First, you'll need a way to measure and record the temperature at two specific points (equidistant from a central heating element) along the stem of your plant:
To build the gauge, you'll need: (1) the PCB; and (2) the components.
Fabricate the PCB
See here for the KiCad PCB files. Follow the directions from your favorite PCB fabrication service (e.g., OSHPark, JLCPCB, Seeed Studio, etc.), and order your desired quantity of PCBs.
Gather the Parts
See here for the parts list. Go to your favorite electronic components distributor (e.g., Digikey, Arrow, Mouser, etc.) and order your desired quantity of components.
Second, you'll need a microcontroller that has an available GPIO pin (3.3V to 5V) and can communicate via I2C. We've found the ESP32 (specifically, the LOLIN D32 Pro dev board) to be great for rapid prototyping with its relatively large onboard RAM, built-in WiFi, and low cost. For field experiments, it might make more sense to use a board with more low-power optimizations.
To reduce wiring, you might want to use a shield to interface your microcontroller with the sapflow gauge connector. See here for our shield design for the D32 Pro.