Effect of light intensity on diurnal differences in the supply of cell wall components to the innermost surface of developing S2 layers of tracheids in Cryptomeria japonica

Abstract

We investigated the effect of light intensity on diurnal differences in secondary wall formation of tracheids. Saplings of Cryptomeria japonica were grown in growth chambers with light intensity cycles set for 12-h high light: 12-h low light by combining two of four light intensity levels: 1.5, 2.8, 4.3, and 10.0 klx. Volumetric changes of differentiating cells were monitored by measuring the tangential strain on the inner bark surface, and the innermost surface of developing secondary walls of differentiating tracheids during the high-light and low-light periods was observed by field-emission scanning electron microscopy. Changes in the aspects of the innermost surface of developing secondary walls and the tangential strain corresponded to changes in the light intensity level. Cellulose microfibrils were clearly observed when the light intensity was high (10.0 or 4.3 klx) and the volume of differentiating cells was low, while abundant amorphous material was observed when the light intensity was lowest (1.5 klx) and the cells were turgid, regardless of the light intensity cycle. These results suggest that the diurnal periodicity in the supply of cell wall components to developing secondary walls is associated with changes in light intensity during the photoperiodic cycle.