Alfalfa Stem Tissues: Cell-wall Development and Lignification

Alfalfa stems contain a variety of tissues with different patterns of cell-wall development. Development of alfalfa cell walls was investigated after histochemical staining and with polarized light using light microscopy and scanning electron microscopy. Samples of the seventh internode, from the base of stems grown on cut stems, were harvested at five defined stages of development from early internode elongation through to late maturity. Internode seven was elongating up to the third sample harvest and internode diameter increased throughout the entire sampling period. Chlorenchyma, cambium, secondary phloem, primary xylem parenchyma and pith parenchyma stem tissues all had thin primary cell walls. Pith parenchyma underwent a small amount of cell-wall thickening and lignification during maturation. Collenchyma and primary phloem tissues developed partially thickened primary walls. In contrast to a recent report, the formation of a ring shaped, lignified portion of the primary wall in a number of cells in the exterior part of the primary phloem was found to precede the deposition of a thick, non-lignified secondary wall which was degradable by rumen microbes. In numerous xylem fibres from the fourth harvest date onwards, an additional highly degradable secondary wall layer was deposited against a previously deposited lignified and undegradable secondary wall. The pattern of lignification observed in alfalfa stem tissues suggests that polymerization of monolignols by peroxidases at the luminal border of the primary cell wall creates an impermeable zone which restricts lignification of the middle lamella region of tissues with thick primary walls.