The use of natural substances to improve fruit crop performances, quality and tolerance towards stress conditions

Sustainable agroecological practices such as the use of natural substances in agriculture have received much attention in last decades. However, conflicting results and lack of crop-specific information are still limiting the application of biostimulants, resistance inducers and biopesticides/botanicals. The aim of the present thesis was to better understand the role of natural substances (e.g. humic acids, seaweed extracts, plant protein hydrolysates, amino acids, vitamins, microelements, chitosan, essential oils) on (i) plant growth and fruit quality promotion, (ii) defense induction  and (iii) antimicrobial activity  in apple and strawberry crop. Therefore, this Ph.D. study was structured into three research topics: i) Application of biostimulants to improve growth, yield and fruit quality in apple and strawberry crop. The gap between organic and conventional farming is represented by some factors (e.g. nutrient availability and uptake) that can limit plant growth, yield and fruit quality. Biostimulant products have been proposed as a solution to overcome these limiting conditions. Preharvest foliar applications of seaweed extracts and alfalfa protein hydrolysate on apple trees cv. ‘Jonathan’ in open field conditions were effective in improving red overcolor of apple fruits at harvest. Total phenol and anthocyanin content were increased in those treated fruits. These findings suggest the involvement of biostimulants in the modulation of metabolism of the phenylpropanoid pathway (chapter 3). Amino acids combined with zinc delayed the development of a particular apple physiological disorder (i.e. Jonathan spot) during storage period, enhancing the rate of marketable fruits. Amino acids showed to be a valid metal-chelating agent and carriers of micronutrients such as zinc. This mineral element could interfere with calcium metabolism and offer protection against oxidative stress by activating antioxidant mechanisms, retarding the occurrence of Jonathan spot disorder (chapter 4). However, no clear indications emerged for vegetative and productive parameters; probably, biostimulants have less (or do not have any) influence on woody plants and in optimal growing status (chapter 3). In another experiment (chapter 5), foliar applications of biostimulants such as seaweed extract, alfalfa protein hydrolysate, vitamins, chitosan and silicon helped strawberry plants cultivated in soilless culture to overcome a stressful condition such as the nutrient limitation. More in detail, plant biomass, chlorophyll content, leaf area, photosynthetic rate and final berry yield were increased in those treated plants. Moreover, high nutritional value (e.g. phenolic compounds) was found in strawberry fruits treated with seaweed extract and protein hydrolysate. ii) Application of elicitors to induce defense responses in strawberry plants against disease infections. The availability of means of disease control is often limited in organic farming. Therefore, this production system is constantly searching new sustainable disease management methods. The tested natural substances proved to be alternatives to synthetic pesticides. Seaweed extracts, alfalfa protein hydrolysate and chitosan showed to reduce powdery mildew incidence and severity on artificially inoculated strawberry plants. As emerged from gene expression analysis, those resistance inducers triggered some SAR- and ISR-related defense responses in treated plants. Essential oils (thyme and juniper) exhibited a preventive and curative action similar to synthetic fungicide against both Botrytis cinerea and Podosphaera aphanis (chapter 6). iii) Application of botanicals/biopesticides to control pathogens in pome fruits. Fire blight as well as postharvest decays such as grey, blue and Monilia mold are responsible for significant losses. Control strategies based on the use of natural products to limit fungal/bacterial development and chemical residues are currently researched. A strong antimicrobial activity of thyme essential oil was observed against Erwinia amylovora infection in in vitro tests. The combination of essential oil with chitosan showed to reduce the Erwinia infection degree in apple seedlings. The antibacterial effect was similar to that obtained in streptomycin-treated seedlings at 4 days after artificial infection. After that time point, the reduced antimicrobial activity of essential oil and chitosan could be related to the degradation of their components (chapter 7). Thyme essential oil exhibited the most pronounced antifungal property against Botrytis cinerea, whereas clove essential oil was most effective against Penicillium expansum in in vitro assays. Both essential oils were effective against Monilia fructigena. Since essential oils applied alone proved to be ineffective in vivo conditions (on apple fruits), encapsulation of essential oils with a polysaccharide matrix (e.g. chitosan) showed to be a valid technique to promote essential oil persistence. Indeed, disease inhibition was higher by combination of chitosan with thyme or clove essential oil against B. cinerea or P. expansum as compared to chitosan and essential oil applied alone (chapter 8). In conclusion, findings from this study suggest that the use of natural substances in apple and strawberry crops can be considered promising tools within the framework of organic farming in order to promote plant growth and fruit quality, as well as to control diseases. Further research should better understand mechanisms of action and optimize the application mode of the tested products.