TXRF analysis of guttation fluids as a new opportunity for the evaluation of mineral elements transport from roots to shoots in plants.

Through the phenomenon known as “guttation”, many plants exudate droplets of xylem sap through the hydathodes, which are specialized foliar structures located at the tips and at the edges of the leaves. Such fluids have been used in the last decades for various purposes, such as the evaluation of the plant nutritional status as well as to assess the presence of insecticide residues in plant crops. Furthermore, since the xylem sap is directly transported from roots towards shoots, the analysis of plants’ guttation fluid can also represent a non-invasive way for investigating the translocation of mineral elements, including potentially toxic elements (PTEs), therefore representing an indirect measure of PTEs uptake from polluted soils. This latter potentiality, however, has been scantly explored. For such purpose, total reflection X-ray fluorescence spectrometry (TXRF) looks particularly appealing for many reasons, above all because of the small volume of sample required. Indeed, the phenomenon of guttation is limited to small drops, never bigger than few microliters. The use of TXRF hence would allow a multi-elemental, highly sensitive and expeditious analysis even of a single guttation drop. In this study, the reliability of TXRF for the analysis of plants’ guttation fluids is under investigation. For this scope, we have collected guttation droplets from plants (e.g. Lolium rigidum, Festuca arundinacea and Piptatherum sp.) grown in PTEs-polluted soils (Pb, As, Cd, Sb and Zn) mesocosms treated with different amendments (compost, biochar and plant growth promoting rhizobacteria-PGPR). The liquid samples were pipetted (5-10 µL) on siliconized quartz carriers previously pipetted with a 2 mg L-1 Y standard solution in order to achieve a final internal standard (Y) concentration of 1 mg L-1. TXRF analyses were carried out using both a Mo-source and a W-source Picofox instrument (Bruker), the latter especially for the quantification of high-Z elements (e.g. Cd), which is one of the most diffused and mobilizable PTE in soils. To date, TXRF results revealed differences in the mineral elements composition of guttation fluids, which appear influenced by soil properties and by the applied amendment. For instance, while no Pb is detected in fluids from mesocosms where Pb is mostly stabilized as insoluble phosphates, it is instead highly concentrated (up to hundred mg L-1) in the sap of plants grown in the soil where Pb is less stabilized. Further, in these latter mesocosms, the W excitation allowed to detect also Cd, especially in PGPR-treated mesocosms. After the completion of TXRF analysis, ICP-MS will be performed for validation. These early evidences may suggest this TXRF-based method for the non-invasive, reliable and expeditious assessment of PTEs mobilization from polluted soils and translocation to plants’ shoots.