Phycomyces

Abstract

ABSTRACIThe mechanical extensibilities of stage IVb Phycomyces were measured before and after a humidified wind stimulus. We find that when the humidity of the wind is greater than that of the ambient air, there is an increase in the mechanical extensibility of the cell wall. We also find that a step decrease in wind humidity results in a decrease in the mechanical extensibility of the cell wall.The avoidance response of Phycomyces was first discovered by Elfving (3) in 1881, and in recent years has been well documented (1, 2, 6). The avoidance response occurs when a sporangiophore is placed within several mm of a solid barrier. The response is observed when the sporangiophore grows away from the barrier. This cell wall bending occurs because the portion of the cell wall nearest to the barrier grows faster than the portion farthest away from the barrier. If a sporangiophore is enclosed by a double barrier, a barrier enclosing both sides of the cell, no bending occurs, but a transient increase in total cell wall growth rate is observed; this phenomenon is known as the avoidance growth response (7). Although the precise mechanism of the avoidance response remains unknown, it has been proposed that the cell wall of the sporangiophore softens in response to an avoidance gas that is emitted from the growing zone (4). The presence of the barrier increases the concentration of the gas near the cell wall surface proximal to the barrier. Recent studies indicate that the avoidance gas may be water (4).Cell wall extensibility studies have shown that changes in the growth rate of the Phycomyces sporangiophore are accompanied by changes in the mechanical properties of the cell wall. Ortega et al. (8) reported that transient increases in both cell wall growth rate and cell wall extensibility occur after the cell is given a saturating light stimulus. Similar increases in growth and extensibility were observed during the avoidance growth response (7). These studies suggest that transient increases in cell wall growth rate result from transient increases in cell wall mechanical extensibility.Cohen et al. (2) reported that mature sporangiophores grow into the wind; this behavior is now known as the anemotropic response. Recent studies using a humidified wind tunnel have shown that as the humidity of the wind is increased, the sporangiophore shows a quantitative increase in its equilibrium bend angle (5). This observation argues against Pellegrino's (9, 10) mass transfer model. According to the model, the equilibrium bend angle should be inversely related to the humidity of the wind. Thus, the anemotropic response should be less dramatic in high humidity than in low humidity. This apparent contradiction might be resolved if it could be shown that a humid wind caused an overall softening of the sporangiophore's cell wall which may have directly enhanced bending. In support of this hypothesis, Gyure et al. (5) reported that after several hours of growth in a high humidity wind, the cell wall begins to weaken under the weight of the ...