Phenolic compound localisation in Polypodium vulgare L. rhizomes after mannitol-induced dehydration and controlled desiccation

Polypodium vulgare L. is a desiccation-tolerant fern that can withstand successive dry periods in its life cycle. To better understand this mechanism, the current study was undertaken to assess the role of phenolic compounds in rhizome dehydration and determine their localisation in the rhizome cells after enforced dehydration in mannitol solution or controlled desiccation with or without abscisic acid (ABA) pretreatment. Phenolic distribution at the subcellular level was studied using gold particle-complexed laccase. Cells from different tissues: cortical parenchyma, endodermis and stelar elements-pericycle, sieve cells and vascular parenchyma were observed under a transmission electron microscope (TEM). The content of phenolic compounds was greater in ABA-untreated rhizomes after enforced dehydration in mannitol solution and subsequent rehydration. After controlled desiccation the phenolic content significantly increased in ABA-untreated rhizomes. A large number of phenolic compound deposits were present in all types of rhizomatous cells. Phenolics were widely distributed in the vacuoles of all cells, and in the secondary cell walls of sieve cells, although scattered labelling was hardly ever observed in the primary cell walls. In dehydrated and plasmolysed cells from the cortex and endodermis, phenolic compounds were present in the apoplastic compartments between the plasma membranes and the cell walls. There is evidence that abscisic acid plays a role as a crucial antioxidant resulting in no damage and a lower level of phenolic increase as compared to ABA-untreated rhizomes. Moreover, the location of phenolics suggests a protective chemical barrier against environmental stresses.