Thesis (M.Sc. (Nutrition))--North-West University, Potchefstroom Campus, 2007.

There is a close relationship between the susceptibility to disease and nutritional state, in the sense that an adequate diet enhances resistance to disease. There is an increasing interest in this beneficial relationship among scientists, food manufacturers and consumers. The trend is moving towards functional foods and their specific health benefits. The results of numerous epidemiological studies and recent clinical trials provide consistent evidence that diets rich in fruits and vegetables can reduce the risk of chronic diseases. These protective effects are mediated through multiple groups of beneficial nutrients contained in the fruits and vegetables, one of these being polyphenol antioxidants. The intake of the polyphenols plays an important role in the reduction and prevention of coronary heart disease (CHD), cardiovascular disease and cancer, as a consequence of their associated antioxidant properties. Fruits contain an array of polyphenols with antioxidant capacity. Polyphenols may be classified in two broad groups namely: flavonoids and non-flavonoids. Flavonoid subgroups in fruits are further grouped as catechins, anthocyanins, procyanidins and flavonol among others. Phenolic acids occur as hydroxylated derivatives of benzoic acid and cinnarnic acid, and are classified as non-flavonoids. Polyphenols have redox properties allowing them to act as reducing agents, hydrogen donators and singlet oxygen quenchers, and thus contribute to the antioxidant capacity of fruits and vegetables. Because of the numerous beneficial effects attributed to these antioxidants, there is renewed interest in finding vegetal species with high phenolic content and relevant biological activities. In view of the importance of these substances towards health and food chemistry, this study will focus on the polyphenol and Vitamin C characterisation and quantification of an indigenous South African fruit, the Kei-apple (Dovyalis cafra), thought to have antioxidant properties. Due to the fact that polyphenol content influences the colour, taste and possible health benefits of the fruit / processed food product, this study will supply valuable information to industry in choosing the best fruit processing methods to attain the desired end product. The exploitation of indigenous South African fruits (Marula and Kei-apple) is receiving increasing prominence, not only due to their health benefits, but also the opportunities these present to rural based economics. Furthermore, this research will serve as a platform for further research on the Kei-apple and other indigenous South African fruits with possible health benefits. Aims: The overall aim of this study is the quantification and characterisation of various nutritionally important antioxidants (polyphenols and ascorbate) in the Kei-apple fruit in its entirety, as well as in its individual fruit components (peel, flesh and seeds). In addition, the total antioxidant capacity of the entire fruit and the various fruit components will be determined in the unfractionated and fractionated fruit extracts. Gas chromatography coupled mass spectrometry (GC-MS) characterisation of the individual polyphenol components will also be analyzed in order to speculate on possible specific health benefits which the Kei-apple may possess. Methods: The study was designed to ensure that a representative fruit sample was collected. Approximately 100 kg Kei-apples were picked in the month of November 2004 from the Bloemhof area in South Africa. A sample of 50 fruits was rinsed and separated into the various components (peel, flesh and seeds). An additional 50 fruits were randomly selected, cleaned and used in their entirety for data representative of the entire fruit. The sample extracts were prepared, after being grounded and lyophilized, by a method described by Eihkonen et al. (1999) using 70% aqueous acetone. The C18-fractionation on the fruit and separated fruit components resulted in four fractions containing (1) phenolic acids; (2) procyanidins, catechins and anthocyanin monomers; (3) flavonols and (4) anthocyanin polymers. The total polyphenol content of the fruit and fruit components as well as the above mentioned C18-fractions were determined by Folin-Ciocalteu's method (Singleton & Rossi, 1965). Both free and total ascorbate concentrations in these samples were determined as described by Beutler (1984), in addition to total sugar content of these via standard methods. Apart from their nutritional interest, both these measurements are necessary for the correction of the total polyphenol concentrations. The total antioxidant capacity of the entire fruit