Theoretical and actual biofuel yields from switchgrass rely on the composition of the starting material, limiting the amount of carbon available for conversion to fuel. Figure 1 summarizes the average lignicellulose content from a recent detailed analysis of genetically diverse, geographically dispersed, primarily mature switchgrass samples (Vogel et al. 2010). Dry biomass from switchgrass consists mostly of cell wall residue (69 ± 6%). On average, most of the remainder is water (9 ± 1%, even with active drying), silica and other minerals (8 ± 2%), proteins (6 ± 3%), and nonstructural sugars (5.5 ± 2.6%), mostly sucrose (Vogel et al. 2010). Sugar polymers make up 75% of the switchgrass cell wall material and on average are composed half of cellulose and half of matrix polysaccharides, which include pectins and hemicelluloses (Fig. 1). Cellulose is a polymer of the 6-carbon sugar glucose; whereas, about 85% of switchgrass matrix polysaccharide consists

of the 5-carbon sugars, xylose and arabinose. The remaining 25% of the mature switchgrass cell wall consists of lignin and other phenylpropanoid — derived constituents.

Actual biofuel yields are dependent not only on the crude content, but also the structure of cell walls. Current models envision that the walls of a growing plant are composed of a network of cellulose microfibrils that are crosslinked to each other by matrix polysaccharides (Cosgrove 1999). While cellulose structure appears to be largely conserved across higher plants, matrix polysaccharides and lignin differ between grasses and other relatively recently evolved Commelinid monocotyledonous species and dicotyledenous species, such as the reference Arabidopsis thaliana and the bioenergy tree, poplar. The similarities and differences between dicots and grasses are especially relevant when researchers consider rational strategies to engineer or select for switchgrass with improved cell wall quality. While functional information gleaned from genetic studies will likely be transferable from dicots in many cases, due to their different compositions we expect some differences in strategies for optimizing grass and dicot walls, as is elaborated below.

Plant cell wall content is intimately connected with the functions of the cells they surround and the age of the tissues in which they reside. Primary cell walls surround every vegetative cell, dictate shape, and control growth (Wolf et al. 2012). Upon differentiation and growth cessation, many cell types develop additional layers of thicker cell wall material. Developmental progression of grass leaves and stems proceeds both temporally and spatially away from the meristems, which are found at the base of each internode and each leaf. Over time, each internode and associated leaf develops and elongates in sequence from the base of the plant upward. Thus the lower leaves and stems are older and characterized by greater secondary growth compared to upper segments (Sarath et al. 2007). Vascular and schlerenchyma cells, in particular, are characterized by secondary wall formation (Sarath et al. 2007; Shen et al. 2009). Indeed, switchgrass cell wall quality (for forage traits) is well predicted by growing degree days (Mitchell et al. 2001), which is a good indicator of morphological development in perennial grasses (Mitchell et al. 1997).