The bacterial canker of kiwifruit caused by Pseudomonas syringae pv. actinidiae is a severe threat to kiwifruit production worldwide. Many aspects of P. syringae pv. actinidiae biology and ...epidemiology still require in-depth investigation. The infection by and spread of P. syringae pv. actinidiae in xylem and phloem was investigated by carrying out artificial inoculation experiments with histological and dendrochronological analyses of naturally diseased plants in Italy. We found that the bacterium can infect host plants by entering natural openings and lesions. In naturally infected kiwifruit plants, P. syringae pv. actinidiae is present in the lenticels as well as in the dead phloem tissue beneath the lenticels, surrounded by a lesion in the periderm which appears to indicate the importance of lenticels to kiwifruit infection. Biofilm formation was observed outside and inside plants. In cases of advanced stages of P. syringae pv. actinidiae infection, neuroses of the phloem occur, which are followed by cambial dieback and most likely by infection of the xylem. Anatomical changes in wood such as reduced ring width, a drastic reduction in vessel size, and the presence of tyloses were observed within several infected sites. In the field, these changes occur only a year after the first leaf symptoms are observed suggesting a significant time lapse between primary and secondary symptoms. It was possible to study the temporal development of P. syringae pv. actinidiae-induced cambial dieback by applying dendrochronology methods which revealed that cambial dieback occurs only during the growing season.
Abstract
Reactivation of axial water flow in ring-porous species is a complex process related to stem water content and developmental stage of both earlywood-vessel and leaf formation. Yet empirical ...evidence with non-destructive methods on the dynamics of water flow resumption in relation to these mechanisms is lacking. Here we combined in vivo magnetic resonance imaging and wood-anatomical observations to monitor the dynamic changes in stem water content and flow during spring reactivation in 4-year-old pedunculate oaks (Quercus robur L.) saplings. We found that previous year latewood vessels and current year developing earlywood vessels form a functional unit for water flow during growth resumption. During spring reactivation, water flow shifted from latewood towards the new earlywood, paralleling the formation of earlywood vessels and leaves. At leaves' full expansion, volumetric water content of previous rings drastically decreased due to the near-absence of water in fibre tissue. We conclude (i) that in ring-porous oak, latewood vessels play an important hydraulic role for bridging the transition between old and new water-conducting vessels and (ii) that fibre and parenchyma provides a place for water storage.
• Background and Aims Frankincense, a gum-resin, has been tapped from Boswellia papyrifera trees for centuries. Despite the intensive tapping and economic interest of B. papyrifera, information on ...the resin secretory structures, which are responsible for synthesis, storage and transport of frankincense, is virtually absent. This study describes the type, architecture and distribution of resin secretory structures of B. papyrifera and its relevance for the ecophysiology and economic use of the tree. • Methods The type and architecture of resin secretory structures present in bark and wood was investigated from transversal, tangential and radial sections of bark and wood samples. The diameter and density (number of resin canals mm⁻²) of axial resin canals were determined from digital images of thin sections across the different zones of inner bark. • Key Results Resin canals form a three-dimensional network within the inner bark. Yet, the intact resin-conducting and producing network is on average limited to the inner 6.6 mm of the inner bark. Within the inner bark, the density of non-lignified axial resin canals decreases and the density of lignified resin canals increases from the vascular cambium towards the outer bark. In the wood, only radial resin canals were encountered. • Conclusions Frankincense tapping techniques can be improved based on knowledge of bark anatomy and distribution and architecture of resin secretory structures. The suggested new techniques will contribute to a more sustainable frankincense production that enhances the contribution of frankincense to rural livelihoods and the national economy.
Trees are long-living organisms that record ecologically relevant information in their xylem, which can be accessed by dendrochronology, the study of tree rings. Wood-anatomical markers, also known ...as wood imprints or signatures, are anomalies in tree rings that can be used for studying specific environmental events as e.g. forest fires, flooding events or erosion events. By dendrochronological dating of tree rings containing wood-anatomical markers, not only the exact year of the triggering events can be reconstructed, but also the season of formation can be estimated by using the position of the marker within the tree ring. Although dendrochronological dating is amongst the most accurate dating tools, the application of wood-anatomical markers is hindered by a lack of fundamental studies that test their temporal accuracy by linking their occurrence to tree-ring growth. In this thesis, the accuracy of wood-anatomical markers that differed in their origin was investigated by a combination of field studies and experiments. First, the temporal resolution of wound dating was studied in relation to dating invasive Anoplophora outbreaks in Japanese maple (Acer palmatum), an ornamental tree occasionally containing larvae of A. chinensis. These xylobiont insects, that eventually kill trees, induce wounds in living trees by forming e.g. exit holes. Second, the formation of wood-anatomical markers that occur in regard to burial and erosion events in drift-sand areas was studied in pedunculate oak (Quercus robur L.). Last, the effect of flooding was studied in young pedunculate oak trees. It was expected that wood-anatomical markers both related to wounding and to environmental changes can be used with an intra-annual resolution.