and various fruit components was determined by measuring the oxygen radical absorbance capacity (ORAC) and ferric reducing antioxidant power (FRAP) of the unfractionated and fractionated extracts. Using GC-MS analysis, the various individual polyhenol compounds contributing to the total polyphenol content of the Kei-apple was separated, identified and quantified. This quantitative data was captured and statistically analysed. The analysis of variation was performed using the Tukey Honest Significant Difference test for post-hoc comparison. ORAC, FRAP and polyphenol Pearson correlation analyses were performed using Statistics (Statsoft Inc., Tulsa, Oklahoma, USA) with significance set at P ≤ 0.05. Results and discussion: This study determined the presence of various nutritionally important antioxidants (polyphenols and ascorbate), the total antioxidant capacity in the entire fruit as well as in the individual fruit components (peel, flesh and seeds) and their polyphenol sub group fractions. Total phenol content: The Kei-apple, in its entirety, has a polyphenol concentration of 943 ± 20.3 mg GAE/100g dry weight. Comparison of the individual fruit components showed the seeds to have the highest total polyphenol concentration with 1990 ± 31.3 mg GAE/100g dry weight, followed by that of the peel, 1126 ± 45.8 mg GAE/100g dry weight and then that of the flesh, 521 ± 1.01 mg GAE/100g dry weight. Total, L-ascorbic (ASC) and L-dehydroascobic (DHA) concentration: The total ascorbate of Kei-apple fruit is 517 ± 0.92 mg/100g dry weight. In contrast to the polyphenol content, the flesh of the Kei-apple had significantly the highest concentration of total ascorbate 778 ± 1.20 mg/100g dry weight, Gascorbic 241 ± 21.0 mg/100g dry weight, as well as Gdehydroascobic 537 ± 22.2 mg/100g dry weight. The ratio of Lascorbic acidltotal ascorbate for the flesh, entire fruit, peel and seed is 0.31,0.43,0.49,0.95, respectively, indicating the seeds are the most stable source of biologically active Vitamin C, with 95% of the total ascorbate occurring as G ascorbate. This is also in line with the total polyphenol content of these components, confirming a polyphenol sparing effect on ascorbate. C18-fractionation extracts: Solid phase (C18) fractionation of the Kei-apple fruit and fruit components showed that the fruit, peels and seeds consist predominantly of phenolic acids, followed by procyanidin, catechin and anthocyanin monomers and thereafter varying amounts of anthocyanin polymers and flavonols. Antioxidant capacity: The antioxidant capacity of the entire fruit and individual fruit components as determined by ORAC, (r=0.76) and FRAP, (r=0.95) significantly correlated with the total polyphenol content, as well as to each other (r=0.88), indicating both to be good predictors of antioxidant capacity. GC-MS polyphenol characterisation of the Kei-apple: Caffeic acid and hydro-p-coumaric acid were seen to be the phenolic acids occurring in the highest concentrations in the Kei-apple fruit. The majority of these are concentrated in the flesh and in the case of caffeic acid, also in the peel. The order of predominance of other major non-flavonoid components in the whole fruit analysis are m-hydroxybenzoic acid > p-hydroxyphenyl acetic acid > 3-methoxy-4- hydroxyphenylpropionic acid > p-coumaric acid. The peel of the Kei-apple, apart from caffeic acid, has exceptionally high concentrations of ferulic acid and also serves as a source of protocatechuic acid. Syringic acid was most prominent in the seeds. Although the total flavonoid concentration in the Kei-apple was low, taxifolin and catechin were identified and the seeds almost entirely accounting for these. Conclusion: From this study it was concluded the Kei-apple is a rich source of antioxidant compounds (polyphenols and ascorbate), with a strong antioxidant capacity, and hence may be associated with health promotion properties, particularly in the prevention of cancer, cardiovascular disease, and neurodegeneration. Additionally, due to the increased scientific and commercial interest in this fruit, it is essential to take into consideration the various factors (agronomic, genomic, pre- and post harvest condition and processing) and tissues. This might affect the chemical composition of the final marketed product, which may play a significant role in determining the polyphenol and ascorbate composition and bioactivity of these compounds during food processing procedures. Hence, the polyphenol composition of the various fruit components should be taken into consideration when selecting a method of fruit processing into the desired end product. / Thesis (M.Sc. (Nutrition))--North-West University, Potchefstroom Campus, 2007.

