  The ecophysiological response of plants to environmental heavy metal stress
is indicated by the profile of its tissue HM concentrations (Cp) versus the
concentration of the HM in the substrate (Cs). We report a systems biology
approach to the modelling of the Cp- Cs profile using as loose analogy, the
Verhulst model of population dynamics but formulated in the concentration
domain. The HM is conceptualized as an ecological organism that `colonizes' the
resource zone of the plant cells driven by the potential supplied by the higher
HM concentration in the substrate. The infinite occupation by the HM is limited
by the eventual saturation of the cellular binding sites. The solution of the
differential equation results in the logistic equation, the r-K model. The
model is tested for 3 metalophillic plants T.erecta, S. vulgaris and E.
splendens growing in different types of substrates, contaminated to varying
extents by different copper compounds. The model fitted the experimental Cp- Cs
profiles well. The r, K parameter values and secondary quantities derived from
them, allowed a quantification of the number of Cu binding sites per cell at
saturation, the sensitivities (affinities) of these sites for Cu in the 3
experimental systems as well as the extraction of information related to the
substrate phyto-availability of the Cu. Thus even though the model operates at
the systems level it permits useful insights into underlying processes that
ultimately derive from the cumulative molecular processes of HM homeostasis.
The chief advantages of the model are its simplicity, fewer arbitrary
parameters, and the non-specification of constraints on substrate and plant
type.
