Regulation of the Degree of Manifestation of Obligate CAM Pathway in Sedum dendroideum by Salinity

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Abstract
Biochemical, physiological, and anatomical features of salt tolerance of Sedum dendroideum Moc. & Sessé ex DC species with obligate CAM photosynthesis were studied at moderate (300 mM NaCl) and high (500 mM NaCl) salinity. Differences in the response of plants to different levels of salinity were revealed. Under moderate salinity, an increase in photosynthetic CAM metabolism was observed, which was accompanied by a significant increase in night-time acidification of cell sap in tissues, the content of PEP carboxylase and proline, and activation of cyclic electron transport around PS I and superoxide dismutase activity. Significant anatomical changes in mesophyll cells of leaf and stem were revealed: compression of vascular bundles, granularity of the cytoplasm, plasmolysis of leaf mesophyll parenchyma cells and their displacement towards the vascular bundle. A number of parameters changed depending on the level of salinity and these changings were more pronounced at high salinity. Significant reduction in leaf biomass, accumulation of sodium ions, increased plasmolysis of leaf cells, and an increase in the volume of water-bearing parenchyma cells of the stem, accompanied by the destruction of their cell walls (lysis) was observed at high salinity. Oxidative stress was observed only under high salinity but was mild, probably due to a significant increase in peroxidase activity, the content of fatty acid esters and phenolic compounds, namely flavans. Thus, an increase in the photosynthetic CAM pathway was observed only under moderate salinity. Under high salinity, the decisive factor in the adaptation process was the activation of the antioxidant system. Implementation of S. dendroideum at different levels of salinity and various adaptation mechanisms, as well as regulation by external factors of the degree of manifestation of the obligate CAM pathway, is discussed.

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