There is a close relationship between the susceptibility to disease and nutritional state, in the sense that an adequate diet enhances resistance to disease. There is an increasing interest in this beneficial relationship among scientists, food manufacturers and consumers. The trend is moving towards functional foods and their specific health benefits. The results of numerous epidemiological studies and recent clinical trials provide consistent evidence that diets rich in fruits and vegetables can reduce the risk of chronic diseases. These protective effects are mediated through multiple groups of beneficial nutrients contained in the fruits and vegetables, one of these being polyphenol antioxidants. The intake of the polyphenols plays an important role in the reduction and prevention of coronary heart disease (CHD), cardiovascular disease and cancer, as a consequence of their associated antioxidant properties. Fruits contain an array of polyphenols with antioxidant capacity. Polyphenols may be classified in two broad groups namely: flavonoids and non-flavonoids. Flavonoid subgroups in fruits are further grouped as catechins, anthocyanins, procyanidins and flavonol among others. Phenolic acids occur as hydroxylated derivatives of benzoic acid and cinnarnic acid, and are classified as non-flavonoids. Polyphenols have redox properties allowing them to act as reducing agents, hydrogen donators and singlet oxygen quenchers, and thus contribute to the antioxidant capacity of fruits and vegetables. Because of the numerous beneficial effects attributed to these antioxidants, there is renewed interest in finding vegetal species with high phenolic content and relevant biological activities. In view of the importance of these substances towards health and food chemistry, this study will focus on the polyphenol and Vitamin C characterisation and quantification of an indigenous South African fruit, the Kei-apple (Dovyalis cafra), thought to have antioxidant properties. Due to the fact that polyphenol content influences the colour, taste and possible health benefits of the fruit / processed food product, this study will supply valuable information to industry in choosing the best fruit processing methods to attain the desired end product. The exploitation of indigenous South African fruits (Marula and Kei-apple) is receiving increasing prominence, not only due to their health benefits, but also the opportunities these present to rural based economics. Furthermore, this research will serve as a platform for further research on the Kei-apple and other indigenous South African fruits with possible health benefits. Aims: The overall aim of this study is the quantification and characterisation of various nutritionally important antioxidants (polyphenols and ascorbate) in the Kei-apple fruit in its entirety, as well as in its individual fruit components (peel, flesh and seeds). In addition, the total antioxidant capacity of the entire fruit and the various fruit components will be determined in the unfractionated and fractionated fruit extracts. Gas chromatography coupled mass spectrometry (GC-MS) characterisation of the individual polyphenol components will also be analyzed in order to speculate on possible specific health benefits which the Kei-apple may possess. Methods: The study was designed to ensure that a representative fruit sample was collected. Approximately 100 kg Kei-apples were picked in the month of November 2004 from the Bloemhof area in South Africa. A sample of 50 fruits was rinsed and separated into the various components (peel, flesh and seeds). An additional 50 fruits were randomly selected, cleaned and used in their entirety for data representative of the entire fruit. The sample extracts were prepared, after being grounded and lyophilized, by a method described by Eihkonen et al. (1999) using 70% aqueous acetone. The C18-fractionation on the fruit and separated fruit components resulted in four fractions containing (1) phenolic acids; (2) procyanidins, catechins and anthocyanin monomers; (3) flavonols and (4) anthocyanin polymers. The total polyphenol content of the fruit and fruit components as well as the above mentioned C18-fractions were determined by Folin-Ciocalteu's method (Singleton & Rossi, 1965). Both free and total ascorbate concentrations in these samples were determined as described by Beutler (1984), in addition to total sugar content of these via standard methods. Apart from their nutritional interest, both these measurements are necessary for the correction of the total polyphenol concentrations. The total antioxidant capacity of the entire fruit and various fruit components was