A comparison of cell-wall-yielding properties for two developmental stages of Phycomyces sporangiophores : Determination by in-vivo creep experiments.

The yielding properties of the cell wall, irreversible wall extensibility (m) and yield threshold (Y), are determined for stage I sporangiophores of Phycomyces blakesleeanus from in-vivo creep experiments, and compared to the values of m and Y previously determined for stage IVb sporangiophores using the same pressureprobe method (Ortega et al., 1989, Biophys. J. 56, 465). In either stage the sporangiophore enlarges (grows) predominately in length, in a specific region termed the "growing zone", but the growth rates of stage I (5-20 urn · min(-1)) are smaller than those of stage IVb (30-70 μm · min(-1)). The results demonstrate that this difference in growth rate is the consequence of a smaller magnitude of m for stage I sporangiophores; the obtained values of P (turgor pressure), Y, and P-Y (effective turgor for irreversible wall extension) for stage I sporangiophores are slightly larger than those of stage IVb sporangiophores. Also, it is shown that the magnitude of m for the stage I sporangiophore is regulated by altering the length of the growing zone, Lg. A relationship between m and Lg is obtained which can account for the difference between values of m determined for stage I and stage IVb sporangiophores. Finally, it is shown that similar changes in the magnitude of m and ϕ (which have been used interchangeably in the literature as a measure of irreversible wall extensibility) may not always represent the same changes in the cell-wall properties.