Manganese (Mn) is an essential micronutrient for plants, but at high concentrations, it can become toxic. However, Eucalyptus tereticornis appears to be remarkably tolerant to Mn, even at levels well above those that would cause harm to other plant species. The mechanism(s) underlying this ability were not understood based on scientific literature. From a study that monitored the Mn absorption in these plants, published in the Journal of Hazardous Materials, researchers from the Department of Plant Biology at the State University of Campinas (Unicamp) and from The Brazilian Synchrotron Light Laboratory (LNLS), from the Brazilian Center for Research in Energy and Materials (CNPEM), demonstrated how E. tereticornis can tolerate and detoxify high levels of Mn in its environment.
The article, entitled “Tissue-level distribution and speciation of foliar manganese in Eucalyptus tereticornis by µ-SXRF and µ-XANES shed light on its detoxification mechanisms” led by Vinicius H. De Oliveira at Unicamp, presents the locations in the plant organism where Mn is accumulated, in what forms this element is assimilated, and even elucidates some of the mechanisms responsible for this ability to tolerate high concentrations of the metal used by E. tereticornis. This characteristic could be explored for environmental remediation purposes, particularly in contaminated soils.
According to LNLS Soil Science Advisor, Dr. Dean Hesterberg, one of the article’s authors, it is not just the total Mn concentration that is important for understanding contaminated soils. “In acidic soils and especially under reducing redox conditions, manganese minerals are more soluble, which generally increases Mn availability for plant uptake. This can impact plants, which mainly absorb dissolved Mn. And, in Brazil, there are many acidic soils”, says Hesterberg.
Synchrotron radiation imaging techniques
To gain evidence of how eucalyptus tolerates Mn-rich soils, researchers mapped the Mn distribution within Eucalyptus tereticornis leaves over time. This was possible through advanced techniques available at the Carnaúba beamline of the electron accelerator and synchrotron light generator, Sirius. The techniques used in the work included synchrotron micro scanning X-ray fluorescence imaging (µ-SXRF) and micro X-ray Absorption Near-Edge Structure (µ-XANES) spectroscopy.
Both use synchrotron radiation, a type of light released when electrons are accelerated to speeds very close to that of light. This usually happens by making them travel in a circular path, through strong magnetic fields, as is the case with the Sirius machine. Synchrotron light is incredibly bright and tunable over a wide range of wavelengths. In this way, the Carnaúba beamline uses light at X-ray wavelengths produced by the Sirius accelerator.
µ-SXRF
The synchrotron micro scanning X-ray fluorescence imaging (µ-SXRF) technique is used to investigate the elemental distribution and composition of materials on a microscopic scale. Fluorescence occurs because when materials are exposed to X-rays, atoms in the sample are excited and emit secondary (or fluorescent) X-rays when de-excited. The energy of these emissions serves as a fingerprint of each chemical element. This allows scientists to identify and quantify the composition of the studied material.
LNLS/CNPEM researcher Dr. Carlos Alberto Pérez, one of the study’s authors, explains a little about how the technique works. “The µ-SXRF works based on X-ray optical equipment. The equipment has a monochromator, a crystal that defines a specific energy for the sample excitation. Another part of the equipment is the nanofocusing of this monochromatic light. This way, an X-ray beam that is about 100 times smaller than a human hair is created”.
Through this beam of light, researchers are able to scan the sample, point by point, which generates an image with thousands of pixels. X-ray fluorescence is emitted as the beam hits each of these points. At the end, the pixels are computed using a program to generate an image, called elemental map.
Elemental maps can be constructed for several specific chemical elements. In the case of the research published in the Journal of Hazardous Materials, the group of scientists assembled the elemental map of Mn in eucalyptus tissues. Thus, they were able to compare the presence of Mn in the plant’s leaf tissues, when it grew with an abundance of Mn and when it grew with normal amounts of the metal.
μ-XANES
Micro-X-Ray Absorption Near Edge Structure (μ-XANES) spectroscopy, in turn, is used to probe the chemical state and electronic structure of specific elements in a sample. It is a sub-technique of X-ray Absorption Spectroscopy (analysis of how a sample absorbs X-rays), focusing on the energy band near the absorption edge of the element being studied. That is why the technique’s name brings the term ‘near the edge’.
Hesterberg says that “unlike µ-SXRF, which is a fixed energy and scanning technique, µ-XANES is a variable energy technique. The absorption edge region is where there is a large increase in X-ray absorption by the sample”.
Analysis of the edge region made it possible to discover the manganese oxidation state, that is, whether the element was in the form Mn²⁺, Mn³⁺ or Mn⁴⁺. Therefore, the technique allowed the researchers to understand if the manganese was in an oxidized form, or in a mineral state, and what coordinating atoms are likely around it. This means understanding what strategies eucalyptus uses to detoxify itself from the metal.
Read more on CNPEM website
Image: Detectors around a sample being measured at the Sirius’ Carnaúba beamline.