A leaf-mounted thermal sensor for the measurement of water content

We present a microfabricated thermal sensor for the real-time measurement of the water content of plant leaves. The device consists of a thin-film micro-heater which induces a temperature gradient within a leaf to which it is attached. Temperature differences (Delta T) between two thin-film thermocouples spaced 50 mu m and 160 mu m from the heater can be related to the thermal properties of the sample under inspection which, in turn, are dependent on the leaf water content. The sensor is fabricated on a flexible polyimide substrate to ensure that it is robust enough to operate reliably without causing visible damage to the leaves. To demonstrate the device's operation, it was clamped to abscised leaves which were allowed to dry over a period of around 6 h. The device's output was then compared to the relative water content of the leaf which was directly measured by weighing the leaf. For the range considered, a linear dependence was observed between Delta T and the water content of the leaf. It is suggested that the changes in Delta T are dominated by the thermal contact resistance between the device and the leaf. The device was subsequently used to monitor the real-time water content of leaves in situ on plants subjected to water stress conditions. (C) 2012 Elsevier B.V. All rights reserved.