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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.

• Parenchyma cells in the xylem store nonstructural carbohydrates (NSC), providing reserves of energy that fuel woody perennials through periods of stress and/or limitations to photosynthesis. If the capacity for storage is subject to selection, then the fraction of wood occupied by living parenchyma should increase towards stressful environments. • Ray parenchyma fraction (RPF) and seasonal NSC dynamics were quantified for 12 conifers and three oaks along a transect spanning warm dry foothills (500 m above sea level) to cold wet treeline (3250 m asl) in California’s central Sierra Nevada. • Mean RPF was lower for both conifer and oak species with warmer dryer ranges. RPF variability increased with elevation or in relation to associated climatic variables in conifers – tree-line-dominant Pinus albicaulis had the lowest mean RPF measured (c. 3.7%), but the highest environmentally standardized variability index. Conifer RPF variability was explained by environment, increasing predominantly towards cooler wetter range edges. In oaks, NSC was explained by environment – values increasing for evergreen and decreasing for deciduous oaks with elevation. Lastly, all species surveyed appear to prioritize filling available RPF with sugar to achieve molarities that balance reasonable tensions over starch to maximize stored carbon. • RPF responds to environment but is unlikely to spatially constrain NSC storage.

Due to land use change, green alder (Alnus alnobetula), formerly restricted to moist slopes, is now expanding to drier sun-exposed sites at the alpine treeline. The highly productive shrub is forming closed thickets, establishing nitrogen-saturated species poor shrublands. To evaluate wood anatomical adaptations to changing environmental conditions, we analyzed vessel characteristics (mean vessel area, MVA; vessel density, VD; and theoretic conductive area, TCA) and axial parenchyma abundance, as well as their distribution in the annual ring at a moist north-facing and a dry south-facing site at the alpine treeline on Mt. Patscherkofel (Central European Alps, Austria). Results revealed that lower soil water availability and enhanced evaporative demand did not affect MVA while VD and TCA were significantly reduced at the dry south-facing site. This suggests that in green alder, vessel size is a static trait whereas vessel number responds plastic. Limited water availability also triggered a significant increase in axial parenchyma, confirming the important role of xylem parenchyma in water relations. Harsh environmental conditions at the distributional limit of green alder may have affected xylogenesis, leading to a near semi-ring-porous distribution of vessels and an accumulation of parenchyma in the late growing season. We conclude that in a warmer and drier climate, growth limitation and physiological stress may set limits to the distribution of Alnus alnobetula at drought-prone sites in the alpine treeline ecotone.

Main conclusion Cell wall fluorescence and immunocytochemistry demonstrate that xylem parenchyma cell walls do not show changes in structure and composition related to gravitropic response comparable to those of tracheids, even when they have lignified secondary cell walls. Tracheid cell walls in compression wood have altered composition and structure which generates the strain responsible for correction of stem lean as part of the gravitropic response of woody plants. Xylem parenchyma cell walls vary among conifer species and can be lignified secondary walls (spruce) or unlignified primary walls (pine). It can be expected that xylem parenchyma with lignified secondary cell walls might show features of compression wood comparable to those of tracheids that have a similar type of cell wall. A comparison of xylem parenchyma cell walls in normal and compression wood in species with lignified and non-lignified parenchyma cell walls provides a unique opportunity to understand the process of reaction wood formation in conifers. Using both UV/visible fluorescence microscopy of cell wall fluorophores and immunocytochemistry of galactan and mannan epitopes, we demonstrate that xylem parenchyma cell walls do not show the changes in composition and structure typical of compression wood tracheids. Adjacent cells of different types but with similar cell wall structure can undergo cell wall developmental changes related to support or defence functions independent of their neighbours. Tracheids are sensitive to gravitropic signals while xylem parenchyma cells are not.

Ultrasonic acoustic emission (UAE) methods have been applied for the detection of freeze–thaw-induced embolism formation in water conduits of tree species. Until now, however, the exact source(s) of UAE has not been identified especially in angiosperm species, in which xylem tissues are composed of diverse types of cells. In this study, UAE was recorded from excised branches of walnut (Juglans regia cv. Franquette) during freeze–thaw cycles, and attempts were made to characterize UAEs generated by cavitation events leading to embolism formation according to their properties. During freeze–thaw cycles, a large number of UAEs were generated from the sample segments. However, the cumulative numbers of total UAE during freeze–thawing were not correlated with the percentage loss of hydraulic conductivity after thawing, suggesting that the sources of UAE were not only cavitation leading to embolism formation in vessels. Among the UAEs, cumulative numbers of UAEs with absolute energy >10.0 fJ strongly correlated with the increase in percentage loss of hydraulic conductivity. The high absolute energy of the UAEs might reflect the formation of large bubbles in the large lumen of vessels. Therefore, UAEs generated by cavitation events in vessels during freeze–thawing might be distinguished from other signals according to their magnitudes of absolute energy. On the other hand, the freezing of xylem parenchyma cells was followed by a certain number of UAEs. These results indicate the possibility that UAE methods can be applied to the detection of both freeze–thaw-induced embolism and supercooling breakdown in parenchyma cells in xylem.

