AN ESTIMATION OF THE TURGOR PRESSURE CHANGE AS ONE OF THE FACTORS OF GROWTH STRESS GENERATION IN CELL-WALLS - DIURNAL CHANGE OF TANGENTIAL STRAIN OF INNER BARK

Diurnal changes of the tangential strain of inner bark was measured an d evaluated as a factor in the generations of tensile growth stress in the longitudinal direction and compressive stress in the transverse d irection in cell walls. The expansion and contraction of living tissue were measured accurately by means of strain gages which were glued in the tangential direction on the inner barks of saplings of sugi (Cryp tomeria japonica D. Don) and kuromatsu (Pinus thunbergii Park.). Tange ntial strains of the inner bark varied strictly with light ('day' vers us 'night') regardless of the periodicity of the lighting. Tangential strain begins to decrease after 30 minutes with light, showing a minim um value after 90 minutes. It increases slowly in the dark and continu es to increase until the next period of light. The topping at the maxi mum decrease in tangential strain during the period of light induces a rapid increase in the strain. These results suggest that the tangenti al strain on the inner bark comes from the deformation of living cells in the phloem and cambium induced by turgor pressure changes due to d ifferences between the xylem water stress and the osmotic pressure of living cells. An estimation from the maximum strain change gave 16-88K Pa of turgor pressure change between daytime and nighttime. It is sugg ested that this pressure change generates an irreversible elongation o f newly-produced cells in the cambium to create some chemical bonds wh ich generate not only tensile growth stress in cellulose microfibrils but also compressive stress in the transverse direction together with lignification during cell maturation.