Hemicelluloses are the second most abundant polysaccharide in plant cell walls and account for one fifth to one quarter of the total dry plant biomass. They are known to be structural hetero-polysaccharides of the plant cell wall associated with cellulose and lignin. Some hemicelluloses are well known C-reserves in seeds of plant species. A possible role of hemicelluloses as mobile carbon reserves in non-reproductive plant organs is still in question. Even if parts of hemicelluloses could be mobilized by plants, this would have major effects on plant carbon relations under changing C-supply. To explore this we conducted a manipulative experiment, in which 16 plant species of four different functional plant types (grasses, perennial herbs, broad-leaved trees and conifers) were grown under either very low (140 ppm), medium (280 ppm) or high (560 ppm) atmospheric CO₂ concentrations in order to induce situations of C-under or C-oversupply. Half of the plants received a mineral fertilizer to ensure sufficient nutrient supply for growth. If hemicelluloses or parts of hemicelluloses are mobile carbon compounds their pool size should respond to altered CO₂ concentrations similar to well-known responses in non-structural carbohydrates (NSC) such as starch and sugars.

Results/Conclusions

Above and belowground biomass of all species increased significantly with increasing CO₂ concentrations with a slightly higher increase for the fertilised individuals. The average NSC concentrations ranged from 5 - 25 % of the total dry biomass in leaves and 1.5 – 25 % d.m. in woody organs. Although markedly lower than at 280 ppm CO₂, considerable amounts of NSC were still present in individuals grown at 140 ppm CO₂. The 560 ppm CO₂ treatment enhanced NSC concentrations in all functional plant groups up to fourfold compared to the 140 ppm CO₂ treatment with starch being the main responsive compound. Each functional plant group revealed specific hemicellulose concentrations and monosaccharide composition of the hemicellulose pools. In contrast to NSC however, the different CO₂ concentrations had no or only minor effects on hemicellulose concentrations and composition in leaves and sapwood of all functional plant types, although some grass species tended to have 5-20 % higher hemicellulose concentrations at high atmospheric CO₂ compared to low CO₂ concentrations. Fertilization had minor effects on NSC but no influence on hemicellulose concentrations. This study shows that for the majority of species, hemicelluloses may not function as significant mobile carbon reserves in non-reproductive plant organs.