Photochemical interactions between pesticides and plant volatiles

Abstract

Among the numerous studies devoted to the photodegradation of pesticides, very scarce are those investigating the effect of plant volatiles. Yet, pesticides can be in contact with plant volatiles after having been spread on crops or when they are transported in surface water, making interactions between the two kind of chemicals possible. The objectives of the present study were to investigate the reactions occuring on plants. We selected thyme as a plant because it is used in green roofs and two pesticides : the fungicide chlorothalonil for its very oxidant excited state and the insecticide imidacloprid for its ability to release the radical NO 2 under irradiation. Pesticides were irradiated with simulated solar light first in a solvent ensuring a high solubility of pesticides and plant volatiles, and then directly on thyme's leaves. Analyses were conducted by headspace gas chromatography-mass spectrometry (HS-GC-MS), GC-MS and liquid chromatography-high resolution mass spectrometry (LC-HRMS). In acetonitrile, chlorothalonil photosensitized the degradation of thymol, -pinene, 3-carene and linalool with high quantum yields ranging from 0.35 to 0.04, and was photoreduced, while thymol underwent oxidation, chlorination and dimerization. On thyme's leave, chlorothalonil was photoreduced again and products arising from oxidation and dimerization of thymol were detected. Imidacloprid photooxidized and photonitrated thymol in acetonitrile, converting it into chemicals of particular concern. Some of these chemicals were also found when imidacloprid was irradiated dispersed on thyme's leaves. These results show that photochemical reactions between pesticides and the plants secondary metabolites 2 can take place in solution as on plants. These findings demonstrate the importance to increase our knowledge on these complex scenarios that concern all the environmental compartments.

The Chemical Landscape of Leaf Surfaces and Its Interaction with the Atmosphere

“…29,264,277−280 Formulation ingredients (e.g., surfactants) and co-occurring volatile and semivolatile metabolites can further influence surface photolysis by acting as photosensitizers or quenchers of reactive species, stabilizing radical intermediates, screening light, or a combination of these processes. 29,121,264,289 By modifying the leaf's wettability, surfactants can also influence shape, density, and crystallinity of the active ingredient's residue, with impacts on its photochemical stability. 29,288 Variation in epicuticular wax chemistry, thickness, and morphology impact photodegradation in a similar manner.…”

“…Overall, photodegradation obeys pseudo-first-order kinetics, with reaction rate constants that change considerably (but not predictably) based on chemistry, surface coverage, and micromorphology of the reaction substrate, as well as co-occurrence of other substances (e.g., Figure ). ,,− Formulation ingredients (e.g., surfactants) and co-occurring volatile and semivolatile metabolites can further influence surface photolysis by acting as photosensitizers or quenchers of reactive species, stabilizing radical intermediates, screening light, or a combination of these processes. ,,, By modifying the leaf’s wettability, surfactants can also influence shape, density, and crystallinity of the active ingredient’s residue, with impacts on its photochemical stability. , Variation in epicuticular wax chemistry, thickness, and morphology impact photodegradation in a similar manner . Wax components may act as photosensitizers or quenchers of reactive species and actively participate in reactions to form “bond residues”, whereas the presence of specific microstructures modifies light transmission and water spreading. ,, SVOC solubility in epicuticular waxes can also impact half-lives, as compounds buried within the cuticle are less susceptible to photodegradation than less lipophilic molecules sitting on its surface .…”

Interactive effects of changes in UV radiation and climate on terrestrial ecosystems, biogeochemical cycles, and feedbacks to the climate system

“…348,112] biocide photodegradation products can be more toxic than their parent compounds. For example, some breakdown products generated by UV-B radiation of the fungicide chlorothalonil and the insecticide imidacloprid on plant leaves are more toxic to fish than their parent compounds [ 344 ].…”

Measuring the photostability of agrochemicals on leaves: understanding the balance between loss processes and foliar uptake

“…3 Glass is an impermeable, clean and uniform substrate, and its polarity and morphology differ from those of real leaves. It does not contain any of the chemical substances that are present in [17][18][19][20][21] or released by 22 plants and that are able to either promote or inhibit photochemical reactions. And nor can it mimic the physical complexity of the leaf surface and all of the redistribution mechanisms that a compound can undergo once applied to that surface, such as specific surface interactions or surface movements.…”