Legume pectic substances and their degradation in the ovine rumen

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Abstract
Red clover and lucerne samples, and their leaf + petiole and stem fractions were sequentially extracted with cold water, chloroform‐methanol, boiling water and boiling 5 g litre−1 ammonium oxalate and the composition of the cold water, hot water and oxalate extracts and the extracted residues determined. Hot‐water‐soluble galacturonan was found to be more highly acetylated and esterified and to be associated with approximately ten times more neutral sugar residues than oxalate‐soluble galacturonan. The major neutral sugars associated with both fractions were arabinose, galactose and rhamnose. Hot‐water‐soluble uronide from stems was also more highly acetylated and associated with more neutral sugar than the corresponding extract of leaf tissue. Despite differences in chemical composition, hot‐water and oxalate‐soluble galacturonan were lost at approximately the same rate from whole plants, and from stem and leaf fractions incubated in nylon bags in the rumen of a sheep. The maximum extent of degradation was reached within 18 h, thereafter little additional material was lost. Hot‐water‐soluble galacturonan, particularly from clover, was degraded to a slightly lesser extent (85–95%) than oxalate‐soluble galacturonan (95–98%). This difference did not relate to an accumulation of acetylated or esterified residues, nor to an accumulation of neutral sugar residues. Loss of associated neutral sugars paralleled the loss of uronic acid from all extracts. Hot‐water and oxalate‐soluble galacturonans were lost at faster rates than those shown by the other structural polysaccharides of the cell wall.