Bruno Andrieu

Extraction of vegetation biophysical parameters by inversion of the PROSPECT + SAIL models on sugar beet canopy reflectance data. Application to TM and AVIRIS sensors

The PROSPECT leaf optical properties and SAIL canopy reflectance models were coupled and inverted using a set of 96 AVIRIS (Airborne Visible/Infrared Imaging Spectrometer) equivalent spectra gathered in afield experiment on sugar beet plots expressing a large range in leaf area index, chlorophyll concentration, and soil color. In a first attempt, the model accurately reproduced the spectral reflectance of vegetation, using six variables: chlorophyll a + b concentration (Cab), water depth (Cw), leaf mesophyll structure parameter (N), leaf area index (LAI), mean leaf inclination angle (θl), and hot-spot size parameter (s). The four structural parameters (N, LAI, τl, and s) were poorly estimated, indicating instability in the inversion process; however, the two biochemical parameters (Cab and Cw) were evaluated reasonably well, except over very bright soils. In a second attempt, three of the four structure variables were assigned a fixed value corresponding to the average observed in the experiment. Inversions performed to retrieve the remaining structure variable, leaf area index, and the two biochemical variables showed large improvements in the accuracy of LAI, but slightly poorer performance for Cab and Cw. Here again, poor results were obtained with very bright soils. The compensations observed between the LAI and Cab or Cw led us to evaluate the performance of two more-synthetic variables, canopy chlorophyll content or canopy water content, for these the inversions produced reasonable estimates. The application of this approach to Landsat TM (Thematic Mapper) data provided similar results, both for the spectrum reconstruction capability and for the retrieval of canopy biophysical characteristics.

Functional–structural plant modelling: a new versatile tool in crop science

Plants react to their environment and to management interventions by adjusting physiological functions and structure. Functional-structural plant models (FSPM), combine the representation of three-dimensional (3D) plant structure with selected physiological functions. An FSPM consists of an architectural part (plant structure) and a process part (plant functioning). The first deals with (i) the types of organs that are initiated and the way these are connected (topology), (ii) co-ordination in organ expansion dynamics, and (iii) geometrical variables (e.g. leaf angles, leaf curvature). The process part may include any physiological or physical process that affects plant growth and development (e.g. photosynthesis, carbon allocation). This paper addresses the following questions: (i) how are FSPM constructed, and (ii) for what purposes are they useful? Static, architectural models are distinguished from dynamic models. Static models are useful in order to study the significance of plant structure, such as light distribution in the canopy, gas exchange, remote sensing, pesticide spraying studies, and interactions between plants and biotic agents. Dynamic models serve quantitatively to integrate knowledge on plant functions and morphology as modulated by environment. Applications are in the domain of plant sciences, for example the study of plant plasticity as related to changes in the red:far red ratio of light in the canopy. With increasing availability of genetic information, FSPM will play a role in the assessment of the significance towards plant performance of variation in genetic traits across environments. In many crops, growers actively manipulate plant structure. FSPM is a promising tool to explore divergent management strategies.

The nested radiosity model for the distribution of light within plant canopies

Abstract We present a new approach to simulate the distribution of natural light within plant canopies. The canopy is described in 3D, each organ being represented by a set of polygons. Our model calculates the light incident on each polygon. The principle is to distinguish for each polygon the contribution of the light coming directly from light sources, the light scattered from close polygons and that scattered from far polygons. Close polygons are defined as located inside a sphere surrounding the studied polygon and having a diameter D s . The direct light is computed by projection. The exchanges between close polygons are computed by the radiosity method, whereas the contribution from far polygons is estimated by a multi-layer model. The main part of computing time corresponds to the calculations of the geometric coefficients of the radiosity system. Then radiative exchanges can be quickly simulated for various conditions of the angular distribution of incoming light and various optical properties of soil and phytolelements. Simulations compare satisfactorily with those produced by a Monte Carlo ray tracing. They show that considering explicitly the close neighboring of each polygon improves the estimation of organs irradiance, by taking into account the local variability of fluxes. For a virtual maize canopy, these estimations are satisfying with D s =0.5 m; in these conditions, the simulation time on a workstation was 25 min for a canopy of 100 plants.

