Stephan Blossfeld

Non-invasive approaches for phenotyping of enhanced performance traits in bean

Plant phenotyping is an emerging discipline in plant biology. Quantitative measurements of functional and structural traits help to better understand gene-environment interactions and support breeding for improved resource use efficiency of important crops such as bean (Phaseolus vulgaris L.). Here we provide an overview of state-of-the-art phenotyping approaches addressing three aspects of resource use efficiency in plants: belowground roots, aboveground shoots and transport/allocation processes. We demonstrate the capacity of high-precision methods to measure plant function or structural traits non-invasively, stating examples wherever possible. Ideally, high-precision methods are complemented by fast and high-throughput technologies. High-throughput phenotyping can be applied in the laboratory using automated data acquisition, as well as in the field, where imaging spectroscopy opens a new path to understand plant function non-invasively. For example, we demonstrate how magnetic resonance imaging (MRI) can resolve root structure and separate root systems under resource competition, how automated fluorescence imaging (PAM fluorometry) in combination with automated shape detection allows for high-throughput screening of photosynthetic traits and how imaging spectrometers can be used to quantify pigment concentration, sun-induced fluorescence and potentially photosynthetic quantum yield. We propose that these phenotyping techniques, combined with mechanistic knowledge on plant structure-function relationships, will open new research directions in whole-plant ecophysiology and may assist breeding for varieties with enhanced resource use efficiency varieties.

Quantitative imaging of rhizosphere pH and CO2 dynamics with planar optodes

BACKGROUND AND AIMS Live imaging methods have become extremely important for the exploration of biological processes. In particular, non-invasive measurement techniques are key to unravelling organism-environment interactions in close-to-natural set-ups, e.g. in the highly heterogeneous and difficult-to-probe environment of plant roots: the rhizosphere. pH and CO2 concentration are the main drivers of rhizosphere processes. Being able to monitor these parameters at high spatio-temporal resolution is of utmost importance for relevant interpretation of the underlying processes, especially in the complex environment of non-sterile plant-soil systems. This study introduces the application of easy-to-use planar optode systems in different set-ups to quantify plant root-soil interactions. METHODS pH- and recently developed CO2-sensors were applied to rhizobox systems to investigate roots with different functional traits, highlighting the potential of these tools. Continuous and highly resolved real-time measurements were made of the pH dynamics around Triticum turgidum durum (durum wheat) roots, Cicer arietinum (chickpea) roots and nodules, and CO2 dynamics in the rhizosphere of Viminaria juncea. KEY RESULTS Wheat root tips acidified slightly, while their root hair zone alkalized their rhizosphere by more than 1 pH unit and the effect of irrigation on soil pH could be visualized as well. Chickpea roots and nodules acidified the surrounding soil during N2 fixation and showed diurnal changes in acidification activity. A growing root of V. juncea exhibited a large zone of influence (mm) on soil CO2 content and therefore on its biogeochemical surrounding, all contributing to the extreme complexity of the root-soil interactions. CONCLUSIONS This technique provides a unique tool for future root research applications and overcomes limitations of previous systems by creating quantitative maps without, for example, interpolation and time delays between single data points.

Disentangling who is who during rhizosphere acidification in root interactions : combining fluorescence with optode techniques

Plant–soil interactions can strongly influence root growth in plants. There is now increasing evidence that root–root interactions can also influence root growth, affecting architecture and root traits such as lateral root formation. Both when species grow alone or in interaction with others, root systems are in turn affected by as well as affect rhizosphere pH. Changes in soil pH have knock-on effects on nutrient availability. A limitation until recently has been the inability to assign species identity to different roots in soil. Combining the planar optode technique with fluorescent plants enables us to distinguish between plant species grown in natural soil and in parallel study pH dynamics in a non-invasive way at the same region of interest (ROI). We measured pH in the rhizosphere of maize and bean in rhizotrons in a climate chamber, with ROIs on roots in proximity to the roots of the other species as well as not-close to the other species. We found clear dynamic changes of pH over time and differences between the two species in rhizosphere acidification. Interestingly, when roots of the two species were interacting, the degree of acidification or alkalization compared to bulk soil was less strong then when roots were not growing in the vicinity of the other species. This cutting-edge approach can help provide a better understanding of plant–plant and plant–soil interactions.

Light for the dark side of plant life: —Planar optodes visualizing rhizosphere processes

BackgroundThe simple term “rhizosphere” is well defined and has inspired numerous studies from a broad field of science since the beginning of the 20th century. However, we still know very little about the spatial and temporal heterogeneity of rhizosphere processes. This is mostly because assessing rhizosphere heterogeneity is not a trivial task. One technology for high-resolution and quantitative imaging of rhizosphere processes is called planar optode technology. This technology can create quantitative maps of key rhizosphere parameters non-invasively and has great potential to reveal new insights into this biogeochemical hotspot.ScopeRudolph and coworkers in this issue of Plant and Soil have used and improved the application of the planar optode technology for mapping rhizosphere pH dynamics. My commentary discusses the advantages and disadvantages of their approach and those of other published studies that deal with other planar optodes (like O2, CO2 or ammonium concentration) compared to conventional techniques.ConclusionsPlanar optodes represent a unique and powerful technology that can be used to investigate a range of rhizosphere processes. For sure there are more steps to take in order to tap the full potential of this technology: we now need concerted interdisciplinary approaches between biology, sensor chemistry and digital image analysis to scope out its full potential.

The Importance of Being First: Exploring Priority and Diversity Effects in a Grassland Field Experiment

Diversity of species and order of arrival can have strong effects on ecosystem functioning and community composition, but these two have rarely been explicitly combined in experimental setups. We measured the effects of both species diversity and order of arrival on ecosystem function and community composition in a grassland field experiment, thus combining biodiversity and assembly approaches. We studied the effect of order of arrival of three plant functional groups (PFGs: grasses, legumes, and non-leguminous forbs) and of sowing low and high diversity seed mixtures (9 or 21 species) on species composition and aboveground biomass. The experiment was set up in two different soil types. Differences in PFG order of arrival affected the biomass, the number of species and community composition. As expected, we found higher aboveground biomass when sowing legumes before the other PFGs, but this effect was not continuous over time. We did not find a positive effect of sown diversity on aboveground biomass (even if it influenced species richness as expected). No interaction were found between the two studied factors. We found that sowing legumes first may be a good method for increasing productivity whilst maintaining diversity of central European grasslands, although the potential for long-lasting effects needs further study. In addition, the mechanisms behind the non-continuous priority effects we found need to be further researched, taking weather and plant-soil feedbacks into account.

The Use of Planar Optodes in Root Studies for Quantitative Imaging

The present state-of-the-art optical sensors – planar optodes – as a technology for noninvasive bioprocess analysis for rhizosphere research are presented. The measuring principle of planar optodes is described very briefly and their advantage over conventional techniques is discussed. The different ways of application of planar optodes for the quantitative imaging of rhizospheric parameters like pH, O2, CO2, or ammonium concentration are described. The core of this chapter is a review of the application of planar optodes in rhizosphere research since the first approach 5 years ago, highlighting the great potential of this technology.