Extensiometric determination of the rheological properties of the epidermis of growing tomato fruit.

This paper examines the rheological properties of the fruit epidermis of tomato (Lycopersicon esculentum L.). This research was conducted because previous work had demonstrated that the rate of tomato fruit growth is determined by the interaction of tissue pressure and epidermal properties. A constant-load (or 'creep') extensiometer was employed in these experiments and the results interpreted using a model which describes creep retardation using a limited number of rheological elements, one of which appears analogous to plant growth and is of similar magnitude to fruit growth rate in vivo. The effects of pH, applied force and boiling upon the individual components of the model have been examined and indicate that several elements are strongly pH-dependent and that this dependency is eliminated by boiling. These results suggest that enzyme activity (plausibly that of one or more expansins) reduces the viscosity of the cell wall over a wide range of time scales. Further consideration of the creep of tomato epidermis in terms of models developed to describe the behaviour of artificial polymers suggests that the types of molecular event described by each rheological element can tentatively be identified and that pH-dependent enzyme activity facilitates both conformer rotation and macromolecular movement within the plant cell wall. These interpretations ascribe considerable importance to the time scale over which creep occurs.