Degradability of cell-wall polysaccharides in maize internodes during stalk development

Limitations to degradation of forage cell-wall constituents in maturing plant tissue are not fully understood, which limits progress in developing cultivars with improved digestibility. We determined in vitro degradability of cell-wall components by rumen microorganisms in developing internodes of maize (Zea mays L.). Plants were grown in the growth chamber and harvested at the 15th-leaf stage of development. Individual internodes were divided in half and separated into rind and pith fractions. Cell-wall degradability was measured on the internode samples and correlated with cell-wall degradability. Degradability of all cell-wall polysaccharide components, except galactose and mannose, declined with maturation of maize internodes. Degradabilities of glucose and xylose residues exhibited large differences between 24- and 96-h fermentation intervals (31 to 58% additional degradation in 72 h), whereas the arabinose and uronic acids potentially degradable in 96 h were already almost completely degraded in 24 h. Maize rind, consisting of epidermal and sclerenchyma tissues and many vascular bundles, was less degradable than the pith (736 vs. 811 g kg(-1), respectively), which had thinner-walled parenchyma tissue and fewer vascular bundles, Lower portions of internodes were younger and more degradable than the upper halves (805 vs, 743 g kg(-1), respectively). Ferulate ether concentration was negatively correlated with cell-wall polysaccharide degradability in young, elongating internodes but not for internodes in which elongation had ceased to occur. This pattern is in agreement with the hypothesis that ferulate cross-linking limits cell-wall degradation, but that the effect is only observable in young tissues before dilution of ferulate ether concentrations by secondary wall deposition of lignin and polysaccharides obscures the relationship.