determined by measuring the oxygen radical absorbance capacity (ORAC) and ferric reducing antioxidant power (FRAP) of the unfractionated and fractionated extracts. Using GC-MS analysis, the various individual polyhenol compounds contributing to the total polyphenol content of the Kei-apple was separated, identified and quantified. This quantitative data was captured and statistically analysed. The analysis of variation was performed using the Tukey Honest Significant Difference test for post-hoc comparison. ORAC, FRAP and polyphenol Pearson correlation analyses were performed using Statistics (Statsoft Inc., Tulsa, Oklahoma, USA) with significance set at P ≤ 0.05. Results and discussion: This study determined the presence of various nutritionally important antioxidants (polyphenols and ascorbate), the total antioxidant capacity in the entire fruit as well as in the individual fruit components (peel, flesh and seeds) and their polyphenol sub group fractions. Total phenol content: The Kei-apple, in its entirety, has a polyphenol concentration of 943 ± 20.3 mg GAE/100g dry weight. Comparison of the individual fruit components showed the seeds to have the highest total polyphenol concentration with 1990 ± 31.3 mg GAE/100g dry weight, followed by that of the peel, 1126 ± 45.8 mg GAE/100g dry weight and then that of the flesh, 521 ± 1.01 mg GAE/100g dry weight. Total, L-ascorbic (ASC) and L-dehydroascobic (DHA) concentration: The total ascorbate of Kei-apple fruit is 517 ± 0.92 mg/100g dry weight. In contrast to the polyphenol content, the flesh of the Kei-apple had significantly the highest concentration of total ascorbate 778 ± 1.20 mg/100g dry weight, Gascorbic 241 ± 21.0 mg/100g dry weight, as well as Gdehydroascobic 537 ± 22.2 mg/100g dry weight. The ratio of Lascorbic acidltotal ascorbate for the flesh, entire fruit, peel and seed is 0.31,0.43,0.49,0.95, respectively, indicating the seeds are the most stable source of biologically active Vitamin C, with 95% of the total ascorbate occurring as G ascorbate. This is also in line with the total polyphenol content of these components, confirming a polyphenol sparing effect on ascorbate. C18-fractionation extracts: Solid phase (C18) fractionation of the Kei-apple fruit and fruit components showed that the fruit, peels and seeds consist predominantly of phenolic acids, followed by procyanidin, catechin and anthocyanin monomers and thereafter varying amounts of anthocyanin polymers and flavonols. Antioxidant capacity: The antioxidant capacity of the entire fruit and individual fruit components as determined by ORAC, (r=0.76) and FRAP, (r=0.95) significantly correlated with the total polyphenol content, as well as to each other (r=0.88), indicating both to be good predictors of antioxidant capacity. GC-MS polyphenol characterisation of the Kei-apple: Caffeic acid and hydro-p-coumaric acid were seen to be the phenolic acids occurring in the highest concentrations in the Kei-apple fruit. The majority of these are concentrated in the flesh and in the case of caffeic acid, also in the peel. The order of predominance of other major non-flavonoid components in the whole fruit analysis are m-hydroxybenzoic acid > p-hydroxyphenyl acetic acid > 3-methoxy-4- hydroxyphenylpropionic acid > p-coumaric acid. The peel of the Kei-apple, apart from caffeic acid, has exceptionally high concentrations of ferulic acid and also serves as a source of protocatechuic acid. Syringic acid was most prominent in the seeds. Although the total flavonoid concentration in the Kei-apple was low, taxifolin and catechin were identified and the seeds almost entirely accounting for these. Conclusion: From this study it was concluded the Kei-apple is a rich source of antioxidant compounds (polyphenols and ascorbate), with a strong antioxidant capacity, and hence may be associated with health promotion properties, particularly in the prevention of cancer, cardiovascular disease, and neurodegeneration. Additionally, due to the increased scientific and commercial interest in this fruit, it is essential to take into consideration the various factors (agronomic, genomic, pre- and post harvest condition and processing) and tissues. This might affect the chemical composition of the final marketed product, which may play a significant role in determining the polyphenol and ascorbate composition and bioactivity of these compounds during food processing procedures. Hence, the polyphenol composition of the various fruit components should be taken into consideration when selecting a method of fruit processing into the desired end product. / Thesis (M.Sc. (Nutrition))--North-West University, Potchefstroom Campus, 2007.

