Jérémie Lecoeur

Competition, traits and resource depletion in plant communities

Although of primary importance to explain plant community structure, general relationships between plant traits, resource depletion and competitive outcomes remain to be quantified across species. Here, we used a comparative approach to test whether instantaneous measurements of plant traits can capture both the amount of resources depleted under plant cover over time (competitive effect) and the way competitors perceived this resource depletion (competitive response). We performed a large competition experiment in which phytometers from a single grass species were transplanted within 18 different monocultures grown in a common-garden experiment, with a time-integrative quantification of light and water depletion over the phytometers’ growing season. Resource-capturing traits were measured on both phytometers (competitive response traits) and monocultures (competitive effect traits). The total amounts of depleted light and water availabilities over the season strongly differed among monocultures; they were best estimated by instantaneous measurements of height and rooting depth, respectively, performed when either light or water became limiting. Specific leaf area and leaf water potential, two competitive response traits measured at the leaf level, were good predictors of changes in phytometer performance under competition, and reflected the amount of light and water, respectively, perceived by plants throughout their lifespan. Our results demonstrated the relevance of instantaneous measures of plant traits as indicators of resource depletion over time, validating the trait-based approach for competition ecology.

A Three-dimensional Statistical Reconstruction Model of Grapevine (Vitis vinifera) Simulating Canopy Structure Variability within and between Cultivar/Training System Pairs

BACKGROUND AND AIMS In grapevine, canopy-structure-related variations in light interception and distribution affect productivity, yield and the quality of the harvested product. A simple statistical model for reconstructing three-dimensional (3D) canopy structures for various cultivar-training system (C x T) pairs has been implemented with special attention paid to balance the time required for model parameterization and accuracy of the representations from organ to stand scales. Such an approach particularly aims at overcoming the weak integration of interplant variability using the usual direct 3D measurement methods. MODEL This model is original in combining a turbid-medium-like envelope enclosing the volume occupied by vine shoots with the use of discrete geometric polygons representing leaves randomly located within this volume to represent plant structure. Reconstruction rules were adapted to capture the main determinants of grapevine shoot architecture and their variability. Using a simplified set of parameters, it was possible to describe (1) the 3D path of the main shoot, (2) the volume occupied by the foliage around this path and (3) the orientation of individual leaf surfaces. Model parameterization (estimation of the probability distribution for each parameter) was carried out for eight contrasting C x T pairs. KEY RESULTS AND CONCLUSIONS The parameter values obtained in each situation were consistent with our knowledge of grapevine architecture. Quantitative assessments for the generated virtual scenes were carried out at the canopy and plant scales. Light interception efficiency and local variations of light transmittance within and between experimental plots were correctly simulated for all canopies studied. The approach predicted these key ecophysiological variables significantly more accurately than the classical complete digitization method with a limited number of plants. In addition, this model accurately reproduced the characteristics of a wide range of individual digitized plants. Simulated leaf area density and the distribution of light interception among leaves were consistent with measurements. However, at the level of individual organs, the model tended to underestimate light interception.

High temperature and water deficit may reduce seed number in field pea purely by decreasing plant growth rate

Seed number, the most variable yield component of legumes is strongly affected by heat stress (HS) and water deficit (WD). The objective of this paper is to investigate whether HS and WD reduced seed number in field pea through their negative effects on biomass production rather than by specific effects on the developing reproductive organs. Several field and glasshouse experiments were carried out in southern France, in which HS and / or WD of various intensities, durations and positions in the plant lifecycle were imposed on several pea cultivars. WD and HS reduced seed number, in an intensity-dependent manner. They also changed the distribution of seeds along the stem. Plants subjected to WD and mild HS had more seeds on the basal phytomers than did control plants, making it possible to exclude direct effects of stress on seed development. In contrast, severe HS resulted in the immediate abortion of reproductive organs. WD and HS also decreased net photosynthesis (Pn), but only during the period of constraint. Quantitative relationships between Pn and soil water status and between Pn and leaf temperature were established. Nevertheless, in all cases there was a single linear relationship between final seed number and plant growth rate during the critical period for seed set (from the beginning of flowering to the beginning of seed fill for the last seed-bearing phytomer). This reflects the reproductive plasticity of pea, which adjusts the number of reproductive sinks in an apparent balance with assimilate availability in the plant.

