ROLES OF STRUCTURAL PHENYLPROPANOIDS IN FORAGE CELL-WALL DIGESTION

Phenolic constituents (lignins and phenolic acids) and carbohydrates a re assembled in a tight architecture which differs according to the pl ant species. During cell wall digestion, the hydrolysis kinetics diffe r between carbohydrates and seem to depend chiefly on the content and organisation of tissue phenolics. Among the phenylpropanoids, ferulic acid is released more quickly than p-coumaric acid. Lignins remain lar gely in the cell walls. They also undergo transformations, chiefly sol ubilisation as lignin-carbohydrate complexes. The limiting effect of l ignins on cell wall degradation increases with increasing content. How ever, their effect on degradation might also depend on qualitative fac tors such as lignin structure and polymer organisation in walls and ti ssues. When various grasses (normal and selected genotypes), or grasse s and legumes are compared, correlations between certain factors such as lignin uncondensed fraction, syringyl units or phenolic acids conte nts and cell wall degradation emerge but not clear causal relationship has been shown. Nonetheless, other structural characteristics, relate d to the alkali reactivity of lignins, seem to have a stronger influen ce on cell wall degradation. Phenylpropanoids seem to act mainly as a physical and (bio)chemical barrier to the action of the microbial enzy mes. In addition, their reactivity as phenolic compounds and their hyd rophobicity seem to play a role. Digestion is not limited only by phen olics. The factors that limit glycanolysis-the accessibility, crystall inity and capillary structure of cellulose and the branching of hemice lluloses-seem to have little or no effect on cell wall degradation in vivo. In contrast, other antiquality substances (tannins, cutin and si lica), plant antomy, environmental factors, factors modulating microbi al growth and animal physiology influence cell wall utilisation. Futur e research in this field should focus on the effects of phenolic struc ture and of cell wall and tissue organisation on carbohydrate degradat ion.