New Laser System Tracks Winter Wheat Fluorescence Dynamics

The system uses a modulated blue laser to measure relative changes in canopy fluorescence quantum yield in winter wheat. This approach allows for continuous, non-contact monitoring of plant physiological status under natural light conditions.

The development of Canopy-LIF addresses a significant limitation in traditional vegetation remote sensing. Conventional spectral indices such as NDVI and SAVI primarily characterize plant structure and biochemical composition. These signals often represent cumulative physiological outcomes over long periods rather than instantaneous changes. In contrast, fluorescence signals are closely linked to the rapid partitioning of light energy between photochemical reactions and thermal dissipation. This makes fluorescence a more sensitive indicator of early environmental stress and dynamic plant function.

The Canopy-LIF system operates by using a 455 nm pulsed laser diode to excite the target canopy. The laser emits short pulses with a repetition frequency of 100 Hz and a duty cycle of 0.05%. This low-duty-cycle excitation minimizes potential disturbances to the plant’s photophysiological status while generating a detectable fluorescence response. The system is designed to work at a fixed distance of approximately one meter from the canopy, illuminating an area of roughly 35 by 30 centimeters.

A key technical challenge in active fluorescence observation is distinguishing the induced signal from intense natural background radiation. Canopy-LIF employs synchronous demodulation to isolate the fluorescence response synchronized with the laser modulation. The system separates the signal into a direct current channel, representing ambient background light, and an alternating current channel, representing the active fluorescence. This separation allows the system to track rapid changes in plant fluorescence without being misled by fluctuations in solar intensity.

To validate the system’s performance, the research team conducted bare-soil observations and Kautsky fluorescence induction experiments. During bare-soil observations, the active fluorescence signal remained low and stable, even when natural radiation changed rapidly. This confirmed that the system does not misidentify background radiation as plant fluorescence. In the Kautsky experiments, the system successfully captured the rapid increase and subsequent decrease in fluorescence following re-illumination of dark-adapted leaves, demonstrating its sensitivity to dynamic physiological changes.

Continuous observations were conducted on a winter wheat plot during the booting-to-heading stage from May 2026. The wheat cultivar was Jimai 22, sown in October 2025. The experiment monitored the canopy over four consecutive clear-sky days, from day of year 132 to 135. During this period, the soil water content in the upper root zone decreased from 66.5% to 53.7%. The Canopy-LIF system recorded active fluorescence at one-minute intervals, providing a high-resolution time series of canopy physiological dynamics.

The results revealed a distinct bimodal diurnal pattern in the active fluorescence signal. The signal increased after sunrise, peaked in the morning, decreased around midday, and rose again in the afternoon. This pattern differs from the unimodal trend of background radiation and photosynthetically active radiation. The midday decrease likely reflects the plant’s photoprotective mechanisms, such as non-photochemical quenching, which dissipate excess light energy when photochemical utilization is limited.

To assess the physiological relevance of the canopy signal, the team compared Canopy-LIF measurements with leaf-scale fluorescence data. Leaf fluorescence was measured using a Moni-PAM fluorometer on two upper-canopy leaves. The time-matched data showed positive correlations between the canopy and leaf signals on all four observation days. Pearson correlation coefficients ranged from 0.65 to 0.83, indicating that the canopy signal tracks relative changes in leaf fluorescence quantum yield. However, the canopy signal integrated responses from multiple leaf layers, resulting in differences in peak amplitude and curve shape compared to the local leaf measurements.

The study also observed a progressive decline in daytime fluorescence levels across the four observation days. This decline coincided with the decrease in soil water content, suggesting a potential link between water availability and canopy fluorescence dynamics. While this association is consistent with known plant stress responses, the authors note that controlled water-stress experiments with sufficient replication are needed to rigorously validate this interpretation. The current findings provide preliminary evidence for the system’s potential in monitoring crop water status.

Canopy-LIF offers several advantages over existing active fluorescence techniques. Unlike leaf-scale PAM instruments, it can observe complex vegetation canopies without physical contact. Compared to laser-induced fluorescence transient systems, it does not require high-energy pulsed sources, enabling long-term continuous observation with lower energy consumption. The system’s fixed excitation and observation geometry reduce interference from changing environmental conditions, making it suitable for field deployment.

Despite its potential, the system has certain limitations. The current validation was conducted over a short working distance and a limited observation period. Long-term field applications will require addressing issues such as laser output stability and optical component contamination. Additionally, the quantitative relationship between the measured signal and absolute fluorescence quantum yield requires further investigation. The system currently serves as a proxy for relative changes rather than a direct measurement of absolute values.

The development of Canopy-LIF represents a significant step toward continuous, non-invasive monitoring of plant physiological status. By providing a stable and sensitive measure of canopy fluorescence dynamics, the system can help detect environmental stress at an early stage. This capability is crucial for precision crop management and understanding plant-environment interactions. Future work will focus on improving instrument stability and validating the system’s response to various stress factors across different crop types.

