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The distribution of aluminum (Al) and other inorganic elements has been mapped at the cellular level in the mature xylem of maple by means of time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging. This method permits to image inorganic elements along with organic molecular species with high spatial resolution, and it was demonstrated that Na, K, Ca, and Mg are almost uniformly distributed in several contiguous growth rings of the air-dried wood sample. In contrast, Al was accumulated heterogeneously and intensely in the cell walls of the secondary xylem, including several growth rings, despite the long-term air-drying during storage. These results are interpreted that Al is immobilized within the cell wall, or it is closely interacted with the components of the secondary xylem. TOF-SIMS mapping also shows that Al has a tendency to be localized around vessels in the inner rings.

The experiments with model compounds revealed that time-of-flight secondary ion mass spectrometry (ToF-SIMS) is able to split the common interunit linkages of lignin, except the 5-5 linkage. In a previous study, ToF-SIMS produced characteristic secondary ions with m/z 137 and 151 (C6-C1 fragments) from the phenylcoumaran-type lignin model compound, the benzofuran ring of which has a β-5 linkage and an α-O-4 linkage. However, it is still unclear whether the fragments are from ring A with the free phenolic OH as a result of the sole cleavage of the α-O-4 link by opening the furan ring or from ring B as a result of the β-5 cleavage. In this study, the phenylcoumaran-type lignin model compound and its deuterium- and/or 13C-labeled analogues were synthesized and analyzed by ToF-SIMS. It could be clarified that the β-5 linkage is not cleaved by ToF-SIMS.

The structure of lignin in the compound middle lamella (CML) of softwood tracheids differs from that in the secondary wall (SW) in regard to the content of condensed structures (5-5′-biphenyl, dibenzodioxocin and 4-O-5′-diphenyl ether). In an early stage of cell wall formation, random coarse networks composed of thin cellulose microfibrils (CMFs), hemicelluloses, and pectin are formed in the CML, then globular p-hydroxyphenyl/guaiacyl lignin (HG-lignin) is deposited quickly into the network. The globular lignin is assumed to be a micellar aggregate of oligolignols folded at the β-O-4 bond with their phenolic ends on the outer part of the aggregate. When 3D clusters of the globules are deposited on the preformed network of polysaccharides, further growth of the oligolignols by endwise addition of new monolignols is spatially limited, so frequent condensation occurs between growing aromatic ends of adjacent HG-oligolignols within the globule and between the wide contact boundaries of the 3D clustered globules to produce a highly condensed supramolecule in CML. In SW, the folded G-oligolignols are deposited slowly in the narrow tubular space surrounding thick CMFs coated with hemicelluloses. Condensation occurs mostly between adjacent growing ends of the oligolignols within the tubular aggregates. Spatial regulation of condensation of folded polylignols is one of the factors producing a different supramolecular structure for CML lignin than for SW lignin.

The distribution of ethyl acetate extracts and 4,8-dihydroxy-5-methoxy-2-naphthaldehyde (compound I), which is a major constituent of the extracts obtained from the blackened heartwood of Diospyros kaki, was analyzed via gas chromatography-mass spectrometry (GC-MS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). According to GC-MS, the extracts and compound I are high in concentration at the pith and at the edges of the blackened heartwood. ToF-SIMS analysis revealed a peak at a mass-to-charge ratio of (m/z) 218, which is characteristic of the ionic form of compound I. The ToF-SIMS imaging of compound I in the blackened heartwood based on m/z 218 shows that compound I is located in parenchyma cells and their neighboring axial elements.

To examine the possible in vivo conversion of coniferin to syringin in the biosynthetic pathway of lignin, variously radio-labeled coniferin was fed to a shoot of Magnolia kobus. When coniferin-[side chain γ- 3 H, Glc-U- 14 C] was administered, only 3 H-activity was detected in syringin isolated from the shoot. The lack of 14 C-activity in isolated syringin suggests that the conversion from guaiacyl to syringyl proceeded at the stage of monolignols, and the glucose moiety is temporally removed during the process of methoxylation. These results suggest the possible role of monolignols in the control of monomer composition of lignin.