Digital Theses Archive


Tesi etd-05022022-112734

Type of thesis
E-mail address
Selenium enrichment of tomato plants: effects on fruit quality, ripening physiology and postharvest behaviour
Scientific disciplinary sector
Istituto di Scienze della Vita - AGROBIOSCIENCES
relatore Prof. TONUTTI, PIETRO
  • biofortification
  • selenium
  • nutriceuticals
  • Solanum lycopersicum
Exam session start date
About one billion people in the world are not obtaining enough selenium (Se) from their daily diet. Severe selenium deficiency is causing health problems including deterioration of breathing, immunity, male fertility and muscle function, including cardiomyopathies such as Keshan and Keshan-Beck diseases. Selenium biofortified plant-derived products, in particular if consumed fresh (as in the case of most of fleshy fruits), are of interest for their capacity to improve human diet and overcome malnutrition challenges. <br>Current techniques of selenium biofortification of plants/plant organs include foliar spraying, substrate or nutrient solution supplementation and seeds soaking. These techniques are effective in delivering selenium to plants, however the results of their application reported in the literature are not consistent, and factors impacting selenium uptake by plants are different and not yet completely understood. Moreover, since some chemical forms of selenium are toxic, the available technologies may imply a risk of environmental pollution. Therefore, there is a need of identifying and testing innovative and safe protocols for Se-biofortification of edible plants. <br>In plants, selenium is not considered essential, however various effects on plant physiology and biochemical composition were reported in the literature. Selenium may affect the composition of different edible organs, including fruits, in terms of polyphenols, carotenoids and amino acids content, as well as accumulation of macro and microelements such as S, Zn, Mn, K, Ca, Mg. Selenium was also found to be able to affect fruit ripening and shelf life, and the response to biotic and abiotic stress in selected species. However, a comprehensive description of the physiological effects of Se enrichment in plants, in general, and in fruits, in particular, is still missing.<br>The main objectives of the present PhD dissertations were those of: i) comparatively evaluating different protocols to enrich tomatoes with appropriate concentrations of Se; ii) characterizing the physiological and metabolic processes of ripening tomatoes in relation to the uptake of Se.<br>Tomato was chosen considering its importance and worldwide diffusion in human diet, and the fact that it represents the model species for ripening physiology studies. <br>In the present PhD dissertation, we compared the existing techniques of Se enrichment by spraying the whole plant with sodium selenate with the innovative use of chemically synthetized selenium nanoparticles, composed of selenium at elemental state. This new approach may be of great interest in terms of metabolic processes (e.g., Se organic compounds) and environmental issues. We also designed and tested two approaches to biofortify harvested fruit: vacuum infiltration and passive immersion.<br>After foliar spraying, selenium was absorbed by aerial plant tissues and translocated to different organs, including fruit. Both selenium nanoparticles and sodium selenate have been found applicable for the biofortification of tomato plants. They increased the nutraceutical properties of tomato fruit, enriching fruit with selenium at levels which are compatible with the amounts of selenium recommended by WHO for a daily diet. Selenium did not cause any significant changes in the qualitative parameters (i.e., dry matter content, soluble solid content, titratable acidity and taste index) of ripe tomatoes.<br>Noticeably, Se content in fruit treated with Se nanoparticles containing elemental Se was statistically comparable to the Se content delivered by the analogous concentrations of sodium selenate. Selenium improved the nutraceutical properties of tomato fruit either directly – due to its increased content, and indirectly – by inducing (or increasing) the production of bioactive compounds.<br>The low concentration of selenium (5 mg L–1) sprayed on the whole plants did not affect the biochemical composition of fruit and, at the same time, enriched them with essential nutrient. The higher concentration of Se (10 mg L–1) mainly affected the accumulation of amino acids, terpenoids, aldehydes, alcohols and carotenoids. Se increased content of GABA, AMP, threonine. Concerning phenol compounds, Se enhanced production of chlorogenic acid, phloredzin and naringenin and lowered the content of coumaric acid. Se decreased accumulation of carotenoid beta-carotene and increased production of apocarotenoid beta-ionone. Se impacted the production of Volatile Organic Compounds (VOCs), especially increased the content of terpenoids and aldehydes. Some of impacted VOCs were previously defined in the relevant literature as compounds associated with consumer liking. Phenols and amino acids that resulted increased by Se were described in the bibliography as bioactive compounds, desirable for human diet. Therefore, we may preliminarily hypothesize that Se may improve not only nutritional properties of tomato fruit, but also their organoleptic qualities. <br>In our attempt to develop alternative protocols to spraying whole plants in the field (or greenhouses) we proved that both vacuum infiltration and passive immersion increased Se content in harvested tomatoes. However, vacuum infiltration caused physiological damage of fruit skin and internal structure, while passive immersion was harmless for fruit. The latter technique may potentially allow to reduce the risk of polluting environments due to the possibility to recycle the same volume of selenium solution multiple times. However, it is still not clear whether selenium applied to the fruit after being detached from the plant induces physiological and biochemical responses similar to those induced by the traditional techniques.<br>We also were interested in assessing whether, as observed in other plant species, Se is effective in protecting tomato plants from biotic and abiotic stress. Se did not affect the incidence of fungal pathogens Botrytis cinerea and Anternaria alternata in inoculated fruit, but the germination level of Botrytis cinerea was significantly suppressed by the highest tested dose of selenium (80 mg L–1) added to the medium in in-vitro trial. Based on our observations, Se added to the substrate did not affect plants resistance against water stress.<br>To explain possible mechanisms of selenium action in plants, we investigated the changes of transcriptome in Se-enriched tomato. RNA-seq results showed that selenium affected the expression of several genes, with a higher incidence in genes involved in MAPK signaling, carotenoid synthesis, flavonoid pathway, ethylene biosynthesis and signal transduction. The results of RNA sequencing were confirmed with validation of individual gene expression using RT-qPCR. <br>Future studies are needed to understand if and how selenium delivered to fruit using different pre- and postharvest methods is converted into organic forms. In order to propose a reliable protocol for Se-enrichment and nutraceuticals enhancement of tomato fruit, it is advisable to provide protocols possibly developed and targeted for different genotypes.<br>