The optical and circuit architecture of Canopy-LIF incorporates specialized hardware to isolate the induced emission under outdoor sunlight. At the receiving end, light collection is handled by a Fresnel lens with a diameter of 160 millimeters and a focal length of 70 millimeters, focusing returned radiation onto the detection module. Before reaching the detector, incoming light passes through an LLP500-800 filter paired with an LPF655 filter, producing an effective transmission band of 655 to 800 nanometers that eliminates direct reflection and scattered light from the 455-nanometer laser. The filtered beam reaches an OTRON OSD100-IC silicon PIN photodiode with an active area of 9.4 by 9.4 millimeters and spectral sensitivity from 400 to 1100 nanometers. Photocurrent is converted to voltage by a low-noise transimpedance amplifier before splitting into dual processing routes. The direct current path uses a 1-megaohm and 1-microfarad filter with a 0.16-hertz cutoff to extract ambient light and prevent circuit saturation, while the alternating current path conditions the 100-hertz modulated active fluorescence signal.

Signal acquisition and processing rely on dedicated electronics paired with structured offline feature extraction. Conditioned signals are sampled by an ADS8681 16-bit analog-to-digital converter operating at 1 megahertz, initiated prior to laser emission to capture twenty pre-trigger baseline samples over a 20-microsecond window. Data stream across an FT232H chip operating in asynchronous 8-bit parallel FIFO mode to an MT7628AN Wi-Fi module, enabling storage on a local SD card or wireless transfer to a host computer. Offline decoding in MATLAB groups raw binary records by timestamp markers and locates valid single-pulse acquisition windows using markers in the ambient channel. For each pulse, the active fluorescence signal is smoothed with a three-point median filter, and the net response is computed by subtracting the full-window minimum voltage from the baseline value. Ambient intensity is calculated as the mean across the window, and all valid pulse measurements within each observation interval are averaged to produce final time-series data.

The agronomic setting at the Gucheng National Field Scientific Observation and Research Station in Dingxing County, Hebei Province, provided controlled conditions for evaluating these optical measurements. Established at 39.14455 degrees north and 115.73785 degrees east, the 2-meter by 4-meter plot was sown with Jimai 22 winter wheat at 340 plants per square meter and fertilized with diammonium phosphate. A single irrigation event of 0.5 cubic meters occurred on day of year 129, after which a rain shelter excluded precipitation to enable natural soil dry-down. Canopy structural traits were monitored at two-day intervals: leaf area index rose from 5.16 on day 126 to 5.34 on day 135, canopy height grew from 67.52 to 71.64 centimeters, and SPAD relative chlorophyll values increased from 54.29 to 59.77. Upper-root-zone soil water was quantified by oven-drying core samples taken at 10-centimeter intervals down to 50 centimeters, calculating relative moisture content against a constant field capacity of 22.7.

Experimental testing verified signal independence from natural sunlight across diverse trial regimes. During bare-soil monitoring, active fluorescence readings remained below 500 throughout the day and showed no synchronous shift when the rain shelter closed at 14:30, despite a steep drop in ambient illumination. The Kautsky induction protocol applied dark adaptation using black cloth from 11:37 to 11:42, followed by three minutes under natural light, and repeated the sequence from 11:45 to 11:50; both iterations triggered a sharp fluorescence rise upon cloth removal before decaying to stable baselines. In comparisons with Moni-PAM leaf observations across the clear days, correlation analysis yielded Pearson values of 0.71 on day 132, 0.65 on day 133, 0.78 on day 134, and 0.83 on day 135, with Spearman coefficients of 0.64, 0.59, 0.71, and 0.82. The corresponding linear-fit regression slopes increased from 0.18 to 0.45 across the measurement dates.

Subject of Research: Development and validation of an active fluorescence system for monitoring canopy photophysiological dynamics in winter wheat.

Article Title: Canopy-LIF: an active fluorescence system for characterizing relative changes in canopy fluorescence quantum yield

Article References: Huang, S., Liu, Z., Huang, P., Li, Y., & Qiang, Y. (2026). Canopy-LIF: an active fluorescence system for characterizing relative changes in canopy fluorescence quantum yield. Smart Agricultural Technology, 15, Article 102722. https://doi.org/10.1016/j.atech.2026.102722

Image Credits: AI Generated

DOI: 10.1016/j.atech.2026.102722

Keywords: Canopy-LIF, active fluorescence, winter wheat, fluorescence quantum yield, pulsed laser excitation, synchronous demodulation, photophysiological monitoring, non-contact sensing, crop stress detection, Kautsky induction, remote sensing, Hebei Province, Jimai 22, soil water content, diurnal variation, Moni-PAM, Smart Agricultural Technology, plant physiology, carbon assimilation, field observation