Das Projekt "Cell wall hemicelluloses as mobile carbon stores in plants" wird vom Umweltbundesamt gefördert und von Universität Basel, Philosophisch-Naturwissenschaftliche Fakultät durchgeführt. Non-structural carbohydrates (NSC) are the most important carbon reserves in most tissues and species. Beside NSC, cell wall hemicelluloses as the second most abundant polysaccharides in plants have often been discussed to serve as additional carbon reserves during periods of enhanced carbon-sink activity. By using a refined extraction method hemicelluloses were quantitatively and qualitatively determined in tissues of different plant functional groups and the ecophysiological importance of hemicelluloses as mobile carbon pools was investigated in two studies. In the first study hemicelluloses were investigated in newly synthesized tissues of 16 species of four different plant functional groups that were grown under extremely low (140 ppm), medium (280 ppm) or high (560 ppm) CO2 concentrations inducing a massive carbon-under or -oversupply. The changed carbon source-sink activities were evidenced by significant increments of biomass and NSC under elevated CO2 concentrations in tissues of all species. In contrast, hemicellulose concentrations remained remarkably stable with varying CO2 supply. Nevertheless, the monosaccharide spectrum of hemicelluloses showed a significant increase in glucose monomers in leaves of woody species as carbon supply increased. In contrast to the growing tissues of the first experiment the second study investigated the potential carbon reserve function of hemicelluloses by using the naturally occurring fluctuation in carbon-sink activity during bud break in mature forest trees (two deciduous species Carpinus betulus L., Fagus sylvatica L. and two evergreen species Picea abies L., Pinus sylvestris L.). NSC concentrations in branch sapwood of the deciduous trees declined strongly immediately before bud break and increased after bud break. Previous seasons' needles exhibited a significant increase in NSC concentrations shortly before bud break which declined again after flushing. Short-term fluctuations in hemicellulose concentrations were found in branch sapwood of Carpinus and previous seasons' needles of Picea and Pinus and correlated with those of NSC suggesting a mobile reserve function of hemicelluloses. The observed variations in hemicellulose concentrations in previous seasons' needles were due to variations in hemicellulose-bound glucose. In conclusion, this thesis showed that during 'de-novo' synthesis of cell-walls, hemicellulose concentrations do not vary strongly with changing carbon supply. Hemicelluloses are therefore unlikely to play a significant role as additional carbon sinks at future elevated atmospheric CO2 concentrations. On the other hand, apart from their predominant structural function, hemicelluloses can serve as additional carbon reserves in mature tissues during times of exceptionally high carbon demand.