Dissertation committee:
1. Σοφία Ριζοπούλου, Ομότιμη Καθηγήτρια (Επιβλέπουσα), Τμήμα Βιολογίας, Ε.Κ.Π.Α.
2. Εμμανουήλ Στρατάκης, Διευθυντής Ερευνών Ινστιτούτου Ηλεκτρονικής Δομής και Λέιζερ (ΙΗΔΛ) του Ιδρύματος Τεχνολογίας & Έρευνας (ΙΤΕ), Ηράκλειο Κρήτης
3. Μαρία-Σόνια Μελετίου-Χρήστου, τ. Επίκουρη Καθηγήτρια, Τμήμα Βιολογίας, Ε.Κ.Π.Α.
4. Κοσμάς Χαραλαμπίδης, Αναπληρωτής Καθηγητής, Τμήμα Βιολογίας, Ε.Κ.Π.Α.
5. Δημήτρης Χατζηνικολάου, Αναπληρωτής Καθηγητής, Τμήμα Βιολογίας, Ε.Κ.Π.Α.
6. Ιωάννης-Δημοσθένης Αδαμάκης, Επίκουρος Καθηγητής, Τμήμα Βιολογίας, Ε.Κ.Π.Α.
7. Ζαχαρούλα Γκόνου-Ζάγκου, Επίκουρη Καθηγήτρια, Τμήμα Βιολογίας, Ε.Κ.Π.Α.
Summary:
Mediterranean ecosystems, characterized by mild rainy winters, followed by long, hot, dry, and sunny summers have peculiar vegetation dominated by evergreen sclerophyllous shrubs and phrygana. These plants adapt to environmental pressures through mechanisms to avoid or mitigate the negative effects of extreme conditions on their leaves.
For many decades human activity has caused marked changes in the composition of the atmosphere with adverse effects on the health and growth of living organisms and ecosystems. The urgent need to monitor and evaluate the management of industrial zones to reduce air pollution has led to the improvement and development of monitoring methods for different habitats. At the same time, biological monitoring methods based on the study of leaf reflectance of plants were used.
The plant species, Ceratonia siliqua (carob), is a typical mediterranean plant which has adaptive mechanisms that make it resistant to drought, salinity, nutrient deficiencies and polluted environments. At the same time, the plant does not have special requirements in terms of soil conditions and cultivation. It is widely used in the food, cosmetics, and pharmaceutical industries as well as for reforestation and other plantings in urban landscapes. This is because of its protection of the soil from erosion and its dense evergreen foliage, making it an important plant in the greek flora.
The focus of this thesis, which was carried out in the Department of Botany of the Faculty of Biology of the National and Kapodistrian University of Athens in collaboration with F.O.R.T.H., was to investigate the optical properties and ecophysiological parameters of carob leaves from different. Additionally, the aim was to find possible differences and to evaluate the long-term results. The ultimate goal was to study of leaf properties as a biomonitoring method for assessing the effect of air quality on plant organisms based on seasonal variations and developmental stages.
Five different (more and less polluted) habitats were selected based on the existence of nearby air quality monitoring stations. Three shoots were collected from each site at the same time, with nine succesively grown compound leaves, along the shoots, with leaflets that were carefully marked (same age, same orientation towards sunlight).
The accumulation of pigments, proline, total sugars, phenolic compounds, starch, and H2O2, as well as SLA in leaves of the urban habitat, regardless of the developmental stage, showed that they received statistically higher values than the leaves of the suburban site. Furthermore, the results indicated that low concentrations of NOx can have a positive effect as a nitrogen source in plants. Abiotic stress triggers the overproduction of active oxygen species (H2O2), and degradation mechanisms (scavenging of Reactive Oxygen Species) involve both enzymatic and non-enzymatic reactions, protecting cells from oxidative damage and thus explaining the high concentrations of H2O2, proline, carotenoids, and phenolic compounds in urban tissue plants. Likewise, the accumulation of MDA in leaves of suburban habitat may be related to the high concentration of ozone, which acts on lipid peroxidation through the enzyme reaction of lipoxygenase and is considered as indicator of lipid peroxidation of membrane lipids by UV radiation. Regarding seasonal variation, young leaves show greater accumulation of proline, carotenoids, phenolic compounds, H2O2 and MDA during periods of stress, while starch increases before the beginning of the vegetative period for possible hydrolysis and transfer of products to the receptor.
Regarding the position of the leaves in the stem, pigment accumulation showed an upward trend from the 1st to the 4th leaf (starting from the top of the shoot) and then stabilized throughout the year defining the 4th leaf as a fully developed leaf.
Leaf thickness, mesophyll thickness, and the ratio of palisade to mesophyll significantly increased in leaves grown in more polluted environments, as dense and compact mesophyll is usually an indication of continuous exposure of the plant to air pollution. However, the thickness of the adaxial, abaxial epidermis and the adaxial, abaxial periclinal walls of epidermal cells showed statistically significantly higher values in leaves from the suburban site. Thus, creating a hydrophobic layer that controls transpiration and acts as a filter to protect the plant from UV and IR radiation during periods of stress. Comparing structural leaf traits of mature and expanding leaves revealed that the young suburban leaves have taken more than 93 % of their final form. This significant finding implies that their growth and differentiation are completed in the shortest time frame when conditions are suitable.
In relation to the light absorption near 1470 and 1900 nm, the water content of the leaves was estimated to be higher in leaves of the suburban site, regardless of the developmental stage. In addition, carob trees maintained their cell turgor pressure, especially during periods of water deficit, since they manage to complete leaf growth in a shorter time, than leaves in the urban environment.
The plant seems to have adapted and shown tolerance to the polluted urban environment and also shown signs of coping with the environmental conditions. The carob tree also exhibited varying responses to different habitats, with respect to specific parameters.
An attempt was made to correlate the results of the accumulation values of the physiological parameters with the concentrations of air pollutants (NOX, O3) in order to create an equation that would correlate these variables. The chlorophyll, starch, and phenolic compound concentration data were successfully fitted to the Michaelis-Menten equation describing enzyme kinetics. This convergence is justified by the fact that the plant responds to increased levels of pollutants, particularly atmospheric nitric oxides, by exhibiting a saturation effect. This method of estimating the accumulation of NO2 is of particular importance and usefulness because the evaluation of air quality by atmospheric methods has a small surface coverage and can therefore lead to erroneous conclusions.
In sum, the laboratory determination of the concentrations of these compounds can provide an indication of air quality in a habitat, potentially making carob trees a significant biosensor for monitoring air pollution.
Keywords:
Ceratonia siliqua, biomonitoring air pollution, anatomical traits, optical properties, Chlorophylls, starch, phenolic copmounds, H2O2, MDA, SLA, abiotic stress
