Crop photosynthesis

In the Crop photosynthesis theme we aim to understand the drivers of photosynthesis and seek for avenues to improve photosynthesis. It thus quantifies the link between the regulation of photosynthesis at cellular level and the ultimate growth of crops. It addresses a key question for future food production: to what extent can improved photosynthesis result in increased yields.

Thesis subjects

Come and prove our models wrong: Shining light on the real differences between C4 and C3 photosynthesis with theory-based experiments

Theory:

C4 species (e.g. maize and sorghum) have a higher photosynthetic capacity than C3 species (e.g. rice and wheat), because C4 crops can maintain a higher CO2 concentration at the site of Rubisco – the enzyme binding CO2 during photosynthesis. However, this comes at a cost of extra ATP; photosynthetic light use efficiency under limiting lights may not be higher in C4 than in C3 species. Our  theoretical models predict that C4 photosynthesis has a more linear light response curve than C3 photosynthesis, and that such a difference in shape has a direct consequence on the optimum vertical light and nitrogen distribution in canopies for a maximized canopy photosynthesis. For instance, compared with C4 species, C3 plants would benefit more from a more uniform light distribution and from the optimum nitrogen distribution in canopy.

Challenges:

  • In this project, you will be challenged to prove whether or not our models are wrong by addressing questions in boththeoryandreality: Theory: what are the consequences of the theoretical model predictions for the canopy photosynthesis of C3 and C4 species?
  • Reality: are these theoretical predictions found in comparable C3, C4 and C3-C4 intermediate species (e.g.Panicum,Moricandia,FlaveriaandAlloteropsis semialata) in actual experiments and measurements of photosynthesis?

Methods:

You will design and conduct a set of simultaneous photosynthesis measurements (gas exchange and chlorophyll fluorescence) under various temperature, CO2 and light conditions on leaves of these plants. At canopy levels, you will measure light and nitrogen profiles once canopy is established. The collected data will be combined with biochemical leaf-photosynthesis models for quantifying critical model parameters and sub-processes. These parameters will be incorporated into a canopy photosynthesis model considering light and N profiles.

Expectations:

In the end, you will be able to use these models as a research tool, for example, (i) to predict leaf photosynthesis for different environment scenarios, and (ii) to conduct a sensitivity analysis for identifying options with which leaf and canopy photosynthesis can be further enhanced. The obtained quantitative understanding of photosynthesis in general may have strong implications for crop production of both sustainable food and bio-energy supply.

Contact:

Dr Xinyou Yin, and Mr Peter van der Putten (Crop Physiology Chair, Centre for Crop Systems Analysis – CSA; Xinyou.yin@wur.nlpeter.vanderputten@wur.nl)

How dynamic is light inside canopies? Intercepting light in the real world

Description:

The conversion of light into biomass and harvestable product is one of the key processes that drive yield. Not only the amount of light intercepted by a crop canopy, but also the distribution of light is of great importance to maximize this so-called light use efficiency. Leaves within a canopy are exposed to a constantly changing light intensity, not only by the incoming sunlight, but also the movement of leaves in the upper part of the canopy. Even though this phenomenon is well known, it has not been well characterized. In theory, an optimal distribution of light within a canopy should increase the light use efficiency. However, these estimates have been made using a static light environment. So, it is the question to what extent the variability of light inside a canopy is of influence to the light use efficiency.

We have developed a methodology to quantify the light environment in a crop canopy and its variability over time. For this, a new open-source, low cost, rugged light sensor (ceptometer) is used to measure light inside a canopy (DOI: 10.3791/59447). The low construction cost of this ceptometer allows measurement with a number of these rugged ceptometers at multiple levels inside a canopy, where the dynamic light environment can be measured and logged under real-world conditions in the field. A field experiment is planned in Cambridge (UK), in collaboration with Dr Johannes Kromdijk (Cambridge University).

With these data, the light environment and its variation inside a canopy can be analysed. Further, this light environment can be modelled using Functional Structural Plant modelling and studied to find ways for optimization of light distribution inside a canopy.

Methods and expectations:

This project will involve hands-on (construction), field work and theoretical work (variation analysis and FSP modelling):

Please note:

  • this project, or parts of this project, can be done as a BSc thesis, MSc thesis or an internship
  • aspects of this project (e.g. field work, data analysis or FSP modelling) can be done as separate thesis projects

Contact:

Dr Steven Driever, Dr Alejandro Morales and Dr Jochem Evers (Crop Physiology Chair, Centre for Crop Systems Analysis – CSA; steven.driever@wur.nlalejandro.morales@wur.nl, jochem.evers@wur.nl)

Assessment of genetic variation in photosynthetic temperature response in wheat

Description:

The response to temperature is a key trait for breeding more resilient crops for future climates, where temperatures are predicted to rise by 2-4 °C by the end of the 21st century (Le Quere et al., 2009, Stocker et al., 2013). The impact of this temperature change on crop production has been estimated to be as much as a 6% reduction per °C (Zhao et al., 2017). 