Magister Scientiae - MSc (Biotechnology) / The mining industry in South Africa is of immense importance as this sector contributes largely to the countries income. In the Limpopo province, a large production of antimony (Sb) is generated per year. Antimony (Sb) is a trace element, which accumulates in the environment through anthropogenic activities, such as mining and smelting industries. Antimony is toxic to all living organisms and plants, and it is found to increase the peroxidation of membrane lipids and encourage an antioxidant response. Sb contamination in plants also accounts for DNA damage. The reduction in yield is due to the disruption of plant metabolism by reactive oxygen species (ROS). To combat abiotic stresses, plants have generated a signalling network that utilises multiple growth regulators that would offer protection against the stress. An increase in ROS is one of the responses to abiotic stresses. ROS is generated in response to the pants interaction with heavy metals, through the Harber-Weiss reaction. ROS compounds include: superoxide, hydrogen peroxide and hydroxyl radicals. Under normal conditions ROS molecules are produced as by-products, however, under stressful conditions the production of ROS molecules are increased to levels where they are detrimental to the plants. Therefore, the accumulation of ROS results in damage to proteins, lipids, carbohydrates and DNA which would lead to cellular death. ROS accumulation is thought to be a result of the disruption in the balance of ROS production and the anti-oxidation systems. The antioxidative system is thus introduced to restore the balance of ROS molecule production and to combat oxidative damage caused by the ROS molecules. The anti-oxidative system consists of various enzymes: superoxide dismutase, catalase, and ascorbate peroxidase and glutathione reductase. Each antioxidant scavenges one or two ROS molecules. / 2020-08-31

>Magister Scientiae - MSc / Sugargraze is a moderately drought tolerant sweet sorghum hybrid which is ideal for grazing, winter stand over and pit silage. A major advantage that Sugargraze has over other forages is its very high sugar content which improves feed quality thus increasing palatability and results in significantly reduced feed wastage. This study explored the influence of hyperosmotic stress on plant development, ROS accumulation, antioxidant capacity and the extent of cell death. Heat shock protein (Hsp70) expression immunoblotting assays were used to demonstrate whether the various treatment conditions induced stress within natural physiological parameters for the experimental material. This was coupled with the separation, visualization and identification of abundant proteins in Sugargraze leaves in response to hyperosmotic stress using two-dimensional gel electrophoresis (2-DE) in combination with mass spectrometry (MS). The results showed that hyperosmotic stress significantly influences plant development by reducing plant biomass and increasing the levels of ROS accumulation, proline content and subsequently reducing total chlorophyll content. An over accumulation of ROS in the form of hydrogen peroxide and lipid peroxidation was observed in the stressed plants which was supported by the extent of cell death. Although an increase in antioxidant enzyme activity (in the form of total enzymatic activity or individual isoform activity) in response to hyperosmotic stress was observed, this increase was not sufficient to counter the deleterious effects caused by the stress conditions hence the decrease in plant biomass and increase in cell death. Western blotting analysis of Sugargraze leaf tissues using Hsp70 antibodies showed that hyperosmotic stress induced Hsp70 expression to levels significantly higher than observed for the control plants. A total of thirteen CBB stained spots were selected for mass spectrometric identification, owing to their good resolution and abundance levels, and of these, nine were positively identified. Identified proteins were divided into functional categories including both known and novel/putative stress responsive proteins. Molecular and physiological functions of some of the proteins of interest identified will be subjected to further investigation via bioinformatic and molecular biology approaches.

>Magister Scientiae - MSc / Salinity is one of the major limiting factors to crop production, which consequently contributes to the risk of reduced food security. Among other factors, food security depends on availability of sufficient and nutritious food for humans. Livestock such as cattle and sheep are fed with various plant-based feeds; with Medicago sativa (commonly known as alfalfa or lucerne) being a very important forage/feed crop, so much that it is regarded as the queen of forage crops. However, alfalfa is severely affected by high soil salinity and thus its growth and yield are drastically reduced in soils with high NaCl content. Among the various alfalfa genotypes/varieties examined in this study, Agsalfa was identified as salt tolerant because it performed better under salt treatment compared to Magna601.

No description available.

