Interaction plante ‐ fongicide systémique

Sommaire L'activité systémique d'un fongicide dépend étroitement de la plante qui peut agir sur les phénomènes de pénétration et de son transport et déterminer la localisation du composé. La pénétration se produit suivant des processus variables en fonction des organes; par la suite le fongicide emprunte la voie apoplastique. Ce type de transport entraîne en général une accumulation du fongicide dans les zones à forte transpiration. Des exemples de variations de l'activité systémique selon l'espèce, la variété, l'organe, l'emplacement sur Forgane, l'âge de l'organe de la plante sont discutés. La transformation du fongicide dans la plante est un phénomène important car elle peut modifier son activité systémique, son activité fongitoxique et, plus généralement, sa toxicité. La plante intervient en fournissant des enzymes (estérases, oxydases) et des substrats réactionnels (acides organiques, acides aminés, glucides). Un composé non fongitoxique in vitvo peut augmenter la résistance de l'hôte à un phytoparasite en modifiant les teneurs en sucres, en auxines, en pectines, en substances naturelles fongitoxiques des tissus de la plante, ou encore en modifiant leur texture et leur structure. Un fongicide systémique peut dans certains cas être phytotoxique (nécroses, nanisme, russeting) ou, à l'opposé, être bénéfique en stimuiant la végétation. Les fonctions végétales touchées peuvent être la respiration, la photosynthèse, les biosynthèses et l'absorption d'éléments minéraux. Resume Interaction between Plant and Systemic Fungicide A systemic fungicide is a chemical product which when applied to one part of a plant is absorbed and then carried to another part of the plant where it (or one of its metabolites) can control a parasitic fungus by direct action upon it. In vivo , the fungitoxic activity is partially connected with its systemic activity which also depends on the plant's effect on it. The plant can affect the penetration, transport and site of action of the fungicide. The Plant's Effect upon the Penetration of the Fungicide Penetration can occur in the seeds, roots, stems, leaves and other organs. For certain species, seed‐treatments result in an important passage of the fungicide through the tguments (e.g. beans benomyl) whereas in other species (e.g. rape, cucumber/benomyl) this rate remains low. The roots take up the fungicides in the same way as they take up water and numerous other soluble substances (apoplastic way). As for leaf treatments, penetration generally involves movement through the cuticle, thus absorption depends upon the plant species, the side of the leaf, its age, etc. The Impact of the Plant on the Transport and Locality of the Fungicide Regardless of the actual site of penetration, it seems that all the systemic fungicides, so far studied, follow the apoplastic way, i.e. they move within the cell‐walls and then pass into the xylem. This type of transport generally produces an accumulation of the fungicide in areas of incense transpiration, Differences in fungicide accumulation can be observed in leaves and stems of woody and herbaceous plants. As for fruit, the variations can be correlated with the density of active stomata. Finally, the age of the plant can also have an effect; it has been shown that accumulation, after a root‐treatment, is greater in young leaves than in old ones, due to the reduced transpiration rate of the latter (herbaceous plants). The plant also affects the stability of the fungicides and it is interesting to study the alterations in the fungicides within the plant tissues. Such alterations can modify both the systemic and fungitoxic properties of the fungicide. The plant may interfere by providing enzymes which lead to hydrolysis, oxidations and other reactions or to the formation of substrates from which conjugates are derived. Simple reactions which may occur include hydrolysis (e.g. cycloheximide acetate, benomyl, oxycarboxin), oxidation (eg. ethirimol, carboxin), disalkylations (e.g. dimethirimol), cyclisations (e.g. carbosy‐methyl DDC, methylthiophanate). In the case of conjugates the plant may provide organic or mineral acids, amino acids and glucides, etc., and esters, amides, ethers and amine‐compounds are formed. All these changes depend, in particular, upon the plant species and the organ under consideration. Systemic fungicides may have secondary effects on plants, e.g. they may produce necrosis (especially where the fungicide accumulates), growth inhibition, russeting in apples and pears, etc. Apart from these harmful effects some benefits such as increased size and weight and modifications in the plant's composition may occur. To be more precise, it has been shown that certain fungicides affect respiration f e.g. oxathiins), photosynthesis (e.g. benzimidazoles), the absorption of minerals (e.g. oxathiins, benzimidazoles), biosynthesis (e.g. oxathiins) and cell‐division (e.g. ethirimol). It should also be remembered that apart from systemic fungicides, sensu stricto , other compounds which increase the host's resistance to phytoparasitic fungi also exist. It can be concluded that between the time of application of the systemic fungicide and its impact there are many phenomena, which we only partly understand and of which the importance depends on many factors including the plant and the systemic fungicide itself.