In the Functional diversity theme we apply evolutionary game theory (e.g. tragedy of commons, kin selection) and ecological theory to better the mechanisms that drive interactions among crop plants and interactions with other organisms (e.g., weeds, pests and pathogens) within crop ecosystems. We use this knowledge to explore opportunities for ecological intensification of agriculture by exploiting spatial and temporal diversity, e.g., by mixed species cropping (intercropping, agroforestry), crop rotation and variety mixtures.
Plants use many cues to get the latest news on their environment, from volatiles released by insect-infested neighbouring plants to light signals from different parts of the spectrum. An important cue for plants to detect competitors and future shading is the ratio between red light (absorbed by foliage) and far-red light (reflected by foliage). In response to a low red to far-red (R:FR) ratio, plants elongate stems and petioles and change leaf angles to keep up with their neighbours. In addition, a low R:FR ratio affects defence levels by repressing the activity of the jasmonate pathway. As a consequence of R:FR mediated defence, insect herbivores feeding on plants exposed to low R:FR perform better. However, recent experimental results in which caterpillars were feeding on low R:FR exposed plants show that these effects are species dependent. Moreover, results of a pilot experiment where caterpillars were fed on an artificial food source, suggest that R:FR can have a direct effect on the growth rate of caterpillars.
In this thesis project you will further explore to what extent herbivores can directly respond to R:FR cues. This will be done by studying herbivore behaviour under contrasting levels of R:FR through various means. For example, one could look at feeding preference or feeding activity under various R:FR regimes. We are looking for a creative, motivated student with good experimental skills. We offer a challenging thesis supervised by enthusiastic supervisors.
Quantifying the priority effect: how important are traits of young root systems for intercrop performance?Item titel
Intercropped crops benefit from facilitation (species helping each other in taking up resources) and niche complementary (species looks for resources in different places or at different moments), which may eventually lead to overyielding. However, negative interactions between two crop species in an intercrop may also happen, when they are competing for resources in the same niche. Below-ground, at the early growth stages when the root systems of both species are not fully developed yet, the soil exploration rate is of vital importance for soil resource competition. This root system exploration at early stages is determined by the seed mass and the time of germination. Hence the advantage due to larger seed mass and/or early germination of one species could result in suppression of root growth of the other species and eventually influence crop performance in later stages. This priority effect on root growth and below-ground competition have hardly been studied in agroecosystems, but mainly in natural ecosystems. In this study, we aim to quantify the priority effect during early development stage for the wheat/faba bean intercropping system. Such knowledge is crucial in the understanding of intercrop performance and the optimization of crop system management for high yielding crops at sustainable input levels.
Types of work:
We offer the interested MSc student the opportunity to participate in a pot experiment on wheat/faba bean mixtures in the greenhouse at Unifarm in the context of a PhD project on intercropping. Shoot and root traits will be observed during the vegetative period. Roots will be scanned and analyzed. Additionally, shoot material will be analyzed for its nitrogen content.
Wageningen (WUR Crop Systems Analysis)
February 2021, for a period of 6 months (specific dates can be negotiated)
Intensive agriculture is an effective pathway to meet the increasing demand for food, feed and fuel in our limited arable land. Intercropping has been proved having higher productivity, better agriculture ecosystem services than monoculture at field level. Regional assessment of the role of intercropping in food security is needed, thus the potential yield of intercropping should be determined and tools need to be developed to assess it under varying conditions. Two years of wheat maize intercropping field trials have been conducted in Wageningen, the Netherlands. The characteristics of plant development and growth, yield and its components, radiation use efficiency, maize photosynthesis rate during flowering period, nitrogen uptake and use efficiency in different planting configurations are going to be analysed. An intercropping model will be parameterized to explore the yield potential for varying planting configurations and growing conditions.
Types of research:
Laboratory work in Unifarm, experimental data analysis, crop modelling
For details on potential thesis subjects contact Wopke van der Werf (firstname.lastname@example.org)
Pyrethrum Tanacetum cinerariifolium and some other species in the genus Tanacetum are perennial species that produce pyrethrins that are used as natural pesticides, especially in organic agriculture. The ecological significance of pyrethrin production is known to be both protection of the plant against insects and, as pyrethrin accumulate in flowers and seeds, protection of offspring against fungal attacks. However, Pyrethrum is also insect pollinated probably primarily by thrips and like other flowering plants the flower is designed to attract pollinators. Pollination is known to be poor in general, however. Thus, an interesting combination arises, as flowers are both made to attract pollinators and at the same are loaded with pyrethrins that deter insects. It has been hypothesized that this represents a so-called push-pull strategy by the plant, in which an ecological balance between sufficient reproduction and annual survival is reached. Roughly this idea entails that these flowers are cleverly designed to attract sufficient pollinators but simultaneously deter potential herbivores so that reproductive success is not diminished. Understanding this dynamic is ecologically highly relevant as it leads to better understanding of why flowers are designed the way they are, but is of major agricultural importance as well, because pollination is also correlated to pyrethrin content and therefore economically important.
