Plasticidade de cajueiros à baixa disponibilidade hídrica no solo: mecanismos fisiológicos, metabólicos e epigenéticos

Abstract

The cashew tree (Anacardium occidentale L.) is a highly relevant species for the Brazilian semi-arid region, which it develops under environmental conditions characterized by a combination of abiotic stresses, such as high irradiance, salinity, water deficit, and high temperatures. Adaptation to this environment involves physiological and molecular mechanisms that regulate acclimation to environmental stress. This thesis investigated the processes that support tolerance and the ability to adjust to water deficit in the cashew tree, integrating different biological levels to identify functional patterns associated with adaptation. In the first chapter, physiological, biochemical, metabolic, and molecular responses were evaluated in two contrasting cashew genotypes subjected to successive cycles of water deficit followed by rehydration. The BRS 226 genotype showed greater osmotic stability during stress, more efficient photosynthetic recovery, and greater functional adjustment capacity after repeated drought exposures. The regulation of the redox state and the progressive activation of the antioxidant system indicated physiological adjustment to recurrent stress, accompanied by extensive metabolic reorganization, with modulation of compounds associated with energy metabolism and cellular adaptation. In contrast, the CCP 09 genotype showed more restricted physiological and metabolic responses, with less recovery capacity after successive stress cycles. The differential modulation of the expression of genes associated with the stress response and the activation of markers re ated to DNA methylation during recovery suggest the participation of epigenetic mechanisms in the physiological reorganization induced by drought cycles. Taken together, the results indicate that the differential tolerance between genotypes is associated with the coordination between physiological, metabolic, and molecular adjustments in response to recurrent water stress. The second chapter investigated the epigenetic dynamics associated with a single water deficit event followed by recovery in cashew seedlings, using enzymatic sequencing of DNA methylation. Although the overall methylation profile remained relatively stable across water conditions, extensive local alterations were detected throughout the genome, indicating epigenetic reorganization associated with the plant's physiological state. These modifications were related to distinct regulatory and metabolic processes between stress and recovery phases, suggesting that DNA methylation participates in the functional modulation of the water response. The observed epigenetic variations accompanied changes in photosynthetic performance and the expression of marker genes, highlighting an association between epigenetic modulation and physiological adjustments to water deficit. In an integrated manner, the results demonstrate that the cashew tree's response to water deficit involves coordination between physiological, metabolic, and epigenetic processes that contribute to functional adjustments of metabolism and the photosynthetic apparatus during stress and recovery. These findings broaden the understanding of the regulatory mechanisms associated with the water response in tropical perennial species and provide a basis for the development of genetic improvement strategies and adaptive management under scenarios of greater climate variability.