One of the main physiological processes affected by temperature is photosynthesis. There is emerging evidence that leaf and canopy photosynthesis acclimate to changing temperature and that photosynthetic temperature responses vary considerably between genotypes. This variation can potentially be exploited, if the genetic and physiological variation can be characterized. However, there is currently no rapid, comprehensive method for assessment of photosynthetic temperature responses, something required for characterizing this genetic variation.

Challenge:

To develop a rapid screen for wheat, focused on CO2 assimilation rate in response to dynamic temperature change.

Methods and expectations:

This project will involve hands-on and theoretical work:

  • Design a rapid screening method for wheat, based on gas exchange and chlorophyll fluorescence techniques
  • Measure the variation of the photosynthetic temperature response in a number of wheat genotypes under controlled conditions
  • Evaluate the most varying parameter(s) of the photosynthetic temperature response among wheat genotypes

Contact:

Dr Steven Driever (Crop Physiology Chair, Centre for Crop Systems Analysis – CSA; steven.driever@wur.nl)

Quantitative analysis of photosynthesis of a super-performer

Description:

During photosynthesis, light energy is converted into chemical energy, which is then mainly used for the fixation of carbon dioxide. Insight into the quantitative aspects of photosynthesis and CO2 exchange has been very much advanced by experimentation and modelling. The classical biochemical model of Farquhar, Von Caemmerer and Berry has paved the way and this model has since been elaborated and improved. Making use of their concepts, it is possible to estimate a wide diversity of biologically relevant photosynthesis-related parameters based on a specific measuring protocol followed by several steps of curve fitting.

Plants of different natural habitats differ in their response to increases in the amount of light intercepted, i.e. plants differ in the decrease of light use efficiency with an increase in incoming radiation. A very good example of a species with a high light use efficiency even at very high levels of incoming light is shortpod mustard (Hirschfeldia incana; formerly Brassica geniculata). It is known for its large environmental plasticity and it grows well at elevated temperatures. It is therefore worthwhile to investigate in great detail the responses of this species to environmental factors influencing the rate of photosynthesis and to quantify in great detail what factors contribute to its high rate of photosynthesis. Equally important it is necessary to compare it with a crop species that is closely related. Based on such a quantitative comparison based on, supported by and enhanced by modelling, we can identify avenues for crop improvement.

Plants performing well under high light intensity may demonstrate a relatively high electron transport efficiency, a large proportion of electrons following more efficient alternative electron pathways, and/or a high Rubisco carboxylation efficiency. In all cases they also require a high rate of physical transport of CO2 to the carboxylation site and a low rate of loss of CO2 that arises from (photo)respiration. The latter aspects are very strongly influenced by biochemistry, leaf anatomical factors and the positioning of different organelles within the mesophyll cells of C3 plants. However, which of these (photochemical, biochemical, physical and anatomical) aspects (co)contribute most to the high photosynthetic productivity and plasticity of Hirschfeldia incana is yet to unravelled.

Methods and expectations:

We offer MSc students the opportunity to work on various aspects of photosynthesis of Hirschfeldia and other Brassicaceae, mainly related to their responses to environmental factors. We offer opportunities for experimental work, modelling topics and combinations of the two methodologies. We prefer combined modelling and experimental approaches. Depending on availability of facilities, the work can take place throughout the year. In most cases, collaboration with other chair groups will be part of the practical arrangements.

Contact:

Prof. Dr Paul Struik (paul.struik@wur.nl) and Dr Steven Driever (steven.driever@wur.nl)

Open-Abs: an open source method to quickly measure leaf absorbtance

Description:

Visible light is readily absorbed by leaves and used in photosynthesis. This light varies greatly in intensity over time and e.g. within a canopy. As leaves acclimate to light, their absorbtion of light is altered. Not only the total amount of light absorbed is changed, but also the absorbtion of different colours of light may change. In order to assess the changes that occur in light acclimation, a quick and easy measurement is needed. Up to date, leaf absorbtance measurements have only been possible with expensive and rare specialist equipment, such as a Taylor integrating sphere. As a result, this type of measurement is rarely performed in plant science. Moreover, there is currently no off-the-shelf equipment that can do this measurement in a simple manner.

