Fluorescence from UAVs as water stress indicator for sugar beet
Water shortage is a critical abiotic stressor limiting crop growth and yield. For better management and minimizing harmful effects on crop production, it is critical to detect water stress timely and accurately. Remote sensing provides spatiotemporal monitoring of crop structural, biochemical, and physiological changes induced by water stress at different scales. The last decade, the characterisation of chlorophyll fluorescence (CF) using advanced (imaging) spectrometers has become an important research topic within the remote sensing domain including the use of Unmanned Aerial Vehicles or drones.
In a recent paper, Lala Wang, PhD at the Laboratory of Geo-information and Remote Sensing, has shown that sun-induced fluorescence (SIF) indicators acquired from UAVs can indicate water stress at the field level for the crop sugar beet. She acquired the UAV images over an experimental field with irrigated and non-irrigated sugar beet plots in the dry summer of 2019. Not only point observations of sun-induced fluorescence were acquired but also hyperspectral and thermal images. This allowed a comparison of different remote sensing based water and drought stress indicators. During the two measurement campaigns, sugar beet plants were affected by drought (June 2019) and by a combination of drought and heat stress (July 2019). A combination of temperature indices, VIs, and soil moisture measurements was used to determine the stress conditions and plant status.
We found that on June 28, when sugar beets were experiencing water stress, SIF indicators all showed a significant response to the recovery of the irrigated sugar beets (p-value < 0.05). On the other hand, on July 24 when both water stress and heat stress affected the crop, SIF indicators weakly tracked the changes induced by the irrigation (p-value < 0.1). The findings suggest that SIF indicators can indicate water stress at the field level, but its value to detect the changes of photosynthetic activities under severe stress needs more investigation.
Sun-induced chlorophyll fluorescence (SIF) is a direct indicator of plant photosynthetic activities and can potentially indicate plant physiological changes caused by water stress. However, the direct effect of water stress on the physiological SIF responses in crops at the field level still needs further research to clearly understand the involved mechanisms. To study this relationship, we made use of Unmanned Aerial Vehicles (UAVs), which are flexible and cost-effective to acquire SIF data at a high temporal resolution. We acquired near-infrared SIF (760nm) and red SIF (687nm) measurements using a UAV platform over irrigated and non-irrigated sugar beet plots. To represent physiological changes in crops, we calculated the apparent SIF yield (SIF normalized by the absorbed photosynthetically active radiation) at 760 and 687nm (SIF760yieldand SIF687yield), the fluorescence emission yield at 760nm (ΦF760), and the SIFratio(the ratio between SIF687and SIF760). ΦF760was estimated using the recently developed NIRvH approach. For an improved interpretation of the response of these SIF indicators, we also acquired additional UAV-based hyperspectral and thermal data. We found that on June 28, when sugar beets were experiencing water stress, SIF687yield, ΦF760, and SIFratioall showed a significant response to the recovery of the irrigated sugar beets (p-value < 0.05). On the other hand, on July 24 when both water stress and heat stress affected the crop, only ΦF760and SIFratioweakly tracked the changes induced by the irrigation (p-value < 0.1). ΦF760had similar changes to SIF760yieldin both June and July, but ΦF760was more sensitive to irrigation. This indicates the importance of correcting for the structural effect when interpreting the SIF response. The findings suggest that SIF indicators can indicate water stress at the field level, but its value to detect the changes of photosynthetic activities under severe stress needs more investigation.