Cell-wall components and lignin biosynthesis in forages
Lignin is the major cell-wall (CW) component that lowers forage digestibility. Negative relationships between lignin and forage quality are caused by interactions and variability of lignin with other CW components. This study was conducted to better understand how lignin content, concentration, deposition, and change in concentration vary with other CW components in maturing stems and to provide preliminary information about phenylpropanoid metabolism in relation to lignification;Regrowth from basal stem tissue was sampled weekly or biweekly from greenhouse-grown forage grass and legume species. Samples were collected for a period of about 8 weeks and analyzed for fiber concentration and content, protein concentration, and phenylalanine ammonia lyase (PAL) activity;On a per pot basis, the sequence of CW component deposition was hemicellulose and cellulose followed by lignin. Graphical representations demonstrated that there was species- and family-specific timing and extent of CW component deposition. In most instances, CW and CW component concentrations increased with regrowth days. Fast increases were followed by a leveling-off of CW concentrations, reflecting rapid CW concentration changes that decreased as tissues matured. Cell wall, cellulose, and lignin concentration changes decreased faster in legumes compared with grasses. Negative changes in hemicellulose concentration implied a dynamic nature of the CW and provided evidence that hemicellulose is diluted by other CW components as tissues mature;In all species, decreasing lignin concentration changes showed a parallel relationship to decreasing PAL activity. Close relationships between PAL activity and changes in lignin concentration were consistent within grass and legume species. On a per pot basis, lignin deposition resembled cumulative PAL total units. These results imply that both the activity and amount of PAL are closely related to change in lignin concentration and lignin deposition in forage tissues.