An experiment was conducted in which different Pyrethrum accessions with unknown variation in Pyrethrin content were grown and were either exposed to thrips or not. Flowers were collected both at the pollination stage (stored in alcohol) and at the ripe stage for seed experiments. The question to be addressed here is if there is correlation between pyrethrin content and the number of thrips on the flowers and also whether there is a relation between the presence of thrips and pollination success. Experiments will be conducted (i) to see whether the presence of thrips has influenced the number of fertile seeds, and (ii) whether the pyrethrin content has affected the thrips numbers. In addition we have a suite of accessions from a wild sister species with a known variation in pyrethrin contents and some distinct differences with pyrethrum. These are growing in a field plot and can be characterized in various ways. Several types of measurements or small experiments are possible.
The project is supvervised by Niels Anten (CSA) en Maarten Jongsma (PRI)
Niels Anten (email@example.com)
Weeds are a serious biotic production constraint in most agricultural production systems. Acting at the same trophic level as the crop, weeds capture resources that cannot anymore be used by the crop. Therefore, leaving weeds uncontrolled will sooner or later lead to considerable reductions in crop yield.
Curative weed control is mainly focussed on weed seedlings and is strongly dominated by the use of herbicides. This heavy reliance on chemical control is considered objectionable because of potential negative side-effects on food safety, public health and the environment. Additionally, cropping systems with a narrow focus on herbicidal control are less sustainable, due to an increased risk regarding the development of herbicide resistance.
Cultural control refers to any adjustment of the general management of the crop that contributes to the regulation of weed populations and reduces the negative impact of weeds on crop production. This preventative approach addresses a variety of life cycle stages and relevant processes, like the weed soil seed bank, seed recruitment, weed seed production and seed predation. Various measures like photo-control, bio-fumigation, mulching, stale seedbeds, transplanting, weed suppressive cultivars and no till systems potentially contribute to this kind of ecological weed management.
The questions that remain are manifold, just to mention a few:
- Which life cycle stage of the weeds can best be tackled?
- How effective and reliable are individual cultural control measures?
- Do the measures combined provide synergistic effects?
- Are weed community changes likely to result from a modified management strategy?
- What is the role of crop rotation in ecological weed management?
Types of research:
- Literature review with follow up analyses of already published data
- Experimental approaches to evaluate individual measures
- Population dynamical models to evaluate control measures and to study changes in weed community composition.
Lammert Bastiaans (firstname.lastname@example.org)
Herbicidal resistance against blackgrass (Alopecurus myosuroides) is frequently reported in regions with continuous cropping of winter cereals. In the Netherlands, the problem is steadily increasing in ‘de Oldambt’ in North-east Groningen. This area is characterized by heavy clay soils, making the production of root crops virtually impossible, whereas the gross margin of many mown crops is simply too low to be an attractive alternative. Consequently, continuous cropping of winter cereals is common practice. The low number of herbicides available for managing weeds in these crops results in repeated use of just a few compounds; an ideal environment for the development of herbicide resistance. Problems with blackgrass have steadily increased in recent years. Integrated weed management strategies, including cultural control measures and crop rotation, are needed to lessen the problem. In this study, the ecological and biological properties of the weed will be investigated and options to undermine the success of the species will be identified. A weed population model will be developed to quantitatively estimate the effectivity of the proposed measures in the long run. Parameterization of the blackgrass model will be based on literature search, expert opinion and experimentation.
Lammert Bastiaans (email@example.com)
Weeds are a serious biotic production constraint in most agricultural production systems. Acting at the same trophic level as the crop, weeds capture resources that cannot anymore be used by the crop. A sustainable way to suppress weed growth in a crop canopy is by improving crop competitiveness. This can be achieved by changing architectural traits of the crop plants that enhance its competitive strength, such as early soil cover, as well as optimized leaf orientation and branching patterns. Crop competitiveness can also be improved by changing population characteristics such as crop population density, presence of secondary competitive crops or the uniformity of crop plant arrangement. In all cases, the balance between competition with weed plants (interspecific competition) and competition among crop plants (intraspecific competition) will determine whether the increased crop competitiveness will result in improved weed suppression.
