Inherent shortcomings in lignin analyses: a critical juncture and the urgent need

Although this chapter attempts to give justice later to what we now know about native lignin macromolecular configuration and modulation thereof, it must be robustly stressed that the analytical methodologies currently available for the study of polymeric lignins (structure, content, and composition) are woefully inadequate, as well as being unsatisfactory from any holistic quantification perspective. Indeed, such inherent shortcomings hold for all of the commonly used protocols — including the ones that we use, such as either NMR, thioaci — dolysis, alkaline nitrobenzene oxidation, or Klason analyses, to give just a few examples. For these reasons, it is important to appreciate and understand the very limited information that is being gained with all these approaches. As discussed below, often only a very small fraction (i. e., 20-40%) of the lignins can be accounted for in a chemically definitive and quantitative manner through degradative analyses.

Such massive deficits and/or discrepancies in the analytical procedures have, in one way or another, existed for nearly a century. For this reason, it is imperative that in the near future new approaches/technologies be urgently developed to determine, for example, the nature and frequency of all interunit linkages in lignin(s) — an essential requirement if we are to fully understand how lignins are, in fact, being formed in vivo.

While these limitations are, in large part, due to both the intractable and polymeric na­ture of native macromolecular lignins, there is yet another confounding feature. That is, with one exception (77), most of the research carried out on lignification (and its modu­lation) to date has not been directed toward any systematic and holistic determination of trends in the lignin-forming processes. Furthermore, there are increasing numbers of re­ports of lignin contents and compositions that are unreliable, and which presumably reflect a lack of scientific rigor and knowledge about lignin/cell-wall chemistries. We would plead that reliable/rigorous chemical and biochemical analyses must be an expectation for lignin investigations as they are for all other fields of (plant) chemistry/phytochemistry.

An additional complicating feature in the analysis of native lignin macromolecular con­figuration is that of the large variations in H, G, and S monomeric compositions and lignin contents between and within plant species. Quite remarkably, no comprehensive attempts to correlate such variability in lignin content/composition with cell wall anatomy have been reported. Yet, this would appear to be useful in terms of potentially identifying simpli — fying/unifying features in lignin macromolecular configuration, e. g., by correlating lignin amounts/composition with overall dimensions of the vascular apparatus and distinct cell types present in different plant lines, such as in the syringyl-rich fiber cell walls, for example.