Divergent abiotic spectral pathways unravel pathogen stress signals across species

Abstract: Plant pathogens pose increasing threats to global food security, causing yield losses that exceed 30% in food-deficit regions. Xylella fastidiosa (Xf) represents the major transboundary plant pest and one of the world’s most damaging pathogens in terms of socioeconomic impact. Spectral screening methods are critical to detect non-visual symptoms of early infection and prevent spread. However, the subtle pathogen-induced physiological alterations that are spectrally detectable are entangled with the dynamics of abiotic stresses. Here, using airborne spectroscopy and thermal scanning of areas covering more than one million trees of different species, infections and water stress levels, we reveal the existence of divergent pathogen- and host-specific spectral pathways that can disentangle biotic-induced symptoms. We demonstrate that uncoupling this biotic–abiotic spectral dynamics diminishes the uncertainty in the Xf detection to below 6% across different hosts. Assessing these deviating pathways against another harmful vascular pathogen that produces analogous symptoms, Verticillium dahliae, the divergent routes remained pathogen- and host-specific, revealing detection accuracies exceeding 92% across pathosystems. These urgently needed hyperspectral methods advance early detection of devastating pathogens to reduce the billions in crop losses worldwide

Avances en teledetección para la prevención y detección temprana de Xylella fastidiosa en el marco de los proyectos H2020 POnTE y XF-ACTORS.

Informational article of the tasks carried out in WP6 of POnTE and WP3 of XF-ACTORS in a professional Spanish magazin

Twenty-three unsolved problems in hydrology (UPH) – a community perspective

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through on-line media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focussed on process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come

Spatial resolution effects on chlorophyll fluorescence retrieval in a heterogeneous canopy using hyperspectral imagery and radiative transfer simulation

Increasing attention is being given to chlorophyll fluorescence (F) for global monitoring of vegetation due to its relationship with physiology. New progress has been made in the methodological and technical aspects of signal retrieval with the recently published low-resolution global maps of fluorescence. Nevertheless, little progress has been made in the interpretation of the F signal when quantified in large pixels, an important issue due to the effects of structure, percentage cover, shadows, and background. High-resolution (40 cm) airborne hyperspectral imagery is used in this letter to assess the retrieval of fluorescence by the Fraunhofer line depth method from pure tree crowns and aggregated pixels. Due to canopy heterogeneity, the F signal extracted from aggregated pixels is highly degraded. A poor relationship is obtained between fluorescence extracted from pure tree crowns (Fcrown) and that quantified from pixels aggregating pure tree crowns, shadows, and background (Faggregated) (R2 = 0.25; p < 0.01). The relationship between F and stomatal conductance (used as a physiological indicator) decreases as a function of aggregation, yielding R2 = 0.69 (p < 0.01) when calculated from pure tree crowns and R2 = 0.38 (p < 0.05) from pixels containing crown, shadows, and soil. This letter demonstrates the need for methods to accurately retrieve a pure-vegetation fluorescence signal from aggregated pixels. The FluorMODleaf and FluorSAIL models were combined with the geometric forest light interaction model (FLIM) model and led to the "Fluor- FLIM" model developed for this letter. Simulations conducted with FluorFLIM obtain predictive relationships between Fcrown and Faggregated pixels as a function of percentage cover, enabling the estimation of pure-crown F from aggregated pixels (R2 = 0.72, p < 0.01)

Almond tree canopy temperature reveals intra-crown variability that is water stress-dependent

Tree water status is often characterized by measuring a few leaves and it is not known to what extent such measurements represent the tree as a whole. We present an assessment of the intra-crown temperature variability and its relationship with water status in two almond cultivars. High-resolution imagery was acquired on 30 June 2009 at 11:30, 14:30, and 16:30h (solar time) with a thermal camera on-board an aircraft over an almond orchard in Kern County, CA, USA. Ten irrigation levels were applied, ensuring a wide variability in water status, and each was replicated eight times. Stem water potential and stomatal conductance were measured on trees of various irrigation regimes at each flight. Significant variation in canopy temperature was found within each crown, probably reflecting differences in stomatal conductance in different parts of the tree crown. The intra-crown standard deviation of canopy temperature (intra-crown sT c) increased from fully irrigated trees to intermediate irrigation levels, diminishing afterwards in the most stressed treatments. Mean canopy temperature was well correlated with stomatal conductance and stem water potential (R 2 above 0.65). In trees that had similar mean canopy temperature, intra-crown sT c correlated well with tree water status. Our results quantified in detail the spatial variability in surface temperatures that exists within almond tree crowns and suggest that the intra-crown temperature variation may be a useful indicator of the onset of tree water stress

