Structure and chemical composition of endodermal and rhizodermal/hypodermal walls of several species

CWM, isolated cell wall material ECW, isolated endodermal cell walls G, guaiacyl monomer H, p-hydroxyphenyl monomer HCW, isolated hypodermal cell walls RHCW, isolated rhizodermal and hypodermal cell walls S, syringyl monomer XV, isolated xylem vessels Endodermal cell walls of the three dicotyledoneous species Pisum sativum L., Cicer arietinum L. and Ricinus communis L. were isolated enzymatically and analysed for the occurrence of the biopolymers lignin and suberin. From P. sativum, endodermal cell walls in their primary state of development (Casparian strips) were isolated. Related to the dry weight, these isolates contained equal amounts of suberin (2·5%) and lignin (2·7%). In contrast, the endodermal cell walls of C. arietinum and R. communis, which were nearly exclusively in their secondary state of development, contained significantly higher proportions of suberin (10-20%) and only traces of lignin (1-2%). The results of the chemical analyses were supported by a microscopic investigation of Sudan III-stained root cross-sections, showing a Casparian strip restricted to the radial walls of the endodermis of P. sativum and well-pronounced red suberin lamellae in C. arietinum and R. communis roots. Compared with recently investigated monocotyledoneous species, higher amounts of suberin by one order of magnitude were detected with the secondary state of development of dicotyledoneous species. Furthermore, the carbohydrate and protein contents of primary (Clivia miniata Reg. and Monstera deliciosa Liebm.), secondary (C. arietinum and R. communis) and tertiary endodermal cell walls (Allium cepa L. and Iris germanica L.) were determined. The relative carbohydrate content of secondary endodermal cell walls was low (14-20%) compared with the content of primary (42-50%) and tertiary endodermal cell walls (60%), whereas the protein content of isolated endodermal cell walls was high in primary (13%) and secondary (8%) and low in tertiary endodermal cell walls (0·9-2%). The results presented here indicate that the quantitative chemical composition of primary, secondary, and tertiary endodermal cell walls varies significantly. Finally, cell wall proteins are described as an additional important constituent of endodermal cell walls, with the highest concentrations occurring in primary (Casparian strips) and secondary endodermal cell walls.