Jean-Luc Regnard

Stress indicators based on airborne thermal imagery for field phenotyping a heterogeneous tree population for response to water constraints

Highlight text Thermal infrared imagery contributes to the phenotyping of crop response to water stress. Based on multispectral images, the Vegetation Index–Temperature (VIT) concept constitutes a relevant approach.

Field phenotyping of water stress at tree scale by UAV-sensed imagery: new insights for thermal acquisition and calibration

Numerous agronomical applications of remote sensing have been proposed in recent years, including water stress assessment at field by thermal imagery. The miniaturization of thermal cameras allows carrying them onboard the unmanned aerial vehicles (UAVs), but these systems have no temperature control and, consequently, drifts during data acquisition have to be carefully corrected. This manuscript presents a comprehensive methodology for radiometric correction of UAV remotely-sensed thermal images to obtain (combined with visible and near-infrared data) multispectral ortho-mosaics, as a previous step for further image-based assessment of tree response to water stress. On summer 2013, UAV flights were performed over an apple tree orchard located in Southern France, and 4 dates and 5xa0h of the day were tested. The 6400xa0m2 field plot comprised 520 apple trees, half well-irrigated and half submitted to progressive summer water stress. Temperatures of four different on-ground stable reference targets were continuously measured by thermo-radiometers for radiometric calibration purposes. By using self-developed software, frames were automatically extracted from the thermal video files, and then radiometrically calibrated using the thermal targets data. Once ortho-mosaics were obtained, root mean squared error (RMSE) was calculated. The accuracy obtained allowed multi-temporal mosaic comparison. Results showed a good relationship between calibrated images and on-ground data. Significantly higher canopy temperatures were found in water-stressed trees compared to well-irrigated ones. As high resolution field ortho-mosaics were obtained, comparison between trees opens the possibility of using multispectral data as phenotypic variables for the characterization of individual plant response to drought.

Genetic variation of morphological traits and transpiration in an Apple core collection under well-watered conditions: towards the identification of morphotypes with high water use efficiency

Water use efficiency (WUE) is a quantitative measurement which improvement is a major issue in the context of global warming and restrictions in water availability for agriculture. In this study, we aimed at studying the variation and genetic control of WUE and the respective role of its components (plant biomass and transpiration) in a perennial fruit crop. We explored an INRA apple core collection grown in a phenotyping platform to screen one-year-old scions for their accumulated biomass, transpiration and WUE under optimal growing conditions. Plant biomass was decompose into morphological components related to either growth or organ expansion. For each trait, nine mixed models were evaluated to account for the genetic effect and spatial heterogeneity inside the platform. The Best Linear Unbiased Predictors of genetic values were estimated after model selection. Mean broad-sense heritabilities were calculated from variance estimates. Heritability values indicated that biomass (0.76) and WUE (0.73) were under genetic control. This genetic control was lower in plant transpiration with an heritability of 0.54. Across the collection, biomass accounted for 70% of the WUE variability. A Hierarchical Ascendant Classification of the core collection indicated the existence of six groups of genotypes with contrasting morphology and WUE. Differences between morphotypes were interpreted as resulting from differences in the main processes responsible for plant growth: cell division leading to the generation of new organs and cell elongation leading to organ dimension. Although further studies will be necessary on mature trees with more complex architecture and multiple sinks such as fruits, this study is a first step for improving apple plant material for the use of water.

Evaluation of wild tomato accessions (Solanum spp.) for resistance to two-spotted spider mite (Tetranychus urticae Koch) based on trichome type and acylsugar content

