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

Alfalfa stem internodes of advanced maturity were used to examine the varia bility among tissues for rate and extent of cell-wall degradation by rumen microorganisms, Thin sections (100 mum) were incubated with rumen fluid in vitro for 0, 2, 4 8, 16, 24, 48, 72, and 96 h. The degradation of tissue ce ll walls was evaluated for each fermentation time interval against a nondeg raded mirror control section by light microscopy. Cell-wall thickness of al falfa stem tissues was measured using scanning electron microscopy for both control and fermented sections. Rate and extent of cell-wall degradation w ere calculated from these measurements. Non-lignified epidermis, collenchym a, chlorenchyma, cambium and primary xylem parenchyma were rapidly and comp letely degraded within the first 8 h of fermentation. Rates of degradation ranged from 0.04 mum h(-1) for thin-walled (0.29 mum) primary xylem parench yma tissue to 0. 11 mum h(-1) for thick-walled (0.90 mum) collenchyma tissu e. The non-lignified secondary wall (1.70 mum) of the primary phloem fibres required 24 h for complete degradation. Cell walls of some lignified tissu es (e.g. pith parenchyma and secondary xylem fibres) were only partially de gradable (9.1 to 65.5%) even after 96 h of fermentation. The primary and se condary 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 m esophyll tissue, However, extent of degradation for the lignified tissues o f alfalfa stems was less than reported for lignified grass stem sclerenchym a. These differences in cell-wall degradation characteristics among tissues within alfalfa and compared to grasses are probably related to cell-wall l ignification and polysaccharide composition of individual tissues.