Divergent sensitivity of red and far-red Fraunhofer line fluorescence in-filling to water stress dynamics in almond orchards

Plant primary responses to water stress involve changes in both transpiration and photosynthesis, with the relationship between these processes varying during the stress. Currently, precision irrigation decisions are based mainly on transpiration as an indicator of water stress. However, this practice can be misleading as crop yield and quality are also affected by stress-related impairment of photosynthesis, which may be hidden in remote sensing thermal data. In this study, we acquired airborne hyperspectral and thermal imagery to evaluate the coupling between transpiration and photosynthesis in almond ( Prunus dulcis ) trees at mild- and high-level water stress and post-rehydration recovery stages, assessing canopy–air temperature difference (T c T a ) as a marker of transpiration, with sun-induced chlorophyll fluorescence (SIF) and fluorescence in-filling across Fraunhofer lines (FLs; FL depth) as indicators of photosynthesis. Leaf-level measurements (e.g. stem water potential (SWP), instantaneous steady-state chlorophyll fluorescence (F t ), quantum yield of photosystem II (Q y ) were acquired to assess almond physiological conditions at the three investigated stress stages. Diurnal responses of SIF and FL depth to water dynamics were further investigated to gain deeper insight into photosynthetic dynamics under varying water stress. Under high-level water stress, we observed significant differences in T c T a , SIF, and FL depths between watered and water-stressed trees, suggesting that transpiration and photosynthesis both declined. Following rehydration, T c T a differences between watered and water-stressed trees disappeared, indicating that transpiration had recovered; however, SIF and FL depths continued to significantly differ across blocks, likely due to unresolved photosynthetic impairment. Under mild stress, midday T c T a , far-red SIF, and several far-red FL depths were able to capture the slight differences between watered and water-stressed almonds due to water deficit levels. As the photosynthetic difference increased, both SIF and FLs exhibited increased sensitivity. Notably, red SIF and FLs exhibited increased responsiveness in the morning, whereas far-red SIF and FLs showed greater sensitivity in the afternoon, suggesting different adaptations of the photosystems to water-stress levels throughout the day. Our findings suggest that the coupling between transpiration and photosynthesis is stress-stage-dependent, and monitoring photosynthetic responses throughout water stress and recovery via remote sensing could help optimise irrigation strategies and mitigate yield losses and their impact on fruit quality.