A model-based analysis of the dynamics of carbon balance at the whole-plant level in Arabidopsis thaliana

Arabidopsis thaliana (L.) Heynh. is used as a model plant in many research projects. However, few models simulate its growth at the whole-plant scale. The present study describes the first model of Arabidopsis growth integrating organogenesis, morphogenesis and carbon-partitioning processes for aerial and subterranean parts of the plant throughout its development. The objective was to analyse competition among sinks as they emerge from patterns of plant structural development. The model was adapted from the GreenLab model and was used to estimate organ sink strengths by optimisation against biomass measurements. Dry biomass production was calculated by a radiation use efficiency-based approach. Organogenesis processes were parameterised based on experimental data. The potential of this model for growth analysis was assessed using the Columbia ecotype, which was grown in standard environmental conditions. Three phases were observed in the overall time course of trophic competition within the plant. In the vegetative phase, no competition was observed. In the reproductive phase, competition increased with a strong increase when lateral inflorescences developed. Roots and internodes and structures bearing siliques were strong sinks and had a similar impact on competition. The application of the GreenLab model to the growth analysis of A. thaliana provides new insights into source-sink relationships as functions of phenology and morphogenesis.

Change with time in potential radiation-use efficiency in field pea

In most crop models, radiation-use efficiency (RUE) is critical to the calculation of yield. Daily biomass production is usually calculated as the product of intercepted solar radiation and RUE which is assumed to be constant. In this work, field experiments were conducted with field pea (Pisum sativum L.) to assess the stability of RUE during the plant growth cycle, in the absence of biotic and abiotic stresses. Two contrasting locations with regard to solar radiation and air temperature were used in France. At each location, various combinations of years, genotypes, sowing dates and densities were studied. The results demonstrated that RUE is not constant during the plant growth cycle but that a consistent sigmoidal pattern can be defined. During vegetative growth, RUE declined from emergence to a low point before flowering. Once in the reproductive stage, RUE increased to reach a maximum level soon after the beginning of seed filling. Thereafter, it declined to zero upon the death of the plant. The level and timing of change in RUE were affected by sowing date, genotype and plant density. However, the sigmoidal pattern was unaffected by these factors and was not related to air temperature. The specific pattern of RUE probably results from the appearance, growth and subsequent senescence of such plant organs as roots, shoots, leaves, pods and seeds and also changes in those meteorological variables which affect photosynthetic plant capacity. Accurate estimation of RUE is important for the quantification of plant productivity in fluctuating environmental conditions and more mechanistic studies on changes in RUE are needed to investigate the causes of these changes and their consequences for plant productivity.

Integrated responses of rosette organogenesis, morphogenesis and architecture to reduced incident light in Arabidopsis thaliana results in higher efficiency of light interception

Plants have a high phenotypic plasticity in response to light. We investigated changes in plant architecture in response to decreased incident light levels in Arabidopsis thaliana (L.) Heynh, focusing on organogenesis and morphogenesis, and on consequences for the efficiency of light interception of the rosette. A. thaliana ecotype Columbia plants were grown under various levels of incident photosynthetically active radiation (PAR), with blue light (BL) intensity proportional to incident PAR intensity and with a high and stable red to far-red light ratio. We estimated the PAR absorbed by the plant, using data from precise characterisation of the light environment and 3-dimensional simulations of virtual plants generated with AMAPsim software. Decreases in incident PAR modified rosette architecture; leaf area decreased, leaf blades tended to be more circular and petioles were longer and thinner. However, the efficiency of light interception by the rosette was slightly higher in plants subjected to lower PAR intensities, despite the reduction in leaf area. Decreased incident PAR delayed leaf initiation and slowed down relative leaf expansion rate, but increased the duration of leaf expansion. The leaf initiation rate and the relative expansion rate during the first third of leaf development were related to the amount of PAR absorbed. The duration of leaf expansion was related to PAR intensity. The relationships identified could be used to analyse the phenotypic plasticity of various genotypes of Arabidopsis. Overall, decreases in incident PAR result in an increase in the efficiency of light interception.

