LIBS in Agriculture
Due to its experimental simplicity, and versatility, LIBS has achieved its high degree of interest particularly in the fields of agricultural science, environmental science, medical science, forensic sciences, and biology. Rapid and accurate detection and prevention of soil contamination (mainly in pollutants of heavy metals) are deemed to be a concerned and serious central issue in modern agriculture and agricultural sustainable development.
This technique has extensively been used to detect various microorganisms, extending the horizon from bacteria, molds, to yeasts, and spores on surfaces, while also being successful in sensing disease-causing viruses. LIBS-based probe has also enabled the successful detection of bacteria in agriculture as well.
The laser-induced breakdown spectroscopy (LIBS) technique offers many advantages for elements analysis. It has gained great popularity in the elemental analysis because of its portability, lightning speed, low cost, nonrequirement for chemicals, minimal or no sample preparation, simultaneous determination of multiple elements, and capability to perform express identification. The technique involves short, high-intensity laser pulses capable of ablating a small amount of material, thereby creating a momentary plasma. An optical fiber collects a portion of the light emitted from the plasma and delivers it to a spectrometer. Te captured spectra are considered a “fingerprint” associated with a sample’s elemental composition.
Huanglongbing (HLB), or citrus greening disease, has been a scourge on citrus industries around the world. Te vector-borne disease is transmitted by two species of psyllids (Hemiptera: Liviidae): the Asian citrus psyllid, Diaphorina citri, and the African psyllid, Trioza erytrea. Like other Hemiptera, psyllids are phloem sap feeders. These two species feed and reproduce on all members of the family Rutacea, which includes all citrus species of commercial importance. During the feeding process, the psyllid transmits the putative phytopathogen Candidatus Liberibacter asiaticus (CLas), a Gram-negative alpha-proteobacterium. Currently, CLas remains unculturable complicating research eforts. In addition to CLas, psyllids host several other bacterial endosymbionts in their gut .
In the last few years, LIBS has been used to study the effects of CLas infection on the nutritional composition of citrus plants. It was recently demonstrated that LIBS can successfully differentiate between CLas-infected and healthy citrus plants by analyzing the major macro-and micronutrients. LIBS analysis showed that CLas significantly decreased the level of calcium, magnesium, and potassium in citrus plants. Recently, it has also been shown that a combination of LIBS and Raman spectroscopy significantly improves discrimination and classification of bacterial species and strains.
In this work, we introduce the LIBS technique for composition microanalysis of D. citri, the vector of citrus huanglongbing. LIBS can reveal the biochemical diferences between CLas-infected and non-infected Asian citrus psyllids for immediate detection of the pathogen. To our knowledge, this is the frst time that a LIBS technique has been directly applied to differentiate between pathogen-infected and pathogen-free vectors.
Schematic representation of laser-induced breakdown spectroscopy system used in this study. A stereomicroscope was modified and attached to the laser source to point laser beams on the abdomen of Asian citrus psyllid.
Differentiation between healthy and CLas-infected Asian citrus psyllid using elements identifed by LIBS. (A) Principal component analysis of all identifed elements (n=22). (B) PCA-loading-plot for all identifed elements. (C) Signal intensity of all elements identifed by LIBS in healthy and CLas-infected Asian citrus psyllid. (D) Principal component analysis of fve signifcant identifed elements (n=22). (E) PCAloading-plot for fve signifcant identifed elements identifed elements. Bars represent standard errors. P-values were calculated using the Student’s t-test.