Assessment of peach trees water status and leaf gas exchange using on-the-ground versus airborne-based thermal imagery

Abstract

The thermal region of the electromagnetic spectrum might provide valuable information for assessing plant water status. Nevertheless, the plant's physiology and the scale of measurement, (e.g. sensor viewing geometry and the canopy aggregation) are critical for quantifiying and monitoring water stress. This study compares the Crop Water Stress Index (CWSI) of a peach orchard obtained using on-the-ground, and airborne-based canopy temperature (T). The temporal evolution of CWSI under mild water stress conditions was assessed for three different irrigation strategies (over-irrigation, OI; farmer irrigation, FI; and non-irrigation, NI). Two aerial campaigns per irrigation season (2017–2018) were performed with an airborne thermal sensor: a first flight under well-watered conditions, and a second flight once mild water stress was developed. At the time of the flights, T and net photosynthesis (P), stomatal conductance (g) and stem water potential (Ψ) were measured on the ground with a hand-held thermal camera, a portable gas exchange system and a pressure chamber, respectively. The canopy temperature obtained from the hand-held thermal camera, averaging the sunlit and shaded parts of the canopy, agreed with that derived from the airborne measurements (Y=1.00X; RMSE= 1.97 K). The CWSI values calculated from both approaches detected peach water status under different irrigation strategies. In general, Ψ was better predicted from the aircraft (R² up to 0.72 for CWSI obtained from the aircraft versus R =0.51 for T ground measurements), whereas the use of ground measurements was preferred for estimating g and P (R² up to 0.73 and 0.74 for T ground measurements versus R =45 and 0.56 for T and CWSI derived from the aircraft). Regardless the approach used for deriving T, and due to the consideration of different meteorological conditions (i.e different dates), CWSI provided a better relationship with Ψ than T, whereas the latter was more closely related with g and P.