Biochemical organization of phytocenoses in mid-European freshwater habitats

In an overall study of the biochemical organization of phytocenoses, an ana lysis 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 phytochemic al data collected showed a tripolar organization based on phenolic pool/min eral pool opposition (land plants versus aquatic plants) and cellulose/lign in 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 discussi on of the results is complicated by the fact that most of the land plants c onsidered are lignous dicotyledons whereas the emerging aquatic species are primarily herbaceous monocotyledons. However, it may be stated that the pa rietal macromolecules of the floating species (dicotyledons or monocotyledo ns) are more dependent upon the environment that on the systematics and tha t lignin dominates cellulose in all the species, as for land plants. As far as the mineral components are concerned, the aquatic plants generall y show more abundant minerals than species on the land (1.5 to 2 fold diffe rence). Potassium content is generally 2.9 % in floating aquatic plants com pared with 2.2 % in emerging plants and only 1.2 % in land plants. This ele ment is therefore considerably concentrated with respect to the ambiant wat er (by a factor of more than 1000) whereas calcium is only weakly concentra ted (by a factor of less than 100). Floating plants contain more biogenic m etalloids than both emerging aquatic species and land species: 3.7 % compar ed with 2.4 +/- 0.4 % for nitrogen, 0.46 parts per thousand compared to 0.2 4 parts per thousand for phosphorus. These results confirm the presence of a biochemical organization of plant c ommunities based on phenols, parietal macromolecules and biogenic elements. However, since the fresh-water medium studied is highly heterogeneous (fro m floating aquatic associations to meso-hygrophylic climacic oak forest), e ach 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 ex tend these observations and confirm the existence of a phytochemical organi zation of phytocenoses, in relation with the notion of "phytochemical tacti cs" and the allocation of trophic resources.