Using a 3-D virtual sunflower to simulate light capture at organ, plant and plot levels : Contribution of organ interception, impact of heliotropism and analysis of genotypic differences

BACKGROUND AND AIMS Light interception is a critical factor in the production of biomass. The study presented here describes a method used to take account of architectural changes over time in sunflower and to estimate absorbed light at the organ level. METHODS The amount of photosynthetically active radiation absorbed by a plant is estimated on a daily or hourly basis through precise characterization of the light environment and three-dimensional virtual plants built using AMAP software. Several treatments are performed over four experiments and on two genotypes to test the model, quantify the contribution of different organs to light interception and evaluate the impact of heliotropism. KEY RESULTS This approach is used to simulate the amount of light absorbed at organ and plant scales from crop emergence to maturity. Blades and capitula were the major contributors to light interception, whereas that by petioles and stem was negligible. Light regimen simulations showed that heliotropism decreased the cumulated light intercepted at the plant scale by close to 2.2% over one day. CONCLUSIONS The approach is useful in characterizing the light environment of organs and the whole plant, especially for studies on heterogeneous canopies or for quantifying genotypic or environmental impacts on plant architecture, where conventional approaches are ineffective. This model paves the way to analyses of genotype-environment interactions and could help establish new selection criteria based on architectural improvement, enhancing plant light interception.

Harvest index increase during seed growth of field pea

Harvest index during seed growth has been reported to increase linearly with time for many crop species. Although the rate of harvest index increase was generally stable across experiments within a species, there are indications that it is sensitive to variations in temperature. The objective of the present study was to compare the harvest index increase for field pea (Pisum sativum L.) across 31 experiments that included a wide range of temperature environments. The change in harvest index within each experiment was well described by a linear increase with time but there was considerable variability among experiments. The increase in harvest index was also calculated as a function of thermal time after flowering. The variability was only marginally reduced. Use of thermal time was, however, superior in identifying a common point for the initiation of seed growth among the experiments.

Influence of intra-shoot trophic competition on shoot development in two grapevine cultivars (Vitis vinifera)

The effect of trophic competition between vegetative sources and reproductive sinks on grapevine (Vitis vinifera L.) shoot development was analyzed. Two international cultivars (Grenache N and Syrah) grown in pots, which were well watered, were studied. A large range of trophic competition levels was obtained by modifying the cluster loads per plant. An analytical breakdown of the branching system was used to analyze the effects of trophic competition. Phytomer production on the primary axis and the probability and timing of axillary budburst were not affected by trophic competition. However, the duration of development and leaf production rate for secondary axes were both significantly affected. The impact of trophic competition differed within the P0-P1-P2 architectural module, locally within the shoot and between cultivars. Trophic competition reduced the organogenesis of secondary axes most strongly close to clusters, on P1-P2 phytomers and in Grenache N. Based on these results, a modeling approach simulating sink strength variation and the local effects of sink proximity would be more relevant than a model considering only development as a function of thermal time or the global distribution of available biomass.

A Stochastic Growth Model of Grapevine with Full Interaction Between Environment, Trophic Competition and Plant Development

Grapevine development is mainly determined by environmental factors whose effects are modulated by its complex topological structure. The trophic relationships between all the organs of the different axes appear to be the main underlying process which drive axis organogenesis in fluctuating environment. A new modelling approach is proposed based on GreenLab formalism in which axis organogenesis is controlled by stochastic processes according to trophic competition between the different axes. In this model, a water budget was also implemented to account for the effects of water depletion. The model was validated at organ and axis scales on a large range of environmental conditions in terms of photosynthetic active radiation, temperature and soil water supply. The efficiency of the model to simulate plant development at a detailed scale proved its ability to further analyse of the retroactions between plant development and the different environmental variables in order to improve crop management.