ROLE OF STRUCTURE · Cellular and extra-cellular pathways for sap flow: their anatomy and physiological significance

Final Activity and Management Report Summary - ROLE OF STRUCTURE (Cellular and extracellular pathways for sap flow: their anatomy and physiological significance)

Better understanding of plant-water relations is essential for appropriate management of forest and agricultural ecosystems. In spite of water's importance, there are surprising gaps in our knowledge of how water is stored and moves in trees. Moreover, the variation in wood tissue design of different taxa and habitats is enormous. The project was designed to further our knowledge of the structure and water-transport physiology in woody plants. An international team of researchers using a combination of modern techniques from the different scientific disciplines of plant anatomy, plant development, and ecophysiology, performed research on the structure, and the water-conductive function of xylem in species with different patterns of hydraulic architecture. A general understanding of the water movement in plants is that water ascends from the lumens of cells, through pits, and into adjacent lumens pulled towards the leaves by the transpiration stream. However, recent evidence on the variability of ultrastructure of pits, ion-mediated control of pit membranes permeability, and the role of aquaporins and living parenchyma show that water transport processes in xylem are more complicated than it was previously assumed. The paradigm that water in xylem is conducted through inert and dead cells needs re-examination. For understanding the mechanisms underlying the dynamic nature of xylem and its ability to regulate flow, we need a profound knowledge of the three-dimensional (3-D) xylem structure. At present, there is a revolution in techniques for 3-D microscopic visualisation and image analysis. Microcasting and confocal microscopy were used by P. Kitin, Marie Curie Fellow, for structural analysis of plant tissues, and these techniques were complemented with work of ecophysiologists in Oregon State University and the Royal Museum for Central Africa in Belgium to investigate the cells and pathways for movement of water and gas in several African and Japanese hardwood species. We reported first steps to understand the potential roles of various vascular cells in the apoplastic and symplastic flow between xylem, cambium and phloem. An essential part of the project included various microscopy work. In order to interpret physiological experiments of plant water movement, it was needed to visualise the water in the water-conducting cells. Therefore, an original cryo-microscopy stage was built that allowed observation by light microscopy of frozen water in xylem conduits, as well as vacuoles and cytoplasm content. We currently use the cryo-system to investigate the occurrence of air embolisms in xylem as well as to visualise phenolics in xylem cells. The project implemented novel methods for investigation of little explored areas of the structure and transport function of wood. We believe we have demonstrated that collaboration among people using novel approaches in several scientific fields is what advances knowledge in each of the fields. Our three-dimensional reconstructions of water conductive paths in xylem, and investigations by cryo-light microscopy of the functionality of different types of xylem, are contributions to the knowledge in the anatomy and ecophysiology of plants, which are fundamental for many areas of the agricultural and forest science.

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