The degree of recalcitrance of cell wall polysaccharides to depolymerization by enzymes, or other hydrolytic reagents, depends on the ability of the enzymes to access their substrates in the walls. This is manifest at two levels. The first relates to the surface area ofthe wall exposed to the hydrolytic agent: the greater the extent of comminution of the feedstock, the greater the surface area exposed as the cellular organization of the plant material is disrupted (99). The second level of constraint is manifest in the organization of the polysaccharides in the
cell walls and in particular to the extent that lignin deposition and its covalent cross-linking to other wall polymers limits enzymatic degradation as discussed in Sections 4.3 & 4.4.

Information is available from studies on forages, particularly grasses and legumes, for ruminant animals on the relative degradabilities (digestibilities) of the walls of different cell types by mixtures of fungal enzymes and by rumen fluid (5, 170-174). In general, non — lignified walls are highly degradable, whereas lignified walls are much less degradable. For example, many of the parenchyma cells in stems and leaves, including mesophyll cells, have thin, non-lignified, primary walls that are highly degradable. Even moderate amounts of diferulate cross-linking between GAXs in non-lignified grass walls apparently do not im­pede wall degradation (175). Thus, an approximate index of the suitability of a plant as a feedstock would be the relative proportion of cells in the vegetative tissue of the mature plant that have lignified walls. The relative proportions of different cell types in a range of C3 and C4 grass species have been reviewed by Buxton and Redfearn (172). For exam­ple, in leaf blades of switchgrass (Panicum virgatum), a warm-season C4 species, 28.9% of the cross-section area is occupied by bundle sheath cells that have moderately thick and weakly lignified walls, 35.7% by mesophyll cells with thin, non-lignified walls, and 2.9% by sclerenchyma fibers with thick, lignified walls (176). Furthermore, there maybe important genetic variations in the relative proportions of the different cell types among various clones of the same species (177) that may explain variations among the clones in their total cell wall degradabilities by polysaccharide degrading enzymes, e. g., in smooth bromegrass (Bromus inermis), a cool-season C3 species (177). The degradability of the cell walls of internodes from 100 lines of the temperate grass (Phalaris aquatica) using fungal enzymes was found to be markedly dependent on the line ranging from 22 to 38% (178).In investigations of the genetic background to digestibility of smooth bromegrass genotypes using fungal enzymes (179, 180), it was found that in high digestibility selections, variation in digestibility was due largely to variation in lignin concentration, whereas in low digestibility selections the influence of lignin concentration was dramatically reduced and esterified FA concentration showed a strong negative relationship with lignin concentration and appeared to be more associated with cell wall digestibility than was lignin.

During the development of forage plant organs, such as flowering stems, the overall degradability of the walls falls because some of the cell types develop lignified secondary walls (181, 182). Although this occurs in most sclerenchyma fibers and xylem tracheary elements, it occurs in only some of the parenchyma cells. Environmental conditions during growth such as temperature extremes, water deficit, nutrient stress, and shade also impact on cell wall composition and digestibility (183).

Interestingly, there are also differences between grasses and legumes in the degradabilities of their lignified walls. For example in legume stems, the lignified walls of the sclerenchyma fibers in the xylem are not degraded by rumen fluid, but in grass stems, the lignified walls of sclerenchyma fibers and parenchyma cells ingrasses show significant degradability (5). However, this degradation affects only the secondary walls and not the underlying more heavily lignified primary wall and middle lamella. The cleavage by feruloyl esterases (184) of ester linkages between heteroxylans and ferulic acid linked to lignin, which occur in walls of grasses but not legumes, maybe responsible for these differences.

Studies using rumen fluid and mixtures offungal enzymes have also shown that suberized layers and cuticles on cell walls in the periderm ofwoody species and epidermal cells of grasses are also barriers to the access of polysaccharide hydrolases to their substrates in the wall and thus hinder their depolymerization (5, 170).