Monitoring of Water Transportation in Plant Stem With Microneedle Sap Flow Sensor

The measurement of xylem sap flow is essential to understanding plant physiology in agriculture. Advanced hydroponics, for instance, would require sap flow measurement to observe the plant reaction to environmental variables, such as sunlight, humidity, and soil water content. However, most conventional approaches for sap flow measurement have been limited to large woody plants. Plants grown in hydroponics, e.g., tomatoes and bell peppers, are smaller and softer, and can hardly survive the invasion of thick thermal probes for flow speed measurement. This report presents a microneedle thermal probe that can be implanted into a small plant for the measurement of sap flow through the xylem. A microscale single hot wire on a single probe is used to benefit from small-scale physics in a simple configuration. The single probe enables minimally invasive measurement with a small thermal impact on plant tissues. We show that the Granier method can be modified to use the single hot wire as a heater and a temperature sensor simultaneously. Tests with a tomato stem result in a universal calibration model that can be applied to the same species. We demonstrate routine measurements of sap flow in a greenhouse tomato tree over a month, opening up the possibility for production scale application.