The kinetics of growth control in a colonial hydroid

Abstract

It is suggested that the cumulative view of growth in which some index of biomass is plotted against time tends to obscure temporal variations in the growth process that might provide evidence of how it is controlled. Experiments with the colonial hydroid Campanularia flexuosa show that the action of a growth control mechanism can be demonstrated by considering changes in specific rates of growth determined at frequent intervals in time. However, it is also necessary to disturb the growth process slightly in order to initiate the action of the control mechanism, and having done so, to isolate the effect of the disturbance on growth and thus the action of the control mechanism. This is done by expressing the specific growth rate of organisms whose growth is disturbed as a percentage of that of control organisms of the same age.Experiments in which various toxic agents have been used to disturb growth show that the response of the control mechanism is indiscriminate, and counteracts inhibition irrespective of the agent responsible. The results of further experiments in which colonies were subjected to changes in the level of inhibitor, or in which the inhibitor was removed after the colonies had equilibrated to it, are consistent with the behaviour of a control mechanism operating by negative feedback. Corroborative evidence for this hypothesis comes from work with a simulation model employing feedback. This interpretation of growth control also helps to explain the results of other experiments in which increased resistance to a toxicant resulted from prior exposure to low levels, and others in which ' catch-up' growth and hormesis were observed.

U-Shaped Dose-Responses in Biology, Toxicology, and Public Health

“…However, there was no effort to place the concept of hormesis within an evolutionary context until the work by Stebbing (73)(74)(75)(76) and, more recently, Parsons (60) and Hart & Frame (37). According to Stebbing (75), hormesis evolved from the well-founded and highly generalizable framework of Selye (67) concerning stress and how biological systems respond to stress, including the well-known "general adaptation syndrome.…”

Interpreting ‘Dose-Response’ Curves using Homeodynamic Data: With an Improved Explanation for Hormesis

“…The idea of re-interpreting the 'dose-response' relationship followed access to the output of growth control mechanisms using perturbation with toxic agents (Stebbing, 1981a). Earlier published versions of the graphical model ( Figure 1) were inaccurate with respect to the form of the cryptic response curve.…”

“…That such an equilibrium exists can be seen by the sudden removal of the inhibitor, which briefly reveals the counter-response as a relaxation stimulation (Stebbing, 1981a). At even lower concentrations, the inhibitor is undetectable to the organism, and elicits no counter response.…”

“…The baseline about which growth rate oscillates is raised such that added stimulation becomes an anticipatory adaptation. The mechanism of adaptation involves the adjustment of preferred growth rates to a higher level (Stebbing 1981a(Stebbing , 2003. Any increase in growth rate inevitably results in a cumulative increase in size over time.…”

Interaction between Short-Term Heat Pretreatment and Fipronil on 2nd Instar Larvae of Diamondback Moth, Plutella Xylostella (Linn)

“…In response to some different low or modest level stresses, organisms have similar response mechanisms. The shortterm high temperature treatment often helps organisms increase other stress tolerances known as cross protection, cross tolerance or cross resistance (Stebbing 1981;Calabrese and Baldwin 2003). For instance, after being treated with high but sublethal temperatures, 4 th instar larvae of mosquitoes, Anopheles stephensi and Aedes aegypti, become more tolerant to propoxur (a carbamate insecticide) (Patil et al 1996).…”