In-vivo Sap Flow Measurement System Using Microneedle Sensor

Abstract

Measurement of xylem sap flow is essential in understanding plant physiology in agriculture. Advanced hydroponics, for instance, 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 thus can hardly survive the invasion of thick thermal probes for flow speed measurement. A microneedle thermal probe system that can be implanted into a small plant was developed for the measurement of sap flow through the xylem. The microneedle sap flow sensor and its measurement method were presented. The thermal property of microneedle probe was studied with analytical model and multiphysics simulation. The microneedle sap flow sensor was fabricated with MEMS fabrication process. The measurement system including electronics and data acquisition setup was prepared. The calibration of the sensor was conducted for xylem flow. Tests with a tomato stem resulted in a universal calibration model that can be applied to the same species. The sensors were installed to greenhouse plants to measure the in-vivo signal for total of 612 days. More than 207 sensors were installed and sap flow and internal relative temperature were measured in field. We demonstrate routine measurements of sap flow in greenhouse plants including tomato plant over a month, opening up the possibility for production scale application. A microscale hot wire on a single probe benefits from small-scale physics with a simple configuration. The single probe enables minimally invasive measurement with a minimal thermal impact on plant tissues.