Adaptive physiological responses of tomato plants to combined abiotic stress and biostimulant application.

Tomato is one of the most cultivated crops in the world. Due to the antioxidant and anti-cancer properties of lycopene and other compounds, tomato consumption and production is still on the rise. However, its productivity is greatly compromised by a wide range of abiotic stresses and, therefore, the production of stress-tolerant tomato lines and the identification of novel strategies to increase stress tolerance are key challenges for modern agriculture. The presence of adverse environmental factors such as extreme temperatures, salinity or drought causes morphological, physiological and biochemical changes in tomato plants. The biotechnological and agronomical methods used to increase tomato tolerance to various abiotic stresses include the selection of tolerant genotypes and the use of management practices, such as the application of biostimulants. An in-depth study of the physiological responses of tomato plants to abiotic stress and to biostimulant application was performed. The first aim of this research was to investigate the mechanisms that control plant physiological responses to high temperature stress, drought and combined stresses in different tomato genotypes in order to select those tolerant to abiotic stress. A second aim was the identification of strategies to increase tomato growth and final yield under stress. To this aim, we focused on the protein hydrolysate-based biostimulant and investigated its ability to induce better performances in plants under heat, drought and combined stresses in different environmental conditions. As for the first aim, the first part of this research focused on the eco-physiological screening of several tomato genotypes under elevated temperatures (Chapter 2) that allowed the selection of two genotypes potentially tolerant to heat stress (LA3120, E42). The response of the selected genotypes was further tested in a growth chamber to better investigate their responses to combined stresses, such as high temperatures and water shortage (50% of water requirements) (Chapter 3). As for the second aim of this thesis, the response of different genotypes grown in open field under elevated temperatures after application of a protein hydrolysate-based biostimulant was analysed. This additional analysis allowed to demonstrate that the use of the biostimulant by fertigation led to better plant performances under elevated temperatures (Chapter 4). The adaptive physiological response to single and combined stresses and biostimulant treatment was also investigated under controlled conditions in the selected genotypes E42 and LA3120 (Chapter 5). Considering that plants grown in open field are subjected to a higher number of different variables compared with controlled environments, in the final part of this work, the performances of the genotype E42 exposed to water deficit and treated with the novel protein hydrolysate biostimulant were evaluated under open field conditions. This final experiment allowed to demonstrate the positive effect of the biostimulant on final yields under water deficit and in different field trails (Chapter 6). Our findings contributed to a better understanding of the morphological and physiological effects of combined abiotic stresses on tomato crop. Additionally, the results obtained in this thesis further demonstrate the effects of protein hydrolysate-based biostimulants on improving plant performances under abiotic stresses. Altogether, results obtained in this thesis provide novel solutions to increase final yields in plants facing the future climate changes