Experimental and modelling work conducted in recent years has shown that substantial gain in crop competitiveness is to be expected by combined optimization of plant and canopies characteristics. The focus of this thesis work is to explore the balance between inter- and intraspecific competition for light in crop-weed canopies, in relation to plant and canopy characteristics. Potential questions to be addressed are: how does canopy uniformity relate to weed suppression? Which crop traits make a crop more competitive with weeds and how does this depend on population density? What is the influence of the moment of weed emergence in this? Which crop plant architecture is optimal for weed suppression? To address such questions, a 3D functional-structural plant (FSP) model of crop-weed interactions is available that simulates growth and development of individual crop and weed plants in a specific arrangement.
Types of work
This work is a modelling study. There is flexibility in which questions will be explored using the FSP model, but it will be limited to competition for light. The model available can be used as it is, requiring only parameter value adjustments, or it can be adapted and modified to accommodate simulation of processes it is currently lacking – this is up to the student’s learning goals. The topic is suitable for students who are interested in plant-plant interactions and ecological weed control, but also for those who wish to develop FSP modelling skills.
Wageningen (WUR Crop Systems Analysis)
Pixel cropping (or pixel farming) is the practice of growing multiple crop species in complex arrangements in which communities of plants are spatially allocated at a fine resolution. The goal is to create diverse configurations of multiple crop species in which the right plant community is allocated to the right location, at the right time, and at the optimal resolution. The concept is grounded in the diversity—productivity theory, and evidence that diversification in agro-ecosystems is necessary to reduce damaging externalities caused by industrial monocultures.
We are currently conducting a preliminary pixel cropping experiment in the field at the Droevendaal Organic Experimental farm in Wageningen. Because we do not yet have the scientific knowledge to design ecologically optimal pixel cropped fields, in the experiment we have implemented superficial design rules: limiting the number of crop species, and allocating all plant communities to a uniform pixel size and shape (50 cm x 50 cm) within a fixed grid. Crops are randomly assigned to each pixel in equal proportion.
The challenge of designing a good pixel cropping plan brings up many questions, such as:
- How do plants selected for monocultures respond (in terms of morphology, yield, service provision, etc.) to being grown in heterogeneous communities?
- What is the optimal pixel resolution for each crop?
- Which crop combinations make good and bad neighbors?
- Which particular plant traits make a species suitable for pixel cropping?
Acquiring the knowledge needed to design optimal pixel plots could involve conducting field experiments in which the complexity of the experimental setup is exponentially amplified to accommodate all possible neighbor and pixel size/shape combinations for the crops of interest. Alternatively, modeling offers a platform to explore designs and interactions out of the field. Our goal is to develop a simulation model that can capture the spatial heterogeneity and species diversity typical for pixel crop designs. The modelling approach to be used here is called functional-structural plant (FSP) modelling, in which individual plants are simulated in inter- and intraspecific competition with neighbouring plants. Ultimately, such a model will allow to address questions on plant response, traits, pixel resolution and optimal neighbour combinations.
The specific aim of this Master thesis is to develop a pixel cropping FPS model that captures the main components of a typical pixel crop system, using data from the pixel cropping field experiments and the modelling tools already available.
Type of work
Modelling, data analysis
Lenora Ditzler (FSE, firstname.lastname@example.org) or Jochem Evers (email@example.com)
Plant density is a key factor in regulating pest and disease epidemics. According to the resource concentration hypothesis plant density will positively affect density of insects and pathogens. With increasing planting density, microclimate alters, movement between plants becomes easier and plant defenses may be lowered because of increased investment in competition. A qualitative scoring of papers publishing the effects of plant density on the abundance of phytophagous insects showed a positive affect of plant density on host abundance in only ~34% of the studies (Rhainds & English-Loeb, 2003). Others reported as positive effect in ~57% of the studies. As the effect of plant density on pest abundance can be both positive and negative, a better understanding is needed to what determines the relationship between host density and pest abundance. A better understanding on the relative contribution of factors that contribute to disease epidemics as a function of host density is crucial in both managed and natural ecosystems.
Type of work
In this thesis project you will do a quantitative review of the literature (meta-analysis) to investigate the overall relationship between plant density and disease prevalence, and to try uncover factors that affect this relationship. This involves a lot of reading, data, statistics and fun.
I am looking for a motivated student that is interested in this relevant topic, and likes to do a theoretical study using literature and statistics.
Bob Douma (firstname.lastname@example.org, 0317-482140)