PhD Dissertation: 

Valentin Kindomihou (2005). Tropical Grasses Silicification: Genetic Interspecific Variation, Influence of Growth Conditions and Relationship with the Foliar Structure. Laboratory of Plant Ecology and Biogeochemistry, Botanical Garden Jean Massart, Interfaculty School of Bioengineers, Free University of Brussels, Belgium. 193 pages.

Promotors: Prof Pierre Meerts (ULB, Belgique) & Prof Brice Sinsin (UAC, Benin).

SUMMARY: This thesis investigates the variation in tropical grasses silicification on the ecological and evolutionary point of view. The review of works examining variation extent in silica concentration identifies three groups of factors influencing the grass silicification, in particular genetic factors (difference in the capacity of root absorption, anatomy and foliar ecophysiological features), endogenous factors (phenology, organs) and the growth conditions. The experimental approach focused the influence of certain growth conditions on the silica concentration (defoliation, moisture, substrate fertility). Results confirm the influence of defoliation and substrate fertility on the silica accumulation, but well underline the complexity of the action of these factors which results in particular in different responses of species. These results suggest that contradictions in published works with regard to the plausible inducible character of silica accumulation are due in some extent to the species*defoliation interaction, and a great results sensitivity to conditions for defoliation application.  Variations in silica content (intrinsic and induced by the growth conditions) relating to the variations in structural and functional characters (specific leaf area, water content, foliar anatomy, etc.) showed correlations which are not entirely constant through all the tests. These are significantly positive with soluble ashes in all tests, but negative with carbon in defoliation. With the relative water content, silica correlated positively in defoliation, but change with phospho- nitrogenized fertilization. This negative correlation remains consistent in both blades and sheaths in interspecific comparison. Difference in the structure of correlations results at the same time from the sampling effects and the large range of the silica contents swept by the various examined species. Pennisetum unisetum is the richest in silica and sclerenchyma, and weights disproportionately in correlations analysis. Our results do not provide a very clear support for the assumption that silica can be substitute to carbon compounds like material of support.

 Key words: grass, tropical, silicification, anatomy, structure, defoliation, fertilization, moisture.