Summary

Field emission scanning electron microscopy and energy-dispersive X-ray microanalysis were used to investigate the generation of growth stress in connection with the deposition of cell wall materials along the innermost surface of radial walls during secondary wall formation of differentiating tracheids of Cryptomeria japonica. Samples were collected when the turgor pressure of the tree was low (16:00 h), and when the pressure was high (6:00 h). In samples collected at 16:00 h, cellulose microfibrils deposited on the innermost surface of radial walls were clearly evident and amorphous material was rarely found. Conversely, in samples collected at 6:00 h, cellulose microfibrils were rarely observed and the amorphous material was prevalent. Cellulose microfibrils were evident, however, after removing the amorphous material with chlorite. The concentration of potassium on the inner surface of radial walls was greater at 6:00 h than at 16:00 h. After chlorite treatment, the concentration of potassium measured in the samples collected at 6:00 h decreased to a level equivalent to that in samples collected at 16:00 h and thus potassium was found to be associated with the amorphous material on the cellulose microfibrils. The amorphous material is probably a matrix of hemicellulose and monolignol, material that is abundant on the inner surface of developing cell walls, especially during expansion (as a result of high turgor pressure) of differentiating tracheids. A matrix of hemicellulose and lignin deposited in the expanded gaps between cellulose microfibrils and daily expansion of the gaps probably produces an isotropic compressive stress (growth stress). This paper demonstrates periodicity in cell wall formation as a result of the cycles of compressive growth in the differentiating cell wall.