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.
A pressure probe method (pressure clamp) was developed to measure transpiration rates of both growing and nongrowing single plant cells, and represents an improvement over the previous pressure probe method (pressure relaxation), which is restricted to nongrowing plant cells (J.K.E. Ortega, R.G. Keanini, K.J. Manica [1988] Transpiration is an important process that has an impact on water transport and growth of plant cells. Transpiration is especially important to the sporangiophores of Phycomyces blakesleeanus, since they can transpire up to 85% of their water uptake (1). In addition, it has been suggested that transpiration rates mediate some of the sporangiophores' sensory responses (2). Therefore, it is somewhat surprising that relatively few investigations have been conducted to measure transpiration rates and to study the transpiration behavior of these sporangiophores (1, 2, 4, 9). Previously, a pressure probe method (pressure relaxation) was developed to measure transpiration rates of nongrowing single plant cells, and the method was demonstrated on nongrowing sporangiophores (stage III) of Phycomyces (9). The 'augmented growth equations' provide the theoretical foundation for this method (7-9). In general, the pressure relaxation method (9) requires that the nongrowing plant cell be removed from its water source, and that the resulting turgor pressure decay due to transpiration be measured with the pressure probe. Ortega et al. (9) -dP/dt = eT.(1) modulus, e, and the relative transpiration rate, T. It is important to note that the pressure relaxation method requires that the magnitude of e be known.In the present article, we introduce another pressure probe method (pressure clamp) to measure the relative transpiration rate and the transpiration rate of single plant cells. This method has advantages over the pressure relaxation method in that the plant cell may be growing or nongrowing, and e is not needed in the determination of either the relative transpiration rate or the transpiration rate. The pressure clamp method is demonstrated on the sporangiophores of P. blakesleeanus for both growing (stage IV) and nongrowing (stage III) cells. Relative transpiration rates and transpiration rates determined for both stages of development are presented and compared. THEORYThe second augmented growth equation (9) provides the theoretical foundation for the pressure clamp method:where V is the cell volume, L is the relative hydraulic conductance, a is the solute reflection coefficient, Air is the osmotic pressure difference, and P is the turgor pressure. Equation 2 states that in a single plant cell, the net relative rate of water uptake, (dV/dt)/V, is equal to the difference between the relative rate of water uptake, L(oA7r -P), and the relative transpiration rate, T. When plant cell enlargement is due primarily to net water uptake, the net relative rate of water uptake, (dV/dt)/V, is equal to the relative volumetric
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