Polyacrylamide ZrOH Hydrogels for Spatially Resolved Sampling of Carboxylates Exuded from Roots Growing in Soil

Sampling and reliable quantification of root exudates from undisturbed soil-grown plant roots remain challenging. We further developed a non-destructive method for the sampling, 2D mapping and quantification of seven carboxylates (aconitate, citrate, fumarate, lactate, malate, oxalate, succinate) exuded from rhizobox-grown plant roots. The method described here employs polyacrylamide zirconium hydroxide hydrogels (ZrOH hydrogels) that uptake all tested carboxylates and can be eluted with an efficiency ranging from 95.3% ± 3.12% to 111% ± 1.99% . The ZrOH hydrogels have a high binding capacity for carboxylates, up to 1.82 µmol cm^-2, depending on the solution pH and carboxylate species, a concentration higher than that usually available in the rhizosphere. Moreover, the bound carboxylates on the ZrOH hydrogels remain stable and can be stored for several weeks at 4 °C before analysis. For the application, plants are cultivated in soil-filled rhizoboxes that allow for easy access with minimal disturbance to the root system. To sample root-released carboxylates, ZrOH hydrogels are carefully applied to the region of interest for 24 h. After retrieving the ZrOH hydrogels, they are cut for mapping purposes, and the gel pieces are eluted for subsequent carboxylate analysis (e.g., via Ion Chromatography-Mass Spectrometry). Our findings indicate that ZrOH hydrogels are effective for capturing and determining carboxylate concentrations in the rhizosphere. The novelty of this method lies in its ability to sample root exudates from intact soil-grown plant roots, as well as the possibility of time-resolved sampling, compared to traditional methods (soil-hydroponic hybrid approach) that are often destructive and allow only single-time sampling. Most importantly, it enables the generation of quantitative, high-resolution, millimetre-scale 2D images, facilitating the visualisation of carboxylate exudation along the root axis and its spatial distribution within the rhizosphere. Additionally, this method facilitates the sampling of root exudates at various growth stages during the growth cycle.