Sur l'organisation biochimique es phytocénoses en milieu dulçaquicole médio- européen

In an overall study of the biochemical organization of phytocenoses, an analysis of the organic components (carbohydrates, tanins, lignin) and mineral components (biogenic metals and metalloids) of 36 land and aquatic vegetal species was conducted in the Dombes (Ain, France), a region of fresh-water ponds in the deciduous biome. The multivariate analysis of the phytochemical data collected showed a tripolar organization based on phenolic pool/mineral pool opposition (land plants versus aquatic plants) and cellulose/lignin opposition (emerging aquatic plants versus floating plants). Although links were detected between the biochemical profiles of the plants and phytosociological divisions or land-encroachment process, the discussion of the results is complicated by the fact that most of the land plants considered are lignous dicotyledons whereas the emerging aquatic species are primarily herbaceous monocotyledons. However, it may be stated that the parietal macromolecules of the floating species (dicotyledons or monocotyledons) are more dependent upon the environment that on the systematics and that lignin dominates cellulose in all the species, as for land plants. As far as the mineral components are concemed, the aquatic plants generally show more abundant minerais than species on the land (1.5 to 2 fold difference). Potassium content is generally 2.9 % in floating aquatic plants compared with 2.2 % in emerging plants and only 1.2 % in land plants. This element is therefore considerably concentrated with respect to the ambiant water (by a factor of more than 1 000) whereas calcium is only wealdy concentrated (by a factor of less than 100). Floating plants contain more biogenic metalloids than both emerging aquatic species and land species: 3.7 % compared with 2.4 ± 0.4 % for nitrogen, 0.46 %o compared to 0.24 %o for phosphorus. These results confirm the presence of a biochemical organization of plant communities based on phenols, parietal macromolecules and biogenic elements. However, since the fresh-water medium studied is highly heterogeneous (from ftoating aquatic associations to meso-hygrophylic climacic oak forest), each of the three sub-groups needs to be taken into consideration to gain a full understanding of this plant community. A discussion is initiated to extend these observations and confirm the existence of a phytochemical organization of phytocenoses, in relation with the notion of "phytochemical tactics" and the allocation of trophic resources.