Silver birch (Betula pendula Roth) is an economically important species in Northern Europe. The current research focused on the molecular background of different xylogenesis scenarios in the birch trunks. The study objects were two forms of silver birch, silver birch trees, and Karelian birch trees; the latter form is characterized by the formation of two types of wood, non-figured (straight-grained) and figured, respectively, while it is currently not clear which factors cause this difference. We identified VND/NST/SND genes that regulate secondary cell wall biosynthesis in the birch genome and revealed differences in their expression in association with the formation of xylem with different ratios of structural elements. High expression levels of BpVND7 accompanied differentiation of the type of xylem which is characteristic of the species. At the same time, the appearance of figured wood was accompanied by the low expression levels of the VND genes and increased levels of expression of NST and SND genes. We identified BpARF5 as a crucial regulator of auxin-dependent vascular patterning and its direct target—BpHB8. A decrease in the BpARF5 level expression in differentiating xylem was a specific characteristic of both Karelian birch with figured and non-figured wood. Decreased BpARF5 level expression in non-figured trees accompanied by decreased BpHB8 and VND/NST/SND expression levels compared to figured Karelian birch trees. According to the results obtained, we suggested silver birch forms differing in wood anatomy as valuable objects in studying the regulation of xylogenesis.

Pierce's disease (PD) in grapevine (Vitis vinifera) is caused by the bacterial pathogen Xylella fastidiosa. X. fastidiosa is limited to the xylem tissue and following infection induces extensive plant‐derived xylem blockages, primarily in the form of tyloses. Tylose‐mediated vessel occlusions are a hallmark of PD, particularly in susceptible V. vinifera. We temporally monitored tylose development over the course of the disease to link symptom severity to the level of tylose occlusion and the presence/absence of the bacterial pathogen at fine‐scale resolution. The majority of vessels containing tyloses were devoid of bacterial cells, indicating that direct, localized perception of X. fastidiosa was not a primary cause of tylose formation. In addition, we used X‐ray computed microtomography and machine‐learning to determine that X. fastidiosa induces significant starch depletion in xylem ray parenchyma cells. This suggests that a signalling mechanism emanating from the vessels colonized by bacteria enables a systemic response to X. fastidiosa infection. To understand the transcriptional changes underlying these phenotypes, we integrated global transcriptomics into the phenotypes we tracked over the disease spectrum. Differential gene expression analysis revealed that considerable transcriptomic reprogramming occurred during early PD before symptom appearance. Specifically, we determined that many genes associated with tylose formation (ethylene signalling and cell wall biogenesis) and drought stress were up‐regulated during both Phase I and Phase II of PD. On the contrary, several genes related to photosynthesis and carbon fixation were down‐regulated during both phases. These responses correlate with significant starch depletion observed in ray cells and tylose synthesis in vessels. During Pierce's disease, Xylella fastidiosa triggers transcriptional changes in Vitis vinifera grapevines and induces major physiological responses, including tylose formation and starch depletion.

Seasonal changes in the accumulation of soluble sugars in extracellular freezing cortical parenchyma cells and deep supercooling xylem parenchyma cells in Japanese white birch (Betula platyphylla var. japonica) were compared to identify the effects of soluble sugars on the mechanism of deep supercooling, which keeps the liquid state of water in cells under extremely low temperatures for long periods. Soluble sugars in both tissues were analyzed by high-performance liquid chromatography (HPLC), and the concentrations of sugars in cells were estimated by histological observation of occupancy rates of parenchyma cells in each tissue. Relative and equilibrium melting points of parenchyma cells were measured by differential thermal analysis and cryoscanning electron microscopy, respectively. In both xylem and cortical parenchyma cells, amounts of sucrose, raffinose and stachyose increased in winter, but amounts of fructose and glucose exhibited little change throughout the entire year. In addition, no sugars were found to be specific for either tissue. Combined results of HPLC analyses, histological observation and melting point analyses confirmed that the concentration of sugars was much higher in xylem cells than in cortical cells. It is thought that the higher concentration of soluble sugars in xylem cells may contribute to facilitation of deep supercooling in xylem cells by depressing the nucleation temperature.

The maintenance of Mg2+ homeostasis in the plant is essential for viability. This review describes Mg2+ functions and balancing in plants, with special focus on the existing knowledge of the involved transport mechanisms. Mg2+ is essential for the function of many cellular enzymes and for the aggregation of ribosomes. Mg2+ concentrations also modulate ionic currents across the chloroplast and the vacuolar membranes, and might thus regulate ion balance in the cell and stomatal opening. The significance of Mg2+ homeostasis has been particularly established with regard to Mg2+'s role in photosynthesis. Mg2+ is the central atom of the chlorophyll molecule, and fluctuations in its levels in the chloroplast regulate the activity of key photosynthetic enzymes. Relatively little is known of the proteins mediating Mg2+ uptake and transport in plants. The plant vacuole seem to play a key role in Mg2+ homeostasis in plant cells. Physiological and molecular evidence indicate that Mg2+ entry to the vacuole is mediated by Mg2+/H+ exchangers. The Arabidopsis vacuolar Mg2+/H+ exchanger, AtMHX, is highly transcribed at the vascular tissue, apparently most abundantly at the xylem parenchyma. Inclusion of Mg2+ ions into the vacuoles of this tissue may determine their partitioning between the various plant organs. Impacts of Mg2+ imbalance are described with respect for both plant physiology and for its nutritional value to animal and human.