Tomato wild relatives are important sources of resistance to many pests of cultivated tomato [Solanum lycopersicum L. (syn. Lycopersicon esculentum Mill.)]. Eleven wild tomato accessions previously identified at AVRDC—The World Vegetable Center as resistant to Bemisia tabaci were evaluated for resistance to the two-spotted spider mite [Tetranychus urticae (Koch.)] based on egg numbers using the leaf disc and Tanglefoot no-choice bioassays, and damage scores in choice bioassays. Highest resistance based on choice and no-choice bioassays was identified in AVRDC S. galapagense accessions VI057400, VI045262, VI037869 and VI037239, and S. cheesmaniae accession VI037240, all of which are new sources of T. urticae resistance. In addition, S. pimpinellifolium accession VI030462 exhibited resistance only in the no-choice bioassay based on egg numbers. Resistance to T. urticae based on the number of eggs from the no-choice bioassays was positively correlated with density of type IV glandular trichomes and negatively correlated with densities of type V trichomes. All resistant accessions accumulated high levels of total acylsugars, which were positively associated with type IV trichomes. There was a significant negative relationship between acylsugar content and T. urticae egg numbers from the no-choice bioassays. There was high correlation between the results from the leaf disc test and the Tanglefoot no-choice bioassay. These findings support the possible presence of broad-based insect and mite resistance in accessions closely related to cultivated tomato.

Multispectral airborne imagery in the field reveals genetic determinisms of morphological and transpiration traits of an apple tree hybrid population in response to water deficit

Highlight This research successfully used image-based spectral indices acquired in the field to assess variability of response to drought in a tree mapping population and to detect the related genetic determinisms.

Are the effects of winter temperatures on spring budburst mediated by the bud water status or related to a whole-shoot effect? Insights in the apple tree

Key messageIn apple, the overwintering bud appears hydraulically isolated from the parent stem. Spring budburst seems more related to a whole-shoot effect than to the water status of the individual bud during winter dormancy.AbstractThe effects of winter temperatures, i.e., during dormancy, on shoot architecture are well known with budburst preferentially in the distal or the proximal part of the parent shoot in cold and mild winter conditions, respectively. However, the link with the overwintering bud water status is still scarcely documented. Our study was developed on four apple (Malus domestica Borkh.) cultivars covering a range of chilling requirements from low (‘Condessa’) to medium (‘Granny Smith’) and high (‘Royal Gala’, ‘Starkrimson’), and maintained in either cold (1,428xa0h below 7.2xa0°C) or mild (99xa0h below 7.2xa0°C) fluctuating winter temperatures. Our aim was to analyze xylem conductance at the stem-to-bud junction, and relative water content and water potential of the bud itself, for buds situated in the distal third of one-year-old shoots. From dormancy to the pre-budburst stage, xylem conductance at the stem-to-bud junction increased or decreased or did not show consistent changes depending on the cultivar and the winter temperature treatment. Whatever the cultivar, there were no significant trends across dates for the effects of winter temperatures on bud water potential and relative water content. Water potential had negative values, between −4.35 and −2.24xa0MPa, across cultivars and winter temperature treatments without a consistent relationship with actual spring budburst frequency. These results suggested that lateral buds were hydraulically isolated from the parent stem during winter until a few days before budburst. We discussed that the temperature-related spring budburst was likely more related to a whole-shoot effect mediated by hormonal, hydraulics and/or sugar signaling, than to the individual bud water status during dormancy.

The volumetric component of individual leaf expansion: Taking into account sub-epidermal tissues in the description of leaf expansion over time

Most leaf development studies at the cell and organ levels have been limited to the leaf surface, with data referring to the leaf surface area and to the number and surface area of epidermal cells. However, leaf sub-epidermal tissues, the palisade and spongy mesophyll, contain the main actors in photosynthesis. The number and thickness of palisade cell layers and the volume occupied by spongy mesophyll (cells and intercellular spaces) affect the accumulation of photosynthates and, as such, whole plant growth. Studies into the leaf phenotype of growth-affected Arabidopsis thaliana mutants have revealed a higher variability in leaf thickness than in leaf surface area. In general, there is no correlation between these two variables, which means that to describe a leaf phenotype, leaf volume has to be taken into account. A method has been developed for high-resolution imaging of leaves in three dimensions usingmultiphoton laser scanning microscopy, and for the analysis of images, providing data on volumes and volumetric proportions of cells and tissues and cell density. The method has been used in the study of A. thaliana leaf expansion from emergence to the onset of senescence for leaves located at different nodal positions in the rosette, completing our knowledge of individual leaf development processes with their volumetric component. The method will further be applied in the study of leaf plasticity in response to the environment for both A. thaliana and apple tree, a model and an agronomic species, respectively. (Texte integral)