Ascorbate (Asc) is a key antioxidant in preventing cardiovascular dysfunction during diseases exacerbated by altered shear stress. According to the literature endothelial responses to hypotonic shock share some characteristics with those induced by shear stress. Thus to study the physiological responses of endothelium to shear stress, the characterization of the Asc release by pig coronary artery endothelial cells in response to hypotonic shock was performed. The pig coronary artery endothelial cells that had been loaded with ^14C Asc and ^3H deoxyglucose, were exposed to buffers of varying osmolality for different time periods and the release of ^14C Asc and ^3H deoxyglucose was examined. Based on various parameters like relative release of ^14C Asc and ^3H deoxyglucose, their rate of release and protein loss, it was decided to use buffer of .67 percent osmolality for 2 min for these characterization studies. The Asc release was authentic and not a result of membrane damage. The hypotonic shock induced Asc release was not due to endogenously released ATP. The inhibition of ATP induced release by anion channel inhibitors niflumic acid and NPPB was complete but only partial in case of hypotonic shock induced release. The release was not inhibited under nominally Ca^2+ free conditions. Additive release by hypotonic shock and ATP or hypotonic shock and Ca^2+ ionophore A23187 suggests that there are two independent Asc release pathways. Asc release by two different mechanisms may help endothelial cells deal with stressful conditions efficiently and preserve endothelial function. / Thesis / Master of Science (MS)

A cours du développement du fruit, son statut oxydatif évolue entraînant une évolution concomitante des activités enzymatiques antioxydantes et ceci en interaction avec des hormones impliquées dans le développement et la maturation du fruit. Ces enzymes antioxydantes sont la superoxyde dismutase (SOD) et la Catalase (CAT) mais également de celles du cycle d’Haliwell-Asada (ascorbate peroxydase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) et gluthation reductase (GR) impliquées également dans le recyclage de l’ascorbate. L’objectif de la thèse est de comprendre les interactions existant entre le stress oxydatif induit par les stress environnementaux au niveau des organes végétatifs et le recyclage de l’ascorbate dans les fruits de tomate (cv Micro-Tom).Dans une première partie nous montrons qu’un déficit hydrique contrôlé et rapide induit une diminution du potentiel hydrique foliaire (Ψh) sans aucun symptôme de stress photo-oxydatif détectable au niveau du PSII après 24h et sans que le statut hydrique du fruit ne soit affecté. Dans ces conditions, on observe toutefois une augmentation du H2O2 dans les fruits et une augmentation de l’activité des enzymes antioxydantes et de celles impliquées de recyclage de l’ascorbate. Par ailleurs, nous montrons une production de NO et de ABA en réponse au stress dans la plante. La localisation de NO a été réalisée par microscopie à fluorescence en utilisant la nouvelle sonde NO, la NO550 (mise au point pendant le doctorat).Afin de déterminer si NO est responsable de la mise en place de la réponse antioxydante du fruits en interaction avec l’ABA et H2O2, une approche pharmacologique a été réalisée. Les résultats montrent une augmentation des activités de ces enzymes en présence de ces trois molécules, avec une plus forte augmentation en présence de NO à chaque fois. Par ailleurs, nous montrons que l'ABA induit la synthèse de NO dans le fruit et non l’inverse. On peut conclure de cette analyse que l’ABA induit par le déficit hydrique est responsable de la synthèse de NO dans les fruits et ce signal va induire l’activation des enzymes antioxydantes en association avec H2O2.Finalement, une approche transcriptomique a été réalisée pour étudier d’une part les gènes induits par NO et les gènes induits par le déficit hydrique au travers du NO. Les résultats suggèrent quele NO est à la croisée de la réponse au stress biotique et abiotiques et pourrait être utilisé pour acclimater les plantes au stress biotiques. / Oxidative status alongside with antioxidant enzymes activities constantly evolve during fruit development. This evolution is closely related to hormones involved in fruit development and maturation. The antioxidant enzymes are superoxide dismutase (SOD) and Catalase (CAT) as well as those of Haliwell-Asada cycle (ascorbate peroxydase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and gluthation reductase (GR) which are also implicated in ascorbate recycling. The objective of this work is to decipher the interactions between oxidative stress induced by environmental stress in vegetative organs and ascorbate recycling in tomato fruits (cv Micro-Tom).Results obtained show that a rapid and controlled water deficit result in a fall in water potential (LΨw) whereas other water parameters remained unaffected and without any photo-oxidative symptoms detected in PS II after 24 hours. However we observed alongside an increase in H2O2 and of the activity of antioxidant enzymes as well as those involved in ascorbate recycling. Furthermore an increase in NO and ABA production was also detected in the plants in response to the stress. NO localisation was realised using fluorescence microscopy using the newly synthesised NO probe NO550 (developed during the thesis).We used a pharmacological approach in order to determine if NO is responsible of the set up of the antioxidant response together with ABA and H2O2. Results show an increase in the activity of the enzymes in contact with the three molecules with every time a greater increase with NO. Furthermore, we show that ABA induces NO production. Those results made us conclude that ABA production induced by the water deficit is responsible of NO synthesis in fruits and may have the action of a signal with activates antioxidant enzymes with the collaboration of H2O2.A microarray analysis was also conducted in order to study the genes induced by NO and the genes induced by water deficit through NO. Results suggest that NO is at the cross road of the response towards biotic and abiotic stress and might be a useful tool to acclimatize plant to stressful conditions.

