Integrated study of climate change adaptation in Canarium madagascariense: water stress response, silicon-mediated tolerance, and elemental bioaccumulation

Madagascar is a global biodiversity hotspot, characterised by a high level of endemism and a strong reliance on medicinal plants for primary healthcare. However, increasing anthropogenic pressures, including deforestation, overexploitation and climate change, are threatening both plant diversity and ecosystem stability. Canarium madagascariense Engl., an endemic and endangered tropical tree, is among the vulnerable species and has ecological, economic and medicinal importance. Despite its significance, its adaptive responses to environmental stress and safety for medicinal use are poorly documented. This thesis aims to investigate C. madagascariense's response to water stress, evaluate the role of potassium silicate (PS) in enhancing drought tolerance, and assess its elemental composition and bioaccumulation patterns. The first part of the study focused on the ecophysiological responses of one-year-old seedlings under different irrigation regimes. Water stress significantly reduced growth parameters, including height and diameter, primarily due to stomatal closure, reduced photosynthesis, and impaired nutrient uptake. Biochemical analyses revealed alterations in soluble sugars and phenolic compounds, indicating the activation of stress response mechanisms. Applying potassium silicate partially mitigated the effects of stress by enhancing antioxidant-related compounds, suggesting a protective role against oxidative damage. The second part of the study investigated the effects of potassium silicate on the drought tolerance of two-year-old seedlings, focusing on their morphological, physiological and biochemical traits. Although PS application did not significantly enhance growth under stressful conditions, it had a positive impact on physiological parameters such as chlorophyll content and stomatal conductance. Furthermore, PS contributed to osmotic regulation by increasing sugar accumulation and improving antioxidant defence systems. Hormonal analysis revealed that PS may play a role in modulating stress-related phytohormones, particularly abscisic acid and auxins. This contributes to improved stress resilience. These findings suggest that PS enhances functional responses rather than structural growth under drought conditions. The third part of the thesis examined the elemental composition of C. madagascariense and its surrounding soil in order to evaluate its suitability for medicinal use. The results showed that the soils exhibited the typical characteristics of tropical systems, with low levels of contamination despite their proximity to mining activities. The plant demonstrated selective uptake of elements, accumulating essential nutrients such as potassium, calcium and magnesium, while limiting the accumulation of potentially toxic elements. Although lead concentrations slightly exceeded the recommended limit, they remained below the threshold for toxic effects on plant growth. Overall, the elemental profile indicates that the species is safe to consume under the conditions studied. In conclusion, this research sheds new light on the adaptive strategies employed by C. madagascariense in response to water stress, while also highlighting the potential of potassium silicate as a means of enhancing drought tolerance in tropical tree species. Furthermore, it contributes to the evaluation of medicinal plant safety by integrating elemental analysis. These findings support the development of sustainable conservation strategies and the safe use of endemic plant resources in the context of climate change