These type of spheres used to be quite expensive, as well as spectrometers of sufficient resolution and accuracy. With the emergence of 3D printing and low-cost miniature spectrometers, this is no longer the case. These advances open the opportunity to develop an affordable open source integrating sphere system that can measure spectral leaf absorbance. The low cost of such system can increase the number of systems and in turn allow for more (rapid) leaf absorbtance measurements to be taken over time and space, providing opportunity to collect large amounts of data that can give new insight into the development and acclimation of leaves to light.

Challenge:

In this thesis you will develop, build and test a reliable, easy to use, open source method to instantly measure spectral leaf reflectance, transmittance and absorbtance, that can be operated under a range of conditions (e.g. in the field).

Methods and expectations:

This project will involve both hands-on and theoretical work

  • Design and build a dual-integrating sphere set-up
  • Test and improve set-up for spectral leaf absorbance measurements, using miniature spectrometers, on a range of different leaf types in a laboratory environment
  • Develop a proto-type integrating sphere system
  • Test the proto-type system in a greenhouse and field environment, to study leaf acclimation to light in a canopy

This thesis topic or parts thereof are suitable for BSc, MSc or internship projects.

Part of the work can be carried out at Cambridge University (UK), as this project will be done in close collaboration with Dr. Johannes (Wanne) Kromdijk at the Department of Plant Sciences, Cambridge University (UK).

Contact:

Dr Steven Driever and Dr Alejandro Morales (Crop Physiology Chair, Centre for Crop Systems Analysis – CSA; steven.driever@wur.nlalejandro.morales@wur.nl)

Effects of elevated CO2 and nitrogen availability on growth, photosynthesis and resource allocation

Description:

In the near future, the concentration of CO2 is predicted to further increase and this will have implications for crop production. Elevated [CO2] has been predicted to potentially enhance photosynthesis and increase productivity. In some species this has already been partly shown to be the case. However, the predicted levels of increase in yield, especially in biomass, are currently not achieved.

A vital requirement for crop productivity is the availability of nutrients such as organic nitrogen. High organic nitrogen input can greatly enhance crop productivity, but a too great supply is detrimental to both plant development and the environment. It is possible that organic nitrogen is a limiting factor for reaching the full potential of increased crop yield under elevated [CO2].

When organic nitrogen is not readily available legumes have the ability to acquire it through a symbiosis with Rhizobium. This symbiosis is energetically costly for the plant, however, with elevated [CO2] and increased photosynthesis, legumes might perform better compared to other plant species since there should be more carbohydrates available to invest in this symbiosis. Nevertheless, it is only possible to increase overall productivity if a plant is able to wisely allocate its resources (such as nitrogen, carbohydrates, etc.).

This project will look into the effects of elevated [CO2], organic nitrogen availability and resource allocation in the model legume Medicago truncatula. The project is a collaboration between Crop Systems Analysis and Molecular Biology.

Methods and expectations:

This project looks into the interaction between elevated [CO2], organic nitrogen, growth and photosynthesis in the model species Medicago truncatula. Potential thesis topics within this project are, for instance:

  • How does the plant allocate its carbon/nitrogen under elevated [CO2] in the presence or absence of symbiosis?
  • At which available organic N level (NO3-and/or NH4) is symbiosis beneficial to the plant’s productivity?
  • How much does photosynthesis need to increase to increase the plant’s N content through symbiosis?

Please note that these topics are examples, do not hesitate to contact us to discuss a topic or area of your interest!

Contact:

Dr Steven Driever (Crop Physiology, Centre for Crop Systems Analysis – CSA; steven.driever@wur.nl)

Dr Wouter Kohlen (Laboratory of Molecular Biology, wouter.kohlen@wur.nl)

What are the effects of nitrogen on the light use efficiency of C4 photosynthesis?

Desription:

Crops such as maize with so-called C4 photosynthesis are among the most productive and the benefits are such, that there are currently efforts to realize C4 photosynthesis in crops like rice. Because of this turbo-charged C4 photosynthesis, the water-use efficiency and nitrogen use efficiency are already quite high, especially under warm conditions. However, when temperatures are low, C4 photosynthesis decreases in efficiency. There are some C4 crops, such as Miscanthus x giganteus, that maintain their photosynthesis under lower temperatures due to an increase in the abundance of their most important photosynthetic enzymes. Increasing these enzymes requires quite some nitrogen, potentially reducing their nitrogen use efficiency.

