STUDY OF THE EFFECTS OF RARE EARTH ELEMENTS ON TOMATO PLANTS GROWN IN HYDROPONIC SYSTEM

RARE EARTH ELEMENTS (REES) ARE A GROUP OF 17 ELEMENTS CONSISTING OF YTTRIUM (Y), SCANDIUM (SC), AND THE 15 LANTHANIDES WITH ATOMIC NUMBERS RANGING FROM 57 TO 71. THE TERM “RARE” IS SOMEWHAT MISLEADING, AS IT REFERS TO THEIR DISPERSED DISTRIBUTION RATHER THAN THEIR ACTUAL ABUNDANCE; INDEED, SOME LANTHANIDES, SUCH AS LANTHANUM (LA), CERIUM (CE), AND NEODYMIUM (ND), ARE RELATIVELY ABUNDANT IN THE EARTH’S CRUST. REES EXHIBIT SIMILAR PHYSICAL AND CHEMICAL PROPERTIES AND ARE WIDELY USED IN SEVERAL INDUSTRIAL SECTORS, INCLUDING GLASS AND CERAMICS PRODUCTION, ELECTRONICS, RENEWABLE ENERGY TECHNOLOGIES, MILITARY APPLICATIONS, AGRICULTURE, AND MEDICINE. DESPITE THEIR INCREASING USE IN AGRICULTURE, THEIR BENEFICIAL EFFECTS ON CROPS HAVE NOT BEEN CLEARLY DEMONSTRATED, NOR HAVE CONSISTENT IMPROVEMENTS IN PLANT GROWTH BEEN ESTABLISHED. IN RECENT YEARS, A GROWING NUMBER OF STUDIES HAVE INVESTIGATED THE EFFECTS OF REES ON DIFFERENT PLANT SPECIES. OVERALL, AVAILABLE EVIDENCE SUGGESTS THAT LOW CONCENTRATIONS OF REES MAY STIMULATE PLANT GROWTH, WHEREAS HIGH CONCENTRATIONS EXERT PHYTOTOXIC EFFECTS AND SIGNIFICANTLY REDUCE PLANT DEVELOPMENT. THIS BIPHASIC RESPONSE IS COMMONLY DESCRIBED AS HORMESIS, A PHENOMENON IN WHICH LOW DOSES OF A TYPICALLY HARMFUL AGENT MAY INDUCE BENEFICIAL OR ADAPTIVE RESPONSES, WHILE HIGHER DOSES BECOME DETRIMENTAL ONCE A THRESHOLD IS EXCEEDED. DESPITE THE INCREASING NUMBER OF STUDIES, THE MECHANISMS OF ACTION OF REES IN PLANTS REMAIN LARGELY UNCLEAR. PROPOSED EXPLANATIONS MAINLY INVOLVE ALTERATIONS IN NUTRIENT UPTAKE, ENZYME ACTIVITY, AND PHOTOSYNTHETIC PERFORMANCE; HOWEVER, THEIR EFFECTS AT THE CELLULAR AND MOLECULAR LEVELS ARE STILL NOT FULLY UNDERSTOOD. IN THIS DOCTORAL THESIS, WE INVESTIGATED THE EFFECTS OF RARE EARTH ELEMENTS ON DIFFERENT PLANT SPECIES, FOCUSING NOT ONLY ON MACROSCOPIC RESPONSES SUCH AS GERMINATION, GROWTH, AND MORPHOLOGICAL CHANGES, BUT ALSO ON PHYSIOLOGICAL AND MOLECULAR PROCESSES. IN PARTICULAR, WE ANALYZED OXIDATIVE STRESS RESPONSES, ANTIOXIDANT DEFENSE MECHANISMS, ELEMENT ACCUMULATION AND TRANSLOCATION, IMPACTS ON THE PLANT-ASSOCIATED MICROBIOME, AND EPIGENETIC REGULATION, WITH A SPECIFIC FOCUS ON DNA METHYLATION DYNAMICS UNDER REES EXPOSURE. OVERALL, THE RESULTS OF THIS THESIS HIGHLIGHT THAT REES, AND IN PARTICULAR GADOLINIUM, ACT AS MULTI-LEVEL STRESSORS CAPABLE OF AFFECTING PLANT PERFORMANCE FROM PHYSIOLOGICAL TO MOLECULAR SCALES. THE INTEGRATION OF PHYSIOLOGICAL, MICROBIOLOGICAL, AND EPIGENETIC DATA PROVIDES NEW INSIGHTS INTO THE COMPLEXITY OF PLANT RESPONSES TO EMERGING ENVIRONMENTAL CONTAMINANTS AND SUGGESTS THAT EPIGENETIC REGULATION REPRESENTS A KEY MECHANISM UNDERLYING BOTH EARLY STRESS PERCEPTION AND LONG-TERM ACCLIMATION. THESE FINDINGS CONTRIBUTE TO A MORE COMPREHENSIVE UNDERSTANDING OF REE–PLANT INTERACTIONS AND UNDERLINE THE NEED FOR FURTHER STUDIES TO CLARIFY THEIR ECOLOGICAL RELEVANCE AND POTENTIAL LONG-TERM ENVIRONMENTAL IMPACTS.