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Inside plants

Tracking the path of silicon used in fertilizers

Inside plantsFertilizing sugarcane, potato, rice and other crops with silicon has helped improve harvest productivity and quality. Studies show that plants’ silicon absorption increases their tolerance to droughts, their photosynthetic capacity and their resistance to pests and diseases. A team of researchers from the Cena, the Department of Nuclear Energy in Agriculture at the University of São Paulo (USP) in Piracicaba went one step further and developed a method that allows one to trace the entire path of the silicon-based fertilizer and to study the absorption, transportation and redistribution processes of the element within the plant (read article on silicon in Agriculture in the 140th edition of Pesquisa FAPESP). “The first step, in order to conduct this study, is to apply an enriched source to one of the silicon isotopes, referred to as a  tracer, with an isotopic composition that is different to that  found in nature,” states Professor José Albertino Bendassolli, from Cena’s Stable Isotope Laboratory, and coordinator of the study.

The isotopes are atoms of the same chemical element that have different atomic weights, i.e. the number of protons and neutrons in the nucleus. The number of protons classifies the element, such as nitrogen, carbon, sulfur or silicon, whereas the variation in the number of neutrons differentiates each one of their isotopes. These isotopes account for the minor differences in the physical properties of the same chemical element. Hydrogen, for example, the simplest atom from a structural point of view, has three isotopes: hydrogen (atomic weight 1), which accounts for over 99% of this gas in nature, deuterium (atomic weight 2), which is what makes the heavy water used to cool nuclear reactors, and tritium (atomic weight 3), which is unstable and radioactive.

“The tracer method with stable isotopes, that do not emit any type of particle or radiation, allows  the transformations to be assessed and the path traced by an element in nature quantitatively and qualitatively,” notes the researcher. This means that the methodology allows  the movement of the silicon to be tracked as it  moves within the plant, i.e., where it accumulates, or if it can move from one leaf to another that lacks this micronutrient. “The tracer also allows researchers to study the plant’s metabolism, both at the cell level and genetically, for example, whether a specific amino acid is a predecessor of a protein,” notes Josiane Toloti Carneiro, who also took part in the study with a post-doctoral project financed by FAPESP.

Restricted absorption
Silicon is the second most abundant element on the Earth’s crust, but it is not entirely available. “The way in which a plant can naturally absorb it is limited; that is what makes fertilization necessary,” states Josiane. Many companies currently use metal slag from different materials, as a source of silicon for agricultural applications. The importance of these fertilizers has grown in the last ten years, though countries such as Japan, China and Korea have used a lot of this chemical element for decades in their rice plantations. Scientific interest about understanding what this element, whose symbol is Si, represents for agriculture has also grown. So much so that since 1999 this topic has been discussed at global congresses; the fourth of these is to be held this year in South Africa.

Studies carried out at Cena focused on two different species: rice (grass) and beans (leguminous), both silicon accumulators. Corn, originally selected for this study, was replaced by beans. The study was carried out basically by analyzing plants grown in a nourishing solution, one without enriched silicon and the other with the 30Si isotope, the heaviest of the three Si stable isotopes (mass 28, 29 and 30) and less abundant in nature. “We found that when silicon was applied to the plant, almost all that was absorbed went directly to the leaves in a short time,” notes Josiane. A lower amount of the element was detected in other parts of the plant. When it accumulated in larger quantities, the researchers removed the source of silicon, found in older leaves, and let new leaves grow to see if it would be redistributed, but this  did not occur. “There was no absorption by new plants, as opposed to what occurs with other fertilizers,” stated Bendassolli. If part of a plant requires nitrogen, for instance, it migrates from where it has accumulated to the area where it is more necessary at that time. “The method is an important tool because though silicon is used as a fertilizer, its physiological function in the plant is not well understood,” says Lílian Aparecida de Oliveira, a biologist who is completing her doctorate at Cena and is involved in the project.

The plants are analyzed using a mass spectrometer, a device that determines the isotopic abundance of the specific chemical element only, with gas fractions. The Cena spectrometer is a single and unique one that was made in Germany in the 60s. Analysis begins with the collection and treatment of the soil or plant sample. Impurities are removed with an acid chemical attack, which separates the silicon fraction in the sample. Reagents are then added to the fraction for the silicon to precipitate and become a salt; this is then decomposed at a high temperature in a vacuum line, to produce and separate silicon tetra fluoride gas, which goes into the mass spectrometer for isotopic analysis.

The method used for spectrophotometric determination of silicon in agronomic samples was published in the Communications in Soil and Plant Analysis journal of the University of Georgia, USA, in June 2007. The part that deals with silicon isotopic determination was accepted by the Analytical Letters journal from New York and is awaiting publication.

Si isotopic determination with mass spectrometers for studies on absorption and mobility in rice and corn crops
Regular Research Awards
José Albertino Bendassolli – USP
R$ 58,941.87 (FAPESP)