CELL-WALL EXTENSION BEHAVIOR OF PHYCOMYCES SPORANGIOPHORES DURING THEPRESSURE RESPONSE

The cylindrical, single-celled sporangiophore of Phycomyces blakesleea nus grows (enlarges) predominantly in the longitudinal direction durin g two stages of development; stage I and stage IVb. Cell enlargement ( cell wall extension) occurs in a distinct region termed the ''growing zone.'' It was previously reported that a large step-up or pulse-up in turgor pressure, greater than approximately 0.02 MPa, will elicit a t ransient decrease in longitudinal growth rate of the stage I and stage IVb sporangiophore. This transient decrease in longitudinal growth ra te is termed the ''pressure response.'' Both the magnitude and duratio n of the pressure response depend on the magnitude of the turgor press ure step-up or pulse-up. Qualitatively, the pressure response is simil ar to the stretch response, which is produced with the application of a longitudinal force (load) on the sporangiophore. In this investigati on, the growth (extension) behavior of the cell wall in the growing zo ne is studied during the pressure response. It is found that both the extension rate of the cell wall in the growing zone and the length of the growing zone decrease during the pressure response, and that toget her they account for the observed decrease in longitudinal growth rate .