Abstract
Woody plants are characterised by a highly complex vascular system, wherein the secondary xylem (wood) is responsible for the axial transport of water and various substances. Previous studies have focused on the dead conductive elements in this heterogeneous tissue. However, the living xylem parenchyma cells, which constitute a significant functional fraction of the wood tissue, have been strongly neglected in studies on tree biology. Although there has recently been increased research interest in xylem parenchyma cells, the mechanisms that operate in these cells are poorly understood. Therefore, the present review focuses on selected roles of xylem parenchyma and its relevance in wood functioning. In addition, to elucidate the importance of xylem parenchyma, we have compiled evidence supporting the hypothesis on the significance of parenchyma cells in tree functioning and identified the key unaddressed questions in the field.
Highlights
Parenchyma—Role and Relevance in Vascular plants have developed a sophisticated and efficient transport system that supplies water and various other substances, including photoassimilates, ions, and hormones, to all plant organs
The vascular system plays an important role in the regulation of various developmental and physiological processes, and owing to the long lifespan and large size of woody plants, it is crucial for the efficient long-distance communication between the distantly positioned roots and the tree’s canopy [5,6,7,8]
Gymnosperm wood primarily consists of tracheids and only a small amount of xylem parenchyma cells, often limited to rays [15,16,18], whereas angiosperm wood consists of vessel elements, tracheids, fibres, and abundant xylem parenchyma fractions [1]
Summary
Parenchyma—Role and Relevance in Vascular plants have developed a sophisticated and efficient transport system that supplies water and various other substances, including photoassimilates, ions, and hormones, to all plant organs. Tracheary elements, which are dead cells, are the major type of wood cells arranged and connected axially to form a long route for transporting xylem sap. These elements can resist large negative pressure because of their thick, lignified secondary cell walls [1,2,14]. Vessel elements are the major conductive cell type in angiosperms, usually wider in diameter than tracheids and arranged axially, one above the other, to form long tubes called vessels [5,16]. Sapwood is found in the outer wood zone, which is the physiologically active part of the secondary xylem It contains living cells and it is the region where xylem sap is transported within tracheary elements. In some groups of conifers, resin canals can be identified [18] (Figure 1)
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