Cohabiting Plant‐Wearable Sensor In Situ Monitors Water Transport in Plant

Abstract The boom of plant phenotype highlights the need to measure the physiological characteristics of an individual plant. However, continuous real‐time monitoring of a plant's internal physiological status remains challenging using traditional silicon‐based sensor technology, due to the fundamental mismatch between rigid sensors and soft and curved plant surfaces. Here, the first flexible electronic sensing device is reported that can harmlessly cohabitate with the plant and continuously monitor its stem sap flow, a critical plant physiological characteristic for analyzing plant health, water consumption, and nutrient distribution. Due to a special design and the materials chosen, the realized plant‐wearable sensor is thin, soft, lightweight, air/water/light‐permeable, and shows excellent biocompatibility, therefore enabling the sap flow detection in a continuous and non‐destructive manner. The sensor can serve as a noninvasive, high‐throughput, low‐cost toolbox, and holds excellent potentials in phenotyping. Furthermore, the real‐time investigation on stem flow insides watermelon reveals a previously unknown day/night shift pattern of water allocation between fruit and its adjacent branch, which has not been reported before.


Materials
Cu film (thickness 6 µm, Jingliang Copper Co.,Ltd,China) Polyimide ( Figure S1. Process for fabricating the Sensor. (a) A Cu film (6 µm thickness) was firstly spin coating (3000 rpm, 45 s) with a layer of polyimide (2 µm) and curried at 200 °C for 2.5 h. Then the Cu film was fixed onto a glass slide coated with a layer of cured PDMS (10 µm) as a temporary adhesive layer, (b) followed by defining Cu serpentine mesh by laser cutting (LPKF U4 laser system, LPKF Laser & Electronics AG, Germany). (c) Then, a 2 µm PI layer was spin-coated (3000 rpm, 45 s) on the top of the mesh. (d) A second laser cutting was applied to remove the PI for exposing the connection pads for the chip components. (e) The thermistor and temperature sensors were welded on the pads with the solder pasting. (f) At last, the whole sensor was transferred with water-soluble tape and secured with a layer of PDMS. The sensor was then ready to connect to the control unit via anisotropic conductive film (ACF) tape.    Here, E is elastic modulus, and ν is Poisson's ratio.      chlorophyll production was at a normal level. These observations suggested that the leaf's sensormounted area was able to uptake air, water, and nutrients and maintain vitality to the same degrees as the unmounted leaf. (b) On the contrary, when a Cu film (resembling the traditional type sensor) was also mounted on the leaf as a control group, the mounted area turned yellow, indicating a decrease in chlorophyll content, possibly due to lack of light or oxygen.

Calibration of the sensor.
Figure S10 Experimental setup for calibration. A fresh-cut watermelon stem (length: 15mm ； diameter：~ 4mm) was connected to a peristaltic pump with a stainless steel ferrule, and a rubber tube after a sap flow sensor was mounted in the middle of the stem. By adjusting the pump pressures, different sap flow rates could be obtained. The water flow through the stem was carefully collected and weighted by a balance to verify the exact flow rates.

In-field Measurements.
Figure S13 In-field monitor of sap flow rate. A healthy watermelon plant during maturation was selected for the study. The plant was grown in a farm field under ambient conditions. A sensor was mounted on a root stem with a diameter of ~ 4 mm. The experiment began shortly before sunrise. For each measurement, the thermistor heated for a short period (120 s), and the resulting temperature variations downstream and upstream were recorded during the followed 360 s. A total of ~ 5 min required for each measurement. To see the low-level sap fluctuation, the monitor was performed continuously for 18 h, between (05:00-23:00).

Study of the responses of watermelon to various environmental stresses.
(1) Temperature, humidity, illumination intensity A healthy watermelon plant in soil was placed in an artificial climate chamber (temperature 25℃, relative humidity 80%, illumination intensity 24000 lux) after a sensor was mounted on the root stem about ~ 4 mm diameter. Then, the sap flow rate was measured under various (25℃, 30℃, 35℃), humidity level (40%, 90%), and illumination intensity (9600 -28800 lux). The plant was kept undisturbed for at least 2 h before the measurement under each condition. Each measurement was repeated 3 times.

(2) Soil moisture level
A healthy watermelon plant in soil with a root stem of about ~ 4 mm was selected for this study.
The plant was kept in-door for 2 days without watering. Then, the sap flow rate was determined before and after watering (15 min later). Each measurement was repeated 3 times. The diameter of the stem is about 5.8 mm.