysenteron represented a considerably higher percentage of all mycorrhizas on the stressed plot (41 % compared to 24 % for the control plot). A comparison of these data with a biometric analysis (Volkmer et al. 1998) shows an interesting coincidence. The local ecotyp "Conventwald" which suffered most under the applied drought stress did not have the obiously more drought-tolerant X. chrysenteron type of mycorrhiza. We thus suggest a correlation between type of beech ectomycorrhiza and drought stress tolerance of the host.

A detailed analysis of plant and fungal carbohydrates and sugar alcohols indicated differences in stress responses between 1997 and 1998. In 1997, were drought stress was severe, reserve polysaccharides in roots, starch and glycogen, were decreased. This was most severe in the clones Conventwald and Zwiefalten and could be related to a respective decrease in net photosynthesis (Buschmann et al. 1998). The 1998 treatment caused a different physiological situation. During this trial carbon allocation toward the root system was less affected, and this coincided with minor decreases of rates of net photosynthesis (Buschmann et al. 1999). Under these conditions the dominating beech mycorrhizas (Xerocomus chrysenteron, Lactarius subdulcis) responded to the more neagtive soil water potential with the accumulation of compatible solutes in an species-secific manner: Xerocomus chrysenteron accumulated large amounts of arabitol while in the case of Lactarius subdulcis it was mannitol. This was balanced by a decrease of the pool sizes of polysaccharides. Stress-adapted mycorrhizas coincided with only minor growth reductions of the host plants. We conclude that well adapted fungus/beech combinations help in coping with drought stress.


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