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BASES GENETIQUES DES VARIATIONS POUR LA STRUCTURE HISTOLOGIQUE DES TIGES DE LUZERNE (Medicago sativa L.)

Abstract : Stems are the essential constitutive elements of the lucerne canopy, and the aerial part which limits forage digestibility. During growth, tissues developp to confer stem rigidity and to allow water transport. Tissues proportion may play a determinant role in their degradability. The aim of this thesis is to determine the genetic bases of histological structures of lucerne stems and the link between histological structures and digestibility of the stems. Two genotypes contrasted in digestibility werre used to quantify with image analysis histological structures along the stem characterised for its morphology. NDF and lignin contents were also determined. The evolution pattern of these traits were observed from the apex to the bottom part of the stem. Genetic varaibility was assessed among and within-cultivars. Six cultivars were evaluated for the among-cultivars variation and 10 genotypes within two cultivars for the within-cultivar variation. The effect of AFLP and SSR molecular markers on histological, morphological and enzymatic solubility of the stem was estimated in a F1 tetraploïd population obtained from a cross between 2 plants chosen in 2 different cultivars. Genetic correlations between all these traits were calculated. The analysis along the stem showed that stem height play an important role in the variation of histological traits, evolving in the same pattern for both genotypes. The increase in NDF and lignin contents in the bottom part of the stem is accompanied by the onset of cambium which is reponsible for the development of new cell layers in secondary tissues (secondary xylem). Few or no genetic variation was found among-cultivars,and the within-cultivar variation was significant and higher for most of the traits. Genetic variation was also found in the F1 population and was the largest one. Molecular markers was significantly associated with the variation of all traits. These markers explained from 6.5 % of the variation of the proportion of pith parenchyma to 40 % of the stem height. Some of the genomic regions were associated with more than one trait. Markers were associated on the same linkage group for the stem height, and the leaf to stem ratio, but also for the stem height and their enzymatic solubility, the proportion of the cortex and the xylem cell wall surface density. Moreover the analysis of the correlations showed a negativ relationship between xylem proportion, xylem cell wall thickness, xylem cell wall surface density and enzymatic solubility of the stems. Enzymatic solubility of the stems was also positively correlated to the proportion of the non lignified tissues. Map saturation and the availability of a QTL software specific to autotetraploid species would permit to consider the improvement of forage digestibility thanks to markers assisted selection. A possible way to improve digestibility would to decrease xylm proportion in the stem by introducing QTLs responsible for the decrease of this trait, and the decrease of xylem cell wall thickness without reducing stem rigidity.
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Submitted on : Friday, March 2, 2012 - 5:49:38 PM
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Françoise Guines. BASES GENETIQUES DES VARIATIONS POUR LA STRUCTURE HISTOLOGIQUE DES TIGES DE LUZERNE (Medicago sativa L.). Amélioration des plantes. Agrocampus - Ecole nationale supérieure d'agronomie de rennes, 2002. Français. ⟨tel-00655243⟩

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