Contrôle de l'oxydation et de la dégradation du pool de vitamine C chez la tomate et impact sur la qualité du fruit et la tolérance au stress. Le métabolisme de l’ascorbate et plus principalement le statut redox du pool d’ascorbate sont impliqués dans la tolérance au stress et dans les processus primaires de croissance et de développement de la plante. La teneur et le statut redox de l’ascorbate chez les plantes sont régulés par (i) ses voies de biosynthèse, (ii) par le cycle ascorbate-glutathion permettant le recyclage des formes semi-oxydées et oxydées de l’ascorbate et (iii) par sa dégradation, l’ensemble de ces processus étant sous le contrôle de l’environnement. Au cours de ce travail de thèse, des méthodes de transgénèse nous ont permis d’identifier, chez différents génotypes de tomate à petit et gros fruits, les bouleversements physiologiques et métaboliques permettant de compenser des modifications de l’activité des enzymes monodéhydroascorbate réductase (impliqué dans le cycle ascorbate-glutathion) et ascorbate oxydase. Nous avons observé d’importantes modifications phénotypiques altérant le rendement en fruits de la plante sous conditions de culture pouvant générer un stress et également en condition normale de culture. Des liens entre l’activité des enzymes précités avec le métabolisme des sucres, la photosynthèse et la conductance stomatique sont révélés. Le déséquilibre entre les activités oxydantes et réductrices de ces enzymes constitue la première étape vers une dégradation de l’ascorbate. Le taux de dégradation se révèle très faible à la lumière, tandis qu’à l’obscurité une forte accumulation des produits de dégradation l’oxalate, le thréonate ainsi que l’oxalyl-thréonate est observé dans les feuilles de tomate. Enfin, l’activité de l’enzyme MDHAR est corrélée au taux de dégradation à l’obscurité. Les travaux de cette thèse mettent en avant l’importance du statut redox du couple ascorbate / monodéhydroascorbate dans les processus de croissance cellulaire et entre dans la régulation du rendement chez la tomate, et influe la dégradation de l’ascorbate. / Ascorbate metabolism and particularly ascorbate redox status are involved in stress tolerance and growth processes of plant cells. The concentration of ascorbate and its redox status are under control of (i) its biosynthetic pathways, (ii) the ascorbate-glutathione cycle allowing recycling of semi-oxidized and oxidized forms of ascorbate and (iii) its degradation rate. These processes are under environmental control. Transgenic lines modified for the activity of monodehydroascorbate reductase (involved in ascorbate-glutathione cycle) and ascorbate oxidase were generated in cherry and large-fruited genotypes of tomato. Physiological and metabolic modifications related to the modification of these enzyme activities were studied. We observed large phenotypic alterations that affected fruit yield under both stress conditions and normal growth conditions. Links between ascorbate recycling and sugar metabolism, photosynthesis and stomatal conductance were also revealed. An imbalance between the oxidizing and reducing activities of these enzymes is the first step leading to ascorbate degradation. We have shown that the degradation rate was very low under light, whereas under darkness the degradation compounds oxalate, threonate and oxalyl-threonate accumulated in tomato leaves. Also, the degradation rate is correlated with MDHAR activity. These results highlight the crucial role of the redox status of the ascorbate / monodehydroascorbate couple in growth processes and yield stability in tomato, and the impact on ascorbate degradation.