Contribution to the study of the molecular physiological response of apple trees to an essential oil-emulsion biopesticide applied by trunk-injection

Abstract

Apple production is among the most important ones in Belgium. Apple trees like any other plants are subject to abiotic and biotic stresses that may cause important economic losses. One example is its major pest, the rosy apple aphid (Dysaphis plantaginea (Passerini)). The common strategy for pest control relies on pesticides application. However, D. plantaginea becomes more and more resistant to these substances. Thus, the Walloon Region started a project to develop a biopesticide based on essential oils which take advantage of their insecticidal properties to come up with an environmentally friendly alternative to these chemical pesticides. This research study focuses on studying the physiological plant response at a molecular scale to this biopesticide directly injected into the trunk. Indeed, a good product for this context has to be efficient against the pest without damaging the plant nor the yield.

The study focused on the plant physiological response to a single essential oil at different concentrations than on comparing if the different EO emulsions already formulated induced the same physiological response. To this purpose, cinnamon essential oil (Cinnamomum cassia J.Presl) emulsions at 1% and 4% were injected in the trunk of two years old apple trees (Malus domestica Borkh). Leaves were sampled at 0h; 1h30; 3h; 4h30; 6h; 24h; 72h and 192h after injecting.

The efficiency of the apple tree redox system against the potential oxidative stress was assessed by measuring two antioxidants: glutathione and carotenoids by high performance liquid chromatography equipped with a FLD detector and by spectrophotometer, respectively. Glutathione play different roles in plants, especially in stress conditions. In this work, the evaluation of glutathione concentration in apple leaves was carried out by using monobromobimane labelling and HPLC-FLD. It presents a good linearity in the concentration range from 25 μM to 200 μM, a great repeatability and a GSH-mbb adduct stability for at least nine days. Furthermore, the leaf physiological status was investigated, on the one hand, with Fv/Fm ratio and F0 determined by a handy plant fluorimeter and, on the other hand, with chlorophylls (a and b) content also evaluated by spectrophotometer. Finally, oxidative damage was considered by the determination of conjugated dienes concentration, a marker of lipid peroxidation by spectrophotometer.

Results on apple leaves first show that it is not an easy task to study the oxidative stress in field conditions. A lot of other factors from the surrounding environment could influence the results including the researcher itself. In this kind of experiment, the challenge is to reflect the situation inside the plant at the time of sampling and not merely report the perturbation inflicted on the field. Moreover, the oxidative burst occurs in the few hours after the injection and it seems that the apple trees are able to manage this stress. Even if studies demonstrate the herbicidal activity of the cinnamon essential oil, it does not in this context. Finally, the plant antioxidant defences are composed by non-enzymatic and enzymatic scavengers. When these defences are not able to counterbalance the oxidative stress, different kinds of damage can be produced. This work was more concentrated on non-enzymatic antioxidant and the photosynthetic performances. Thus, the study has to be continued.