Characteristics of sun-induced fluorescence in monocot and dicot crop patches with different NDVI and canopy closure

Although monitoring the health of the crops grown in our fields is almost as old as plant production itself, it is only now that technological development (remote and proximal sensing) enables us to look at the links between different processes. In Kartal, Hungary, measurements of the physiological performance of plants were carried out in crop fields in two consecutive years on five clearly discernible spots; in May 2020 on winter wheat and in June 2021 on sunflower. The five patches were selected on the basis of their distinct NDVI values from satellite imagery and/or canopy closure and identified using GPS coordinates. The following parameters were measured or derived: soil moisture, leaf area index (LAI), sun-induced fluorescence (SIF), vegetation indices (FCVI, PRI, MTCI, WI, NDRE), plant water, chlorophyll, and carotenoid content. To quantify the latter, three leaves from the plants on each patch were processed.

Our aim was to look for differences in the function and condition (stress state) of plants in patches that were markedly different. Field observations reflected the results of our instrumental measurements. For example, the average LAI values of the patch with the apparently tallest and greenest, i.e. most vital wheat plants, were more than twice as high as those obtained from patches that were visually either nutrient deficient or stressed (low and yellow plants). Similar conclusions could be drawn from the PRI values. The lowest value (-0.063) was derived from the patch with yellowish plants, while  the highest (0.0235) was in the greenest patch. Principal component analysis (PCA) of the variables reflected slight differences between the two observed stands as well because yellowish, stressed vegetations were not found in the sunflower stand, even though the canopy closure was markedly different. PCA loadings for SIF B (at 685 nm in the O₂ B-band) oriented in the direction of scores from the yellow plant patches, while SIF A (at 761 nm in the O₂ A-band) oriented in the direction of scores from the healthy, green plant patches in the wheat stand, reflecting potential reabsorption by chlorophylls in healthy, larger canopy patches. Loadings of the two SIF metrics did not differ in orientation in the sunflower stand, both of them oriented in the direction of smaller canopy closure and smaller physiological activity plots, that is, plots characterized with a larger share of the excitation energy loss as fluorescent light emission.

Moreover, the SIF values measured with the Piccolo Doppio spectrometer were compared with the values obtained by using radiative transfer model simulations with the Soil-Canopy Observation of Photochemistry and Energy fluxes (SCOPE) model. By comparing our data with a simplified test condition, we can draw conclusions about the sensitivity of the model for a set of input parameters.