Maize Leaves Turn Away from Neighbors

In commercial crops, maize (Zea mays) plants are typically grown at a larger distance between rows (70 cm) than within the same row (16–23 cm). This rectangular arrangement creates a heterogeneous environment in which the plants receive higher red light (R) to far-red light (FR) ratios from the interrow spaces. In field crops, the hybrid Dekalb 696 (DK696) showed an increased proportion of leaves toward interrow spaces, whereas the experimental hybrid 980 (Exp980) retained random leaf orientation. Mirrors reflecting FR were placed close to isolated plants to simulate the presence of neighbors in the field. In addition, localized FR was applied to target leaves in a growth chamber. During their expansion, the leaves of DK696 turned away from the low R to FR ratio signals, whereas Exp980 leaves remained unaffected. On the contrary, tillering was reduced and plant height was increased by low R to FR ratios in Exp980 but not in DK696. Isolated plants preconditioned with low R/FR-simulating neighbors in a North-South row showed reduced mutual shading among leaves when the plants were actually grouped in North-South rows. These observations contradict the current view that phytochrome-mediated responses to low R/FR are a relic from wild conditions, detrimental for crop yield.

Dynamics of Light and Nitrogen Distribution during Grain Filling within Wheat Canopy

In monocarpic species, during the reproductive stage the growing grains represent a strong sink for nitrogen (N) and trigger N remobilization from the vegetative organs, which decreases canopy photosynthesis and accelerates leaf senescence. The spatiotemporal distribution of N in a reproductive canopy has not been described in detail. Here, we investigated the role of the local light environment on the spatiotemporal distribution of leaf lamina N mass per unit leaf area (SLN) during grain filling of field-grown wheat (Triticum aestivum). In addition, in order to provide some insight into the coordination of N depletion between the different vegetative organs, N dynamics were studied for individual leaf laminae, leaf sheaths, internodes, and chaff of the top fertile culms. At the canopy scale, SLN distribution paralleled the light gradient below the flag leaf collar until almost the end of grain filling. On the contrary, the significant light gradient along the flag leaf lamina was not associated with a SLN gradient. Within the top fertile culms, the time course of total (alive + necrotic tissues) N concentration of the different laminae and sheaths displayed a similar pattern. Another common pattern was observed for internodes and chaff. During the period of no root N uptake, N depletion of individual laminae and sheaths followed a first-order kinetics independent of leaf age, genotype, or N nutrition. The results presented here show that during grain filling, N dynamics are integrated at the culm scale and strongly depend on the local light conditions determined by the canopy structure.

Multiple pathways regulate shoot branching

Shoot branching patterns result from the spatio-temporal regulation of axillary bud outgrowth. Numerous endogenous, developmental and environmental factors are integrated at the bud and plant levels to determine numbers of growing shoots. Multiple pathways that converge to common integrators are most probably involved. We propose several pathways involving not only the classical hormones auxin, cytokinins and strigolactones, but also other signals with a strong influence on shoot branching such as gibberellins, sugars or molecular actors of plant phase transition. We also deal with recent findings about the molecular mechanisms and the pathway involved in the response to shade as an example of an environmental signal controlling branching. We propose the TEOSINTE BRANCHED1, CYCLOIDEA, PCF transcription factor TB1/BRC1 and the polar auxin transport stream in the stem as possible integrators of these pathways. We finally discuss how modeling can help to represent this highly dynamic system by articulating knowledges and hypothesis and calculating the phenotype properties they imply.

