Chemistry of developing bordered-pit rims in balsam-fir trees

Rims of bordered pits form on the primary walls of radially enlarged cambial derivatives prior to the onset of general secondary-wall formation. A recent report (Botany, 2014, 92(7): 495-511) raised the possibility that the chemical composition of the rim might be different from that of the secondary wall. To investigate this, early-stage nonfluorescent and late-stage autofluorescent rims were separated from cambial derivatives of Abies balsamea (L.) Mill. and purified to homogeneity by density-gradient centrifugation. Solid state nuclear magnetic resonance spectroscopy, Raman microspectroscopy, combined gas chromatography - mass spectroscopy, enzyme digestion, and chemical resilience data support the interpretation that cellulose alone is the microfibrillar polysaccharide of nonfluorescent early-stage rims. A lignin is additionally present in late-stage rims, and it evidently bonds with cellulose because rims are extraordinarily resistant to hydrolysis by either enzymes or strong acid.