2019 ASHS Annual Conference

Non-destructive methods to quantify crop growth can provide a valuable tool in both research and production settings. Quantifying canopy size can be done using a variety of imaging techniques, with regular color (red/green/blue, RGB) imaging being the most common approach. However, separating canopy from background is not always easy using RGB imaging and different methods may be needed depending on the background in the image or the color of the leaves. To circumvent this issue, we developed an imaging approach that takes advantage to the fluorescence emitted by chlorophyll. The energy of about 1 to 3% of photons absorbed by leaves is re-emitted as photons in the range of ~690 to 740 nm. This fluorescence coming from plants is easy to photograph: plants are exposed to blue light and images are taken using a monochrome camera with a 680 nm long-pass filter (i.e., only photons with wavelengths > 680 nm can pass through the filter). This assures that the camera can only detect fluorescence from chlorophyll. One complication is that the chlorophyll in algae fluoresces similar to that in plants, so image processing may be needed to separate algae from leaves. This can be achieved by comparing images collected under both blue and white light: algae are more pronounced under blue than under white light. Alternatively, algicides have proven effective in suppressing algae without harmful effects on plants. Comparisons of leaf area measurements using the fluorescence imaging versus a leaf area meter indicate that the fluorescence imaging is almost perfectly correlated with standard leaf area measurements (R² = 0.998). Chlorophyll fluorescence imaging can also be used to monitor ripening of fruits that contain chlorophyll in their unripe state. The decrease in fruit chlorophyll levels during ripening is easily quantified using this approach. The hardware costs for a chlorophyll imaging system are ~$1,000 and the system is easy to assemble.