The following experiments were performed in an attempt to elucidate any possible changes in cell wall extensibility that may occur as a result of a saturating light stimulus. In each experiment, the same individual sporangiophore was used to ensure measurement of real changes within any given sporangiophore and not some random variation that may occur between sporangiophores. MIATERIALS AND METHODSThe sporangiophores were grown in glass shell vials containing 4.0%0 potato dextrose agar with 1.0%-yeast extract under diffuse incandescent light. The sporangiophores were grown and tested at 22 C. Tensile loads were applied with an Instron Model T.M. tension-compression machine equipped with a Model A load cell set at high sensitivity (2 g full scale) to stage IVb sporangiophores at a constant displacement rate of 2.54 mmj"min. These rates ensured that any measured changes between a light-stimulated and dark-adapted state (dark-adapted in red light for 40 min) could not be explained as an artifact of increased growth rates. Before each tensile test the sporangiophore was dark-adapted for at least 40 min in a box (15 cm X 15 cm X 13 cm) constructed of transparent red Plexiglas. This box contained a light bulb (Sylvania, 25 w, 120 v, soft white light) which could be turned off and on from outside the box. All stimuli were pulse-up stimuli of 10-sec or 2-min duration. Tensile loads were transmitted to the sporangiophore using a 0.2-mm Nichrome wire hook which passed first through a small hole in the Plexiglas box and was then attached directly under the sporangium (Fig. 1).The data for determining extension as a function of tensile load were obtained using two separate methods. First, the imposed displacement was calculated from the product of the machine extension rate and the time required to reach a particular tensile load. This method does not measure the actual extension of the sporangiophore since part of the imposed displacement is used to deform elastically the loading hook, but this method does measure actual changes in the extension of the sporangiophore since the extension of the loading hook is a linear function of load. In the second method, the actual extension was measured from photographs of the sporangiophore with starch markers on the growth zone. The photographs were taken with a 35-mm Nikon FTN single lens reflex camera equipped with bellows ex-333 www.plantphysiol.org on May 10, 2018 -Published by Downloaded from
Phycomyces sporangiophores respond to four distinct physical stimuli: gravity, light, stretch, and an avoidance stimulus. Saturating the organism with a light stimulus so that it does not respond to any additional light program does not decrease its ability to respond to an avoidance stimulus. This demonstrates that the organism has the potential to respond after a saturating light stimulus and that the an avoidance stimulus acts at some point past or parellel or parellel to the light-receiving mechanism.
The mechanical behavior of the sporangiophore of Phycomyces has been studied using a variety of loading methods: an internal hydrostatic pressure (9), combined gravitational and centrifugal forces (3), and a constant axial load (4). These investigations showed that at small loads (less than 20 mg), the cells deformed elastically; that is, the amount of extension upon loading was equal to the amount of contraction upon unloading after correcting for growth (4), and the extensibility of the cell was found to decrease uniformly below the sporangium (3, 4, 9).During normal development of the sporangiophore, both the distribution and geometry of growth vary as a function of the stage of development (2, 5). The recorded photographically using a 35-mm Nikon FTN single lens reflex camera equipped with a bellows extension, 50 mm f 2.0 lens and strobe light. The load-extension behavior of the sporangiophore was determined through the growth zone by measuring the position of pairs of starch markers on the photographs at 25X magnification and correlating with the loadtime record. The fractional extension or engineering strain in each segment below the sporangium was obtained by measuring the change in marker spacing and dividing this by the initial marker separation. The results are reported in terms of axial load versus fractional change in length during loading and unloading. The total length of the sporangiophores studied ranged from 4 to 6 mm in stage II and from 6 to 10 mm in stage IV. A typical test required a total time of 6 min for loading and unloading at 0.254 mm/min crosshead speed and approximately 1 min for loading and unloading at 2.54 mm/ min crosshead speed.The load versus extension of a typical stage IV Phycomyces is shown in Figure la; the load increases uniformly as the cell wall extends, and upon unloading the cell wall behaves quite nonelastically. For convenience, the experiments reported here were obtained at pulling speeds of 0.254 mm/min, which is about 5X its maximal growth rate. The same results were obtained from a pulling speed of 2.54 mm/min, thus insuring that our nonelasticity is not an artifact occurring because of growth. Rotation of the sporangium or stalk was not observed during loading or unloading, although striction in the loading hook could have masked the observation of rotations.From the data already given one can readily calculate the spatial distribution of extensibility of the sporangiophore since it is a direct function of the slopes of the solid lines given in Figure la. The extensibility calculated in this manner is in full agreement with the results of Roelofsen (9) in which he used the "iron lung" procedure to measure extensibility. Figure lb shows a load versus extension plot of a typical stage II sporangiophore. Note that its mechanical behavior is very similar to the nongrowing stalk region of stage IV.The reported elastic extension at low loads (4) followed by permanent nonelastic deformation at higher loads is typical of the viscoelastic behavior exhibited by many pol...
Phycomyces sporangiophores grow away from stationary objects, a phenomenon known as the avoidance response. Evidence is presented suggesting that a growth-stimulating gas is emitted from the sporangiophore and is then swept to the leeward side by air currents resulting in higher gas concentration on that side. The presence of a stationary barrier decreases the passive movement of the gas away from the leeward side. It is proposed that an increase of this gas on one side causes that side to grow faster. Indirect evidence suggests that the gas is water vapor.
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