Natural deep eutectic solvents present in plant exudates? A case study on the saps of Drosera species

Many plants produce exudates as defense mechanism to hamper pests and pathogens. Plant exudates include latexes, resins, gums, root exudates, and also biofluids produced by carnivorous plants. Plants exudates consist of, among others, polysaccharides, lipids, proteins, organic acids and bases, sugars, and amino acids. All these small molecules are NADES components. In this chapter, the chemical compositions of plant exudates, especially leaf exudates, and Drosera plant biofluids will be elaborated. Metabolic profiling approaches based on ¹H NMR and GC–MS were performed on the biofluids of the Drosera species. The ¹H NMR spectra showed a high level of myo-inositol and sugars in the biofluids as well as some organic acids. To further confirm the identity of the sugars and to identify some minor components, GC–MS was employed. This gave similar results as the ¹H NMR spectroscopy. All the tested exudates were found to contain sugars, such as fructose, glucose, arabinose, sucrose, and xylose; organic acids, such as glucuronic acid and ascorbic acid; and myo-inositol. Those Drosera ingredients are known to be strong hydrogen-bond donors. Based on the information obtained from the ¹H NMR and GC–MS results, new NADESs compositions were proposed and they were tested in vitro, of which indeed five combinations formed liquids. However, considering that these Drosera biofluids consist mainly of water and other materials, it may indicate that the stable high viscosity droplets typical for the Drosera species must be due in part to other physical-chemical forces, such as gel formation by polysaccharides with calcium ions. A possible role of the NADES-like liquids could be in stabilizing proteins, or the formation of biofilms enabling a faster digesting of the prey. The myo-inositol that was clearly a major compound in the exudate might be involved in protecting the leaves and the sugar containing exudate from herbivory. The occurrence of some common products found in fermentation processes might indicate that microorganisms are present in the biofluid.