Stress relaxation property of the cell wall and auxin-induced cell elongation

A stress-relaxation method has been developed to measure the mechanical property of the plant cell wall, as a physically defined terms. In the method, the stress relaxation property of the cell wall is simulated with a Maxwell viscoelastic model whose character is represented by four parameters; the minimum relaxation time, To, the relaxation rate, b, the maximum relaxation time, Tm and the residual stress, c. Thus, the mechanical property of the cell wall is represented by the four parameters. Physical and physiological meanings of the parameters are discussed. Auxin effects on the parameters were also studied. The cell elongation is simply thought to be extension of the cell wall under a force. The extension of the cell wall can be simulated by the mechanical property of the cell wall. However, the calculated extension was found to be incomparable to the real cell growth, indicating that there has to be other factors limiting the rate of cell growth. Major factors governing cell growth are discussed to be the cell wall mechanical property, the osmotic potential and water movement in the apoplast. A possibility to predict cell expansion with the three factors was discussed and a novel equation representing cell growth was obtained: $$1/R = 1/R_w + 1/R_p $$ whereR is the rate of cell elongation,R _w is the rate of cell wall extension due to the osmotic pressure andR _p is the rate of cell elongation determined by water conductivity.