Summary In trees, dead and living cells of secondary xylem (wood) function collectively, rendering cell‐to‐cell communication challenging. Water and solutes are transported over long distances from the roots to the above‐ground organs via vessels, the main component of wood, and then radially over short distances to the neighboring cells. This enables proper functioning of trees and integrates whole‐plant activity. In this study, tracer loading, immunolocalization experiments and inhibitor assays were used to decipher the mechanisms enabling transport in wood of Acer pseudoplatanus (maple), Fraxinus excelsior (ash) and Populus tremula × tremuloides (poplar) trees. We show that tracer uptake from dead water‐conducting vessels, elements of the apoplasm, to living vessel‐associated cells (VACs) of the xylem parenchyma of the symplasm system proceeds via the endocytic pathway, including clathrin‐mediated and clathrin‐independent processes. These findings enhance our understanding of the transport pathways in complex wood tissue, providing experimental evidence of the involvement of VACs and endocytosis in radial uptake from vessels.

Trees owe their growth in girth to cambial activity that gives rise to secondary conducting tissues. Since cambium originates from the primary vascular meristem, called procambium, the main objective of the present study was to analyse the changes occurring during transition from procambium to cambium in Diospyros lotus L., characterized by the occurrence of double-storied cambium. In this species, procambium and primary vascular tissues are present only within the bud, whereas just below this level, cambium and secondary vascular tissues occur. Procambium differentiates as a continuous cylinder, which as a whole transforms into cambium during further development. Commonly used criteria distinguishing procambium and cambium — that is, the appearance of periclinal divisions, intrusive growth of meristematic cells, presence of rays, and primary versus secondary nature of derived vascular tissues — were analysed in D. lotus. All these features, however, were present in procambium, making the sharp distinction between both stages of vascular meristem based on these criteria difficult. In addition, the formation of fusiform and ray initial storeys in D. lotus was shown to appear in the third year of cambium activity and become more distinct with cambium age. The double-storied structure may result from the genetic control of cell fate and (or) the presence of epigenetic patterns determining cellular arrangement.

In recent decades, significant changes have been observed in the atmospheric pollutant emissions and deposition in the mountain regions of Central Europe. Pollution caused significant deforestation, but the level of tree damage differs, even between neighboring locations. Thus, it is particularly important to examine the relationships between pollutant deposition rate and detrimental changes in the subalpine spruce ecosystems, and to correlate these with the intensity and structure of pollutant deposition. Radial wood cores were extracted at breast height from trees located in different montane areas: the Sudetes and both Western and Eastern Carpathians. Specific features of the secondary xylem, i.e., annual ring width as well as the width and lumen of early-and latewood tracheids, were analyzed for three decades: before the intense of environmental pollution (1950-1960), during essential stage of the ecological disaster (1980-1990) and after cessation of pollution (2000-2010). The narrowest annual rings and earlywood tracheids were formed in the 1980s in areas where fog was the main source of the pollutant deposition. In conclusion, it has been shown that fog could be an important factor increasing the destructive role of pollutants on wood formation. It was manifested in decreasing of cambial activity resulting in formation of narrower annual rings as well as in reduction in radial dimensions of earlywood tracheids.

The development of woody plants is related to the continuity of the procambium and cambium. Whether such a continuity is present in plants with successive cambia, especially in those, where the first cambium is formed outside the primary vascular bundles, has not been analyzed so far. Therefore, we studied the development of vascular meristem in Celosia argentea, in which the first and successive cambial cylinders arise outside the primary bundles and, intriguingly, in the literature are interpreted as developmentally independent structures. Our results showed that in C. argentea, the outermost procambial cells maintain their meristematic characteristics during differentiation of vascular bundles and divide periclinally, forming the zone of procambium-derived cells outside the primary bundles. This zone comprises parenchyma cells bordering the bundles, and a continuous ring of the incipient cambial cells neighboring the primary cortex. Later in the development, the ability to preserve the outermost cells in the cambium undifferentiated is repeated during the formation of successive cylinders of cambia. Together, our results clearly point to the developmental continuity of the procambium and successive cambia in C. argentea, despite their seemingly spatial distinctiveness. We postulate that the mechanism demonstrated in C. argentea is universal and orchestrates the development of successive cambia in other plant species.