Computer stereo plotting for 3-D reconstruction of a maize canopy

A method for building a three-dimensional (3-D) model of a vegetation canopy is presented. It is based on stereovision, the purpose of which is to reconstruct scenes in 3-D space from a pair of images taken from different viewpoints. We applied this method to maize photographs recorded from above by two photographic cameras and obtained a 3-D model of the vegetation. This model enables us to analyse the canopy geometrical structure and to carry out different simulation procedures. Estimations of leaf position and orientation, and of the leaf area distribution, are shown. Possible ways to automate and accelerate the processing, and some applications, are discussed.

Coupling a 3D virtual wheat (Triticum aestivum) plant model with a Septoria tritici epidemic model (Septo3D): a new approach to investigate plant–pathogen interactions linked to canopy architecture

This work initiates a modelling approach that allows us to investigate the effects of canopy architecture on foliar epidemics development. It combines a virtual plant model of wheat (Triticum aestivum L.) with an epidemic model of Septoria tritici which is caused by Mycosphaerella graminicola, a hemi-biotrophic, splashed-dispersed fungus. Our model simulates the development of the lesions from the infected lower leaves to the healthy upper leaves in the growing canopy. Epidemics result from the repeated successions of lesion development (during which spores are produced) and spores dispersal. In the model, canopy development influences epidemic development through the amount of tissue available for lesion development and through its effects on rain penetration and droplets interception during spore dispersal. Simulations show that the impact of canopy architecture on epidemic development differs between canopy traits and depends on climate. Phyllochron has the strongest effect, followed by leaf size and stem elongation rate.

Modeling maize canopy 3D architecture: Application to reflectance simulation

The aim of this study is to develop a 3D model of maize (Zea mays) canopy structure for accurate reflectance simulations. We focus on fully developed maize plants without paying attention to the reproductive organs. Several experiments are used to describe the dimension, shape, position and orientation of the leaves and stems. They correspond to a wide range of cultural practices. Empirical models are proposed to derive the position, size, and shape of the leaves and stems as a function of three main variables: the final number of leaves, the final height of the canopy and the cumulated leaf area per plant that is considered as a vigor index. Leaf orientation is described through the curvature of the main rib and the cross section used to represent the undulation of the lamina. The statistical distributions of the parameters of each structure sub-models are investigated. It allows to generate realistic computerized plant and canopy 3D architecture representation. The canopy structure is then used to compare the SAIL reflectance model to the reflectance simulated with PARCINOPY which is a monte-carlo ray tracing model. The combined use of detailed canopy architecture models and quasi exact radiative transfer models such as PARCINOPY appears to be a very convenient tool to evaluate more simple reflectance models applied to specific vegetation types. Results show a general good agreement between the two models, except for the exponential hot-spot function used in the SAIL model, and the directionality of the multiple scattering.

Evaluation of an improved version of SAIL model for simulating bidirectional reflectance of sugar beet canopies

Abstract The processing of remote-sensing data requires simple but accurate models of directional reflectance of the vegetation canopy. In this study, a reflectance model for a homogeneous canopy is evaluated over an extensive set of radiometric measurements performed on sugar beet canopies. The model corresponds to the Scattering by Arbitrary Inclined Leaves (SAIL) model (Verhoef, 1984) in which the term for first order scattering is corrected for hot-spot and leaf specular reflectance. Leaf optical properties are calculated using the PROSPECT model (Jacquemond and Baret, 1990). Experimental data correspond to a two-year experiment and express a large variability of leaf area index, chlorophyll concentration and soil background optical properties. In the first data set, reflectance was measured about midday under vertical viewing in five optical Thematic Mapper bands. In the second data set, both vertical and oblique measurements (zenith angle 45°, four azimuth angles) were performed from sunrise to sunset in the three SPOT bands. Except for leaf cuticle reflectance, structure and optical variables were measured in the field or adjusted to field measurement, independently of reflectance calculations. Although the structure of sugar beet canopies departs strongly from a turbid medium, a good agreement with measurements was obtained in the case of vertical, north and south view directions. However, the model underestimated the measurements close to the hot-spot direction. In the near infrared, there was also some underestimation of canopy reflectance in the opposite direction to the hot-spot. Possible reasons for these differences are discussed.