Airborne thermal imagery to detect the seasonal evolution of crop water status in peach, nectarine and Saturn peach orchards

In the current scenario of worldwide limited water supplies, conserving water is a major concern in agricultural areas. Characterizing within-orchard spatial heterogeneity in water requirements would assist in improving irrigation water use efficiency and conserve water. The crop water stress index (CWSI) has been successfully used as a crop water status indicator in several fruit tree species. In this study, the CWSI was developed in three Prunus persica L. cultivars at different phenological stages of the 2012 to 2014 growing seasons, using canopy temperature measurements of well-watered trees. The CWSI was then remotely estimated using high-resolution thermal imagery acquired from an airborne platform and related to leaf water potential (ΨL) throughout the season. The feasibility of mapping within-orchard spatial variability of ΨL from thermal imagery was also explored. Results indicated that CWSI can be calculated using a common non-water-stressed baseline (NWSB), upper and lower limits for the entire growing season and for the three studied cultivars. Nevertheless, a phenological effect was detected in the CWSI vs. ΨL relationships. For a specific given CWSI value, ΨL was more negative as the crop developed. This different seasonal response followed the same trend for the three studied cultivars. The approach presented in this study demonstrated that CWSI is a feasible method to assess the spatial variability of tree water status in heterogeneous orchards, and to derive ΨL maps throughout a complete growing season. A sensitivity analysis of varying pixel size showed that a pixel size of 0.8 m or less was needed for precise ΨL mapping of peach and nectarine orchards with a tree crown area between 3.0 to 5.0 m2

A PRI-based water stress index combining structural and chlorophyll effects: assessment using diurnal narrow-band airborne imagery and the CWSI thermal index

This work advances the evaluation and interpretation of the Photochemical Reflectance Index (PRI) as an indicator of water stress, over a range of canopy structures and pigment content levels. Very high resolution (VHR) narrow-band multispectral (10 cm) and thermal (20 cm) imagery was acquired diurnally, in four airborne campaigns conducted over an experimental vineyard site undergoing three different irrigation treatments. Field measurements of leaf stomatal conductance (Gs) and leaf water potential (Ψleaf) were acquired concurrently with the airborne campaigns and compared against the Crop Water Stress Index (CWSI), a widely accepted, thermal-based indicator of water stress, and against narrow-band multispectral indices calculated from pure-vegetation pixels. The study proposes a new formulation, a normalized PRI (PRInorm), in which the standard PRI index is normalized by an index that is sensitive to canopy structure (Renormalized Difference Vegetation Index, RDVI) and by a red edge index that is sensitive to chlorophyll content (R700/R670). The hypothesis investigated is that the new index, calculated as PRInorm = PRI/[RDVI · R700/R670], not only detects xanthophyll pigment changes as a function of water stress, but also normalizes for the chlorophyll content level and canopy leaf area reduction induced by stress. Results demonstrated that when comparing PRInorm against stomatal conductance (r2 = 0.79; p < 0.001) and leaf water potential (r2 = 0.77; p < 0.001) measured at midday, the new index performed better than the standard PRI (r2 = 0.52 and 0.49, respectively). Further, when using the four flights conducted during the diurnal experiment, the relationships with stomatal conductance also showed the superior performance of PRInorm (r2 = 0.68) as opposed to PRI (r2 = 0.4). The proposed normalized PRI was highly related (r2 = 0.75; p < 0.001) to the thermal indicator of water stress, CWSI, which was used here as a benchmark. In comparison, the standard PRI index was found to be significantly related to CWSI (p < 0.001), although the relationship was weaker (r2 = 0.58) than that obtained for PRInorm. In summary, this study demonstrates that PRInorm isolated better than PRI the physiological changes against a changing background of altered pigments and structure, tracking more precisely the diurnal dynamics of the stomatal aperture. Simulations conducted, using leaf and canopy radiative transfer models to elucidate these results, showed that PRInorm is more linearly related to canopy pigment content than the standard PRI, and was more capable of differentiating between stress levels, providing better insight into the results of this diurnal study