Potential of UAV-based sun-induced chlorophyll fluorescence to detect water stress in sugar beet

Abstract

• SIF parameters from a UAV were used to detect crop water stress. • UAV-based hyperspectral and thermal data assisted the interpretation of SIF response. • The selected SIF indicators were able to track drought recovery after irrigation. • The fluorescence emission yield was more sensitive to drought than the apparent SIF yield. 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 (760 nm) and red SIF (687 nm) 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 687 nm (SIF 760yield and SIF 687yield ), the fluorescence emission yield at 760 nm (ΦF 760 ), and the SIF ratio (the ratio between SIF 687 and SIF 760 ). ΦF 760 was 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, SIF 687yield , ΦF 760 , and SIF ratio 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, only ΦF 760 and SIF ratio weakly tracked the changes induced by the irrigation (p-value < 0.1). ΦF 760 had similar changes to SIF 760yield in both June and July, but ΦF 760 was 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.