Main content area

Carbohydrate composition of alfalfa cell walls isolated from stem sections differing in maturity

Hatfield, R.D.
Journal of agricultural and food chemistry 1992 v.40 no.3 pp. 424-430
Medicago sativa, stems, carbohydrates, biomass, polysaccharides, cell wall components
As plants mature, changes occur in the allocation of biomass between cell solubles and cell walls. Generally a larger proportion of the biomass accumulates in the cell-wall fraction. This study was undertaken to determine the cell-wall carbohydrate composition of alfalfa (Medicago sativa L.) stem sections of differing maturity. Stems were harvested from greenhouse-grown plants at bud stage and subdivided into apical, upper, and lower nodes and internodes. Cell walls isolated from each stem region were fractionated into pectic polysaccharides, alkaline-soluble polysaccharides, and cellulose. As expected, the proportion of pectic material in the cell wall decreased from apical to lower node regions. The extent of this decrease, approximately 50% in the lower regions, reflects the change in patterns of polysaccharide synthesis. Sugar composition of the pectic fractions did not change except in the lower regions, which contained higher proportions of xylose. For each stem region, 33% of the total pectic material was recalcitrant to calcium ion chelator extraction. This material is held in the cell-wall matrix by covalent linkages, some of which were alkaline labile. However, the majority of the recalcitrant pectic material was not solubilized until delignification of the wall. The ratio of alkaline-soluble polysaccharides to cellulose did not change with maturity of the stem section. The composition of the alkaline-soluble fractions changed with respect to stem regions. Apical nodes and internodes contained higher proportions of arabinose, galactose, and rhamnose residues. There was also a trend toward increased uronosyl residues in the lower stem regions within the alkaline-soluble fractions.