CELL-WALL FRACTIONATION OF ALFALFA STEM IN RELATION TO INTERNODE DEVELOPMENT - BIOCHEMISTRY ASPECT

In addition to morphological factors (height of the plant, proportion of stems), alfalfa quality is related to several biochemical factors, especially the content and monomeric composition of the cell wall comp onents. This work was aimed at studying internode development in relat ion to the chemical composition and fractionation of cell walls. Analy ses were performed on the fourth apical internodes (elongating), the t wo following ones (onset of cambial activity), and bottom internodes ( mature). Deposition of guaiacyl type lignin occurred in the pectin ric h cell walls of apical internodes. The onset of cambial activity corre sponded to cell wall accumulation of syringyl-guaiacyl lignin, xylans and/or glucuronoxylans, and cellulose. Such events were related to the thickening of secondary cell walls which proportion increased in matu re internodes. Chemical fractionation of the cell walls allowed the re lease of a high content of water soluble pectins from apical internode s, whereas alkalis were more efficient in extracting heteroxylans from bottom internodes. A partial release of lignin from the cell walls al so occurred during the extraction steps. Lignin solubilization was enh anced by the cleavage of labile-ether lignin structures and appeared c losely related to the extent of internode differentiation along the st em. Data gained from chemical fractionations evidenced that lignin-xyl an-pectic complexes accumulated progressively in secondary wall rich t issues.