Biosensors (i.e. online plant sensors) can be succesfully used as diagnostic tools to evaluate the physiological plant health status and are, therefore, of practical importance for the development of advanced phytomonitoring systems. A promising sensor is the LVDT, which can be used to measure online and with great precision stem diamter variations, including radial stem growth. The LVDT measurement reveals useful information on both the plant-water relations and the carbon metabolism. The aim of this research is to develop a mechanistic model which allows to unambiguously interpret the LVDT-signal and, hence, the underlying physiological plant processes.