At the same time, the light use efficiency of photosynthesis is reduced under low temperature due to a phenomenon called alternative electron transport. This is a form of photosynthetic electron transport (as part of the so-called light reactions) that is not used for the assimilation of CO2, but for something else. The alternative electron can be quite substantial and reduces the light use efficiency of photosynthesis which is detrimental to the growth and production of the plant. However, this alternative electron transport could potentially be used to provide energy for processes in the leaf other than CO2 assimilation. One of such processes is the reduction of nitrate to ammonia, which is energetically quite costly. This could potentially benefit the plant, by making more nitrogen available, for instance for the formation of additional photosynthetic enzymes and increase its productivity.

In this project, we will focus on the effects of the availability of different forms of nitrogen (nitrate and ammonia) to young maize plants and assess the role of nitrate reduction in relation to photosynthesis and alternative electron transport. If effects of different nitrogen forms are found, this will be investigated further in response to stress, such as low growth temperature.

Methods and expectations:

This project will involve a combination of a growth experiment in a controlled environment and physiological experimentation with gas exchange and chlorophyll fluorescence techniques. Additionally, leaf material will be analyzed for its (different forms of) nitrogen content.

Depending on the chosen duration of the thesis, research interest and background of the candidate, the thesis topic can focus on any or part of the following topics:

  • What is the effect of nitrate and ammonia on the leaf N composition of young maize leaves?
  • What is the relation between growth temperature, nitrate and photosynthetic light use efficiency of young maize leaves?
  • Does alternative electron transport increase in response to limiting ammonia availability in young maize leaves compared to Miscanthus leaves?

Please note that these topics are examples, do not hesitate to contact us to discuss a topic or area of your interest!

Contact:

Dr Steven Driever (Crop Physiology, Centre for Crop Systems Analysis – CSA; steven.driever@wur.nl)

Improving crop light-use efficiency: Exploiting differences in the energy budget among subtypes of C4 plants to boost crop productivity

Theory:

C4 crops of agricultural importance all belong to the NADP-ME subtype, and this subtype has been the template for C4 introductions into C3 crops to improve their productivity. However, the ATP cost for the C4 cycle in both NADP-ME and NAD-ME subtypes accounts for >40% of the total ATP requirement for CO2 assimilation. These high ATP costs, and the associated need for intense cyclic electron transport (CET) and low intrinsic quantum efficiency of CO2-assimilation, are major constraints in realising strong improvements of canopy photosynthesis and crop productivity. Based on mathematical modelling, we have recently proposed a C4 ideotype with low chloroplastic ATP requirements present as in the non-domesticated PEP-CK subtype. The ideotype is a mixed form of NAD(P)-ME and PEP-CK types, requires no CET, and its theoretical quantum efficiency is much higher than that of a crop C4 type. Its cell-type-specific ATP and NADPH requirements can be fulfilled by local energy production. The ideaotype is projected to have ca. 10% yield advantage over current C4 crops, and >50% advantage over C3 counterparts. The ideotype provides a unique (theoretical) case of improved quantum efficiency, thereby paving a new avenue for improving photosynthesis in both C3 and C4 crops. This thesis work aims to first identify whether there are C4 species in nature, super CO2-assimilators, that are identical or close to the designed C4 ideotype. We then apply a set of experimental approaches, which will be combined with model analysis. This combined approach can identify the key physiological requirements for the C4 ideotype. The ideotype key-requirements and any super CO2-assimilators can be exploited for improvement of the photosynthetic performance and boost productivity of crops.

Methods:

Based on our recent Viewpoint (https://nph.onlinelibrary.wiley.com/doi/abs/10.1111/nph.17011) in this area, you will screen C4 species of different subtypes and then conduct a set of simultaneous photosynthesis measurements (gas exchange and chlorophyll fluorescence) under O2 and light conditions on leaves of these plants. The collected data will be combined with biochemical C4 leaf-photosynthesis models for quantifying critical model parameters and sub-processes. This information will be used to identify if there are species identical or close to the theoretical ideotype.

Expectations:

In the end, you will be able to use the models as a research tool for identifying options with which leaf photosynthetic light-use efficiency can be improved. The obtained quantitative understanding of differences in photosynthesis among C4 subtypes may have strong implications for improving production of both C3 and C4 crops.

Contact:

Dr Xinyou Yin, and Mr Peter van der Putten (Crop Physiology Chair, Centre for Crop Systems Analysis – CSA; Xinyou.yin@wur.nl)