ABSTRACT
Figure 4.1 Position of lignin inside vascular plants.However, in its native state or as found in plants, it is important to the life of vascular plants (Chakar and Ragauskas, 2004). In this regard, lignin is a main constituent of the cell walls of vascular plants, especially in woody tissues where it is present in high amounts, it acts as a binding agent that provides compressive strength and bending stiffness (Holmgren et al., 2009). The lignin is an amorphous phenolic complex that is the second in abundance after cellulose; it accounts for approximately 30% of aromatic biopolymers that represent in the plant biomass (Lapierre et al., 2000). Besides that, lignin plays a crucial part of several properties that make this biopolymer relatively unique in nature. Within this context, the functional significance of lignin has been until now mainly associated with mechanical support of plant organs, improved sap conduction through lignified vascular elements, efficient water transport between cell walls, and the resistance of thermal degradation and fungi attack by defense mechanisms (Boudet, 2000). Other prospective functions have also been investigated, such as the control of cell wall extension via the lignifications of the primary cell walls, which are in a dynamic state of extension and concomitant apposition of new wall layers (Müse et al., 1997).
