6. Lignin

Abstract

Next to cellulose lignin is the most abundant and important polymeric organic substance in the plant world. The incorporation of lignin into the cell walls of plants gave them the chance to conquer the Earth's land surface. Lignin increased the mechanical strength properties to such an extent that huge plants such as trees with heights of even more than 100 m can remain upright.Lignin is a characteristic chemical and morphological component of the tissues of higher plants such as pterodophytes and spermatophytes (gymnosperms and angiosperms), where it typically occurs in the vascular tissues, specialized for liquid transport and mechanical strength (e.g. xylem, 2.1.). Primitive plants such as fungi, lichens and algae are not lignified, while it is still questionable whether mosses contain true lignin or only phenolic compounds which also yield a nonhydrolyzable residue during acid treatment (Freudenberg 1968;Sarkanen, Hergert 1971). More recent investigations on several mosses (e.g. Sphagnum magellanicum) indicate that they contain no lignin and that lignin occurrence is indeed restricted to vascular plants (Erickson, Miksche 1974;Miksche, Yasuda 1978;Nimz, Tutschek 1977).The amounts of lignin present in different plants is quite variable. While in wood species the lignin content ranges from 20 to 40% (->3.3.) aquatic and herbaceous angiosperms as well as many monocotyledons (e.g. horse-tail species) are less lignified (Neish 1968;Sarkanen, Hergert 1971;Krüger 1976).Additionally the distribution of lignin within the cell wall (-» 8.2.) and the lignin contents of different parts of a tree are not uniform. For example high lignin values are characteristic for the lowest, highest and inner parts of the stem, for softwood branches, bark (-H> 9.2.) and compression wood (-» 2.3.). The lignin contents of needles and leaves are described inconsistently as high or low, possibly depending on their state of development (Wardrop 1971;Uprichard 1971;Howard 1973;Miksche, Yasuda 1977;Luger, Gampe 1978).In most cases of wood utilization lignin is used as an integrated part of wood. Only in the case of pulping and bleaching (-» 16.) is lignin more ore less released from wood in degraded and altered forms, representing a large potential carbon source of more than 35 million tons of carbon per year world-wide for chemical and energy purposes (-» 18.6.).