La photoprotection, pour réduire l'utilisation des produits phytosanitaires – ECOPHYTO

Modernisation of agriculture has contributed to considerable increased yields and to the improvement of product quality. Among the advance, the use of chemical control products has proved to be extremely efficient, reliable and convenient. But, the use of pesticide is nowadays more and more being questioned, especially in France following the Grenelle's conference guidelines. After pesticide spraying, the active ingredient can either enter the inner leaf, being volatilized or phototransformed. All these reactions need to be investigated, especially photochemistry which is a potentially important degradation pathway. This pathway leads to a fast decline of pesticides. To balance, pesticides are overdosed. The second drawback of pesticide photolysis is the production and the accumulation of by-products whose toxicity is unknown. The use of photoprotecting agents is a promising technique to reduce the amount of pesticide applied and the formation of unidentified and potentially toxic by-products. But photoprotecting agents proposed until now do not meet requirements: good efficacy and no toxicity. In the ECOPHYTO project, we aim to develop a new class of photoprotecting agents extracted from plants. These new agents are original (patent PCT based on the US provisional n°60/886544), they are environmentally friendly and not harmful for crops. Two crops will be investigated (maize and grapevine) and three types of pesticide will be considered (herbicides, fungicides and insecticides). Reactions will be studied in the laboratory scale using a model system (which mimics plant surfaces) detached leaf and plant and in the field scale for final validation. The goal of this project can be summarized in three action points, combining photochemistry, analytical photophysics, vegetal physiology and agronomy for a comprehensive view of phenomena: 1. Understanding and quantifying pesticide fate on crops after treatment. We shall mainly focus on photochemistry and pesticide penetration into leaves. During the two past years the LPMM has implemented an experimental method to evaluate pesticides photodegradation and validated it with tests on leaves and outdoor measurements. LPMM made a great effort to study reactions under environmental relevant conditions: simulated solar light, low pesticide concentrations, use of formulated pesticides and importance of water quality. ESD has recently developed a powerful technique to depict the mechanisms involved in cuticle penetration. It is based on two-photon excitation microscopy (TPEM). This is a non destructive technique that allows visualizing chemicals in plant cuticles through the novel use of both the plant and the compound autofluorescence. The collaboration of the two complementary teams will allow the enhancement of our understanding of the photochemical mechanisms involved at the leaf surface. 2. Development of a new class of photoprotecting agents. The originality of the proposed compounds consists on their origin and their nature; they are hydro-alcoholic plant extracts. They are natural compounds and should not be harmful for crops and the environment. They will decompose by chemical and biological processes once released in the environment. Their ability to reduce pesticide photodegradation was demonstrated by LPMM with photochemical tests on model wax films. The way the natural plant extracts slow down pesticide photodegradation is not yet fully elucidated. They certainly have a sunscreen effect, but these natural plant extracts which contain polyphenolic compounds could also have some antioxidant properties. The plant extracts could also be plant elicitors – i.e. they may activate the plant defence mechanism. This potential effect could increase the interest of the proposed photoprotectors; it will be investigated in the project, first by physiological parameters (ERTAC) and antioxidant status of the plants and by monitoring the response of grapevine leaves inoculated with fungi (SV). Finally, we shall visualize the penetration of photoprotecting agents into leaves by TPEM. 3. Demonstrating pesticide dose rate reduction thanks to photoprotection. It is difficult to demonstrate the relevance of a pesticide dose reduction because it needs to be applied to all types of climatic and geographic conditions. To reinforce our demonstration we will proceed as follows. The three types of pesticides will be investigated independently: herbicide on a representative weed of maize (ERTAC), fungicide on pathogen downy mildiou of grapevine and insecticide on the major European pest of grape, the European grapevine moth (SV). First, we shall do tests in growth chamber in order to adjust the application dose with photoprotection by dichotomy. Correlations between the gain in pesticide half life and reduction dose factors will be calculated and statistically tested. The last step will be field assays. This is an original aspect of our project.