Alfalfa stem tissues: rate and extent of cell-wall thinning during ruminal degradation

Abstract

Alfalfa stem internodes of advanced maturity were used to examine the variability among tissues for rate and extent of cell-wall degradation by rumen microorganisms. Thin sections (100 μm) were incubated with rumen fluid in vitro for 0, 2, 4, 8, 16, 24, 48, 72, and 96 h. The degradation of tissue cell walls was evaluated for each fermentation time interval against a nondegraded mirror control section by light microscopy. Cell-wall thickness of alfalfa stem tissues was measured using scanning electron microscopy for both control and fermented sections. Rate and extent of cell-wall degradation were calculated from these measurements. Non-lignified epidermis, collenchyma, chlorenchyma, cambium and primary xylem parenchyma were rapidly and completely degraded within the first 8 h of fermentation. Rates of degradation ranged from 0.04 μm h −1 for thin-walled (0.29 μm) primary xylem parenchyma tissue to 0.11 μm h −1 for thick-walled (0.90 μm) collenchyma tissue. The non-lignified secondary wall (1.70 μm) of the primary phloem fibres required 24 h for complete degradation. Cell walls of some lignified tissues (e.g. pith parenchyma and secondary xylem fibres) were only partially degradable (9.1 to 65.5 %) even after 96 h of fermentation. The primary and secondary xylem vessels appeared to be completely nondegradable. The observed rates of cell-wall degradation for nonlignified alfalfa stem tissues were two to five times faster than previously estimated for nonlignified grass mesophyll tissue. However, extent of degradation for the lignified tissues of alfalfa stems was less than reported for lignified grass stem sclerenchyma. These differences in cell-wall degradation characteristics among tissues within alfalfa and compared to grasses are probably related to cell-wall lignification and polysaccharide composition of individual tissues.