Alfalfa Stem Tissues: Cell Wall Deposition, Composition, and Degradability

maturation on stem quality may have the largest potential benefit. Declining cell wall degradability of alfalfa (Medicago sativa L.) The tools of molecular biology offer the potential of stems with maturation limits the nutritional value of alfalfa for ruminants. This study characterized changes in cell wall concentration, altering the development of alfalfa stems in very precise composition, and degradability by rumen microbes resulting from ways to improve forage quality. It has long been asalfalfa stem tissue proliferation and development during maturation. sumed that lignin limits cell wall degradability because The seventh internode from the shoot base of three alfalfa clones lignin concentration of stems increases during maturawas sampled after 12, 17, 21, 31, and 87 d of regrowth in 1996 and tion and degradability declines (Jung and Deetz, 1993). 21 and 31 d in 1997. Cross sections were examined by light microscopy Shifting the composition of lignin toward a lower syrinfor tissue development, and after 48-h in vitro degradation. Cell wall gyl:guaiacyl monolignol ratio is another possible route concentration and composition of the internodes were determined by to improving alfalfa stem quality because this type of the Uppsala dietary fiber method, and cell wall degradability by rumen lignin has been associated with more degradable cell microbes was measured after 12 and 96 h. All stem tissues were pectinrich and nonlignified at the two youngest maturities in 1996, except walls (Cherney, 1990). Therefore, modifying lignin biofor primary xylem vessels which had lignified and thickened walls, synthesis is a popular strategy for improving the cell and the internode was actively elongating. Primary xylem was the wall degradability of alfalfa and other forages (Bernard only tissue not degraded from immature stems. The 21-d-old inVailhe et al., 1996; Baucher et al., 1999). Another stratternodes had completed elongation and begun secondary xylem prolifegy for potentially improving the quality of alfalfa stems eration. Secondary xylem lignified immediately, and lignification of might be to increase pectin content of the cell wall primary phloem and pith parenchyma began when elongation ended. because this polysaccharide is rapidly and extensively As tissues lignified, their cell walls became undegradable. Maturation degraded in the rumen (Chesson and Monro, 1982; Hatincreased stem proportion consisting of undegradable secondary xyfield and Weimer, 1995). lem, and cell wall polysaccharide composition shifted from predominantly pectin toward cellulose. Degradability of pectin remained high While these cell wall traits may represent reasonable regardless of maturity stage, but cellulose and hemicellulose degradtargets for improving alfalfa stem quality, nontissueabilities declined as secondary xylem proliferated. Degradability of specific genetic transformations of lignin biosynthesis alfalfa stems would be improved if the amount of lignified secondary to reduce plant lignin content have generally resulted xylem was reduced. in agronomically nonviable plants (Piquemal et al., 1998; Tamagnone et al., 1998). This negative result is not surprising, given the diversity of functions (mechanical W alfalfa is a high-quality forage crop for support, water transport, disease resistance) ascribed ruminant livestock, digestibility of alfalfa declines to lignin (Higuchi, 1990). While almost every tissue in severely with maturity (Nordkvist and Aman, 1986; grasses will lignify to some extent (Engels and SchuurBuxton and Brasche, 1991). This decline in forage qualmans, 1992), lignification of alfalfa stems occurs in a ity results from the combination of two separate events limited number of tissues (Wilson, 1993; Vallet et al., during maturation. First, as legumes mature the leaf: 1996; Engels and Jung, 1998). Similarly, cell wall polystem ratio declines because stem material accumulates saccharide content of alfalfa stem tissues vary. There is at a faster rate than leaves, and lower leaves senesce and limited information concerning differences among fordie (Aman and Nordkvist, 1983; Nordkvist and Aman, age tissues in cell wall concentration, composition, and 1986). This is important to forage quality because stems development (Chesson et al., 1997; Grabber et al., 1991; contain more cell wall material than leaves, and cell Hatfield et al., 1999). Successful implementation of mowalls are less digestible by ruminants than are cell solulecular biology methods to improve alfalfa quality will ble components (Buxton and Brasche, 1991; Jung et require a more refined understanding of cell wall strucal., 1997). Secondly, as stems mature they accumulate ture and degradability for the diversity of tissues that higher concentrations of cell wall material and the decomprise the stem. gradability of these cell walls by ruminants decreases Our objectives were to describe the developmental (Buxton and Brasche, 1991). Improving the forage qualpattern of cell walls in the various tissues that comprise ity of alfalfa will require alterations in one or both of the alfalfa stem, characterize the shifts in cell wall conthese maturation events, but given the large contribucentration and composition resulting from differences tion of stems to alfalfa yield, changing the impact of in tissue proliferation and development during maturation, and explain the pattern of declining cell wall deH.G. Jung, USDA-ARS Plant Science Res. Unit and U.S. Dairy gradability associated with plant maturation based on Forage Res. Center Cluster, Dep. of Agronomy and Plant Genetics, differences in proliferation and development of stem 411 Borlaug Hall, 1991 Upper Buford Circle, Univ. of Minnesota, St. Paul, MN 55108; F.M. Engels, Dep. of Plant Sciences, P.O. Box 341, tissues. A single, specific internode of developing alfalfa Wageningen Univ., Wageningen, The Netherlands. Received 30 Apr. stems was chosen as a model for stem tissue develop2001. *Corresponding author ([email protected]). ment. Three alfalfa genotypes were included to provide variation in cell wall development and/or degradability. Published in Crop Sci. 42:524–534 (2002).