Manuscript for Arborist news dd 2-10-2007 Phloem nodes deface trees and shrubs in urban environments

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Manuscript for Arborist news dd 2-10-2007
Phloem nodes deface trees and shrubs in urban environments
André A.M. van Lammeren and Fons van Kuik
In recent years it was observed that Fraxinus excelsior in the city of Alphen aan den Rijn in the Netherlands formed a kind of node-like protuberances on their trunks. Up to 30% of the ash-trees are affected in the urban environment. Hemispherical outgrowths of various sizes were scattered all over the stem surface. The outgrowths varied in size from almost unnoticeable to structures up to 3 cm in diameter (Fig. 1 and 3). Inspection of other tree species and shrubs revealed that many more species developed hemispherical outgrowths. Among the species investigated are ornamentals such as Hamamelis (American witch-hazel), production species such as beech and ash, and agronomic important species such as apple, olive and fig (see Table 1). Most species are of temperate climate regions but others are of Mediterranean origin. With respect to their geographical distribution, more and more severely affected specimens were found in urban environments than in rural areas.

Whether the nodes are a threat for the physiology and or health of the trees on the short or long term, or why they increase in frequency in recent years is not clear yet but they doubtless damage the tree trunks from the esthetical point of view. Therefore it was decided (by the administration of Alphen aan den Rijn) to investigate the internal structure and development of the malformations. Material was sampled from the trees with a hollow pipe and processed at the Laboratory of Plant Cell Biology of Wageningen University and Research Centre, Wageningen, The Netherlands. Samples were fixed, sectioned with a microtome and microscopic slides were prepared. Sections of outgrowths of various sizes were investigated with light microscopy to understand their structure and development. From all trees and shrubs listed in Table 1 samples were investigated. The organization of the outgrowths appeared remarkable similar in all species investigated.

Structure of phloem nodes

Outgrowths consist of a spherical inner region of xylem tissue surrounded by a phloem layer. Comparing sections of small and large outgrowths reveals that the increase in size is caused by the activity of a vascular cambium (Figs 2 and 4). Its activity is the deposition of xylem and phloem cells by which the node changes from a hardly discernable structure into a sphere which protrudes and defaces the stem surface. The vascular cambium deposits xylem elements towards the inner side and phloem elements to the outside. The outgrowths of most plant species exhibit a deposition of xylem elements in ring wise fashion (Fig. 5). Dependent on the plant species, diffuse or ring-porous xylem is being formed. Both xylem and phloem parts of the outgrowths consist of ray cells originating from ray initials, and fibres, vessels and parenchyma originating from fusiform initials (Fig. 5). The differentiated fusiform cells however do not have the longitudinal orientation known in normal secondary xylem but are found in most varying orientation (Fig. 5). As a result all the transport elements of the outgrowths, i.e. xylem vessels, tracheids and phloem vessels, are of no use in longitudinal transport.

Origin of nodes

Where do the outgrowths start their development? Investigating median sections of outgrowths showed that the very centre of the outgrowth often contains a sclereid or fibre that belongs to the first formed phloem tissue (Fig. 6). This finding implies that these cells are the ones around which the tuber formation has started and secondly that the tubers are of phloem origin, initially. Therefore we propose the name ’phloem nodes’ although their contents is largely xylem. In several cases an additional cluster of degenerated cells is observed in the centre of the node. Formation starts very close to the outer surface of the stem, within a distance of 500 m.

The Figs 1 to 6 depict data from ash but as can be seen from Figs 7 to 12 phloem nodes of apple (Figs. 7 and 8), fig (Figs 9 and 10), and Elaeagnus ebbengei (Figs. 11 and 12) and are very alike as is the annular organization in the xylem (Fig. 10).

The first cells surrounding the very centre of the phloem nodes are shaped in an unordered pattern (Figs 6, 8 and 12). It is not clear whether this is the shape of the initials preceding the differentiation, or the result of (tip)growth once the cells were deposited. Another question which remains to be elucidated is the speed of development. Are the growth rings seasonal rings, or do more rings develop each year? If the former is the case, the growth is rather slow which is in contradiction with the observation that the outgrowths are only seen recently. Future research will focus on these items and in addition to the question what causes phloem tuber formation.

Request for correspondence

As we are interested in knowing the geographical distribution of this phenomenon and the range of plant species affected, we do appreciate you cooperation in sending us a message when you observe similar phloem nodes in your surroundings. Please use the e-mail addresses mentioned in this paper to contact us.

Adresses of the authors

Dr. A.A.M. van Lammeren is senior scientistassociate professor at the

Laboratory of Plant Cell Biology of

Wageningen University and Research Centre,

Arboretumlaan 4

6703 BD Wageningen
Ir A.J. van Kuik is a senior scientist Crop Protection of Wageningen UR Applied Plant Research

Flowerbulbs, Nurserystock & Fruits P.O. Box 85 2160 AB Lisse The Netherlands

Acer saccharinum ‘Pyramidale’, maple

Aesculus hippocastanum, horse chestnut

Edgeworthia chrysantha, Oriental paperbush

Elaeagnus ebbingei, Ebbing's Silverberry

Fagus sylvatica, beech

Ficus carica, fig

Fraxinus excelsior, cv ‘Atlas’ and ‘Westhofs glorie’, ash

Gleditsia triacanthos, Honey locust

Hamamelis interm. ‘Diane’ and ‘Feuerzauber’, American witch-hazel

Ilex aquifolium, holly

Malus sylvestris, ‘Elstar’, apple

Olea europea, olive

Platanus acerifolia, plane

Tilia sp., linden

Table 1. Species of trees and shrubs that develop ‘phloem nodes’ of which the internal structure is strikingly similar.

Legends to the figures.

Fig 1. Stem of ash, Fraxinus excelsior ‘Atlas’, with an overview of phloem nodes at various stages of development. Note the sites where nodes were sampled (red arrows)

Fig. 2. Section of a phloem node of ash in an early stage of development. Bar is 1 cm.

Fig. 3. Detail of phloem nodes on ash at early (left) and late (right) stages of development.

Fig. 4. Phloem node of ash at a late stage of development. Note the growth rings which are indicated by arrows. Bar is 1 cm.

Fig. 5. Micrograph of the woody region of a phloem node of ash with xylem arranged in a ring like fashion. Note that vessels exhibit various orientations. Bar is 100 m.

Fig. 6. Micrograph of the very centre of a phloem node of ash with xylem cells deposited around a group of yellow necrotic cells. The arrow points to an enclosed phloem fibre or sclereid. Bar is 1 cm.

Fig. 7. Micrograph of a phloem node of apple. Bar is 1 cm.

Fig. 8. Detail of the xylem in the inner part of the phloem node of apple showing a concentric deposition of fusiform cells around more centres. Bar is 1 cm

Fig. 9. Overview of transversal section of a phloem node of fig (Ficus carica) again originating from the outer phloem cell layers. Bar is 500 m.

Fig. 10. detail of phloem fiber of fig showing an annular organization of the xylem elements. The arrow points to the cambium that deposits xylem and phloem cells to the node. Bar is 1000 m.

Fig. 11. Overview of phloem node of Elaeagnus ebbengei. Bar is 0.5 cm.

Fig. 12. Detail of the very centre of the node of Elaeagnus ebbengei with enclosed sclereids originating form the outer phloem layers. Bar is 100 m.

Figures 1-8 (from upper left to lower right

Figures 9-12 (from upper left to lower right)

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