In dendrogeomorphology, abrupt changes in wood anatomy are frequently used to date the exact year of burial and exposure events. However, few studies have addressed the precision and underlying ...mechanisms of these changes. In a field experiment, performed in a drift-sand area in the Netherlands, we buried the stems of mature pedunculate oak trees (Quercus robur L.) up to a height of 50cm and analysed the responses in ring width and earlywood-vessel characteristics, while monitoring the course of temperature above and below the soil surface.
After 3 years of stem burial, we found no significant differences in ring width and earlywood-vessel characteristics between control and buried trees both above and below the burial level. Burial however strongly reduced temperature amplitude and the occurrence of sub-zero temperatures around the buried stems. All buried trees formed epitropic roots that grew upward into the new sediment layer, but no adventitious roots were formed on the buried stems. Irrespective of the burial treatments, we found that the mean ring width was largest at the original stem base and lowest at breast height. In contrast, vessel sizes were significantly larger at breast height compared with the stem base. Differences in vessel density barely differed between years and heights.
In our field experiment on mature pedunculate oak trees, the burial of stems by 50cm of drift sand did not induce any local growth suppression or detectable changes in wood anatomy. As wood-anatomical changes in response to burial have previously been reported for trees that had formed adventitious roots, we stress the role of adventitious-root formation as a possible trigger behind the local changes in wood anatomy, reflecting a functional change of a buried stem towards a root. Based on our field experiment, it seems unlikely that years of shallow or moderate burial events (≤50cm) can be reconstructed using the wood structure of buried stems. As epitropic roots develop quickly after burial, dating such roots may potentially yield better estimates of burial events. Further research on the relation between adventitious root and changes in stem anatomy is needed to ascertain the precision of dating sand-burial events using tree rings.
Shrubs and trees with secondary phloem tissue produced by successive cambia mainly occur in habitats characterized by a periodical or continuous lack of water availability. The amount of this ...secondary phloem tissue in stems of Avicennia trees rises with increasing soil water salinity and decreasing inundation frequency. Hence, increased water storage in secondary phloem tissue produced by successive cambia was put forward to be advantageous in harsh environmental conditions. It was however never tested whether the secondary phloem cells over the entire stem of woody species showing this wood anatomical feature are indeed water-filled as expected. In this preliminary and pioneering study, we use magnetic resonance imaging (MRI) to visualize the stem water content of three species with successive cambia, the mangroves Avicennia marina and A. officinalis and the non-mangrove Bougainvillea spectabilis. Measurements were conducted in living plants. We tested the hypothesis that not only the outermost phloem tissue has high water content but also the secondary phloem tissues over the entire stem from the bark inward to the pith, herewith serving as water storage sites. We can conclude that all secondary phloem tissue of both Bougainvillea and Avicennia has high water contents. This aligns with the contribution of secondary phloem tissue produced by successive cambia to ecological success in conditions of physiological drought. Further study should however be done to understand the mechanisms through which this secondary phloem tissue contributes to the water household of plants in conditions of water shortage.
During winter dormancy, temperate trees are capable of only a restricted response to wounding. Depending on the ambient temperature during winter dormancy, wounded trees may start ...compartmentalization, e.g. by producing inhibitory compounds, but it is thought that processes involving cell proliferation, such as the formation of callus and wound xylem, are delayed until the next growing season. We investigated the effect of two contrasting temperature regimes on early reactions of Acer palmatum trees to wounding during winter bud dormancy. Stems of A. palmatum trees were wounded and stored under an ambient temperature of 4 or 15 °C for 3 weeks during winter bud dormancy. We then studied wound reactions in the living bark, cambial region and xylem. In the 4 °C treatment, wound reactions were virtually absent. In the 15 °C treatment, however, trees reacted to wounding by dieback of the cortex and phloem and by the formation of ligno-suberized layers. In the cambial zone, cambial dieback occurred and callus tissue and wound xylem were formed locally, close to the wound margins. In the xylem, compartmentalization took place by deposition of inhibitory compounds in fibre cells and vessel elements. We conclude that temperature is an important factor in wound reactions during winter dormancy, and may even induce proliferation of callus and wound xylem within a 3-week period. It therefore seems likely that trees that have been wounded during dormancy in areas with mild or warm winters might cope better with wounding, as unlike trees in cold environments, they may compartmentalize wounds even during winter dormancy.
During winter dormancy, temperate trees are capable of only a restricted response to wounding. In an experiment, we investigated the effect of wounding on Acer palmatum trees during winter-bud dormancy and found that in the cold (4 °C) temperature treatment, wound reactions were virtually absent. In the warm (15 °C) treatment, however, trees reacted actively to wounding within a three-week period by e.g. forming callus and local wound xylem. We conclude that temperature is an important factor in wound reactions during winter dormancy and may even induce the formation of callus and wound xylem within a three-week period.
PDF
