The Mistletoe Viscum Album

The Mistletoe Viscum album Female fruiting plant of Viscum album ssp. album, February 2008. Henning S Heide-Jørgensen1 2015 - www.viscum.dk Fig. 1. Flowering male plant of European Mistletoe (Viscum album ssp. album), early April 2011. The author Content Henning S. Heide-Jørgensen was born in 1942. He Introduction page 2 received a Candidatum Scientiarum from the Uni- The haustorium - 3 versity of Copenhagen in 1970 and PhD in 1974 in Distribution in Scandinavia - 9 ecological plant anatomy. He was Associate Profes- Hosts and taxonomy - 10 sor at the Institute of Plant Anatomy and Cytology, The shoot system - 10 University of Copenhagen and Guest Researcher at Flowering and pollination - 12 the University of Victoria, B, C. in1990-1991, and Fruit, seed, and seed dispersal - 14 returned as Research Assistant at University of Co- Embryo and germination - 23 penhagen until retirement in 2007. Publications in- Host reactions and self-parasitism - 26 clude studies in xeromorphic plants, the plant cuticle, Myths, superstition, and medicine - 28 carnivorous plants, parasitic plants, and the biologi- Literature - 31 cal consequences of climate change in Greenland. Copyright Introduction This publication may not be reproduced without The present paper on the European mistletoe is written permission from the author. One printout based on ’Danish and other Nordic parasitic plants’ for personal use is allowed. Links to the paper are published at the web-site www.viscum.dk in 2014. allowed. Copyright to the illustrations belongs to the The paper is mainly expanded on illustrations, di- author and for Fig. 18 to H. Adsersen (H. A.), Figs. scussions on vegetative morphology, pollination, 57-58 to Emil Lütgen, and Fig. 92 to I. Drozdowski. and dispersal. Figures can be enlarged 200-300%. 2 Fig. 2. Viscum album in its first year where the ra- Fig. 3. Young Viscum album 4-5 years after seed de- dicle has produced a holdfast. The shrunken re- position. The holdfast of the primary haustorium can mains of the cotyledons are still visible. If both a still be seen (long arrow) while the cotyledons and the radicle and a hypocotyl are present, is unclear. first set of foliar leaves are lost. Short arrow, scar of cotyledon located 90° from scar after 1st foliar leaf. The European mistletoe, Viscum album L. (Front Mistletoes are hemi-parasites having trees as page and Fig.1), is a fascinating parasitic plant and hosts. Hemiparasites are characterized by containing a pleasure to follow year round if one is so lucky to the green matter chlorophyll which performs photo- have it growing just outside the window. It has its synthesis after addition of water, carbon dioxide, and own family Viscaceae together with six other gene- solar energy. This means the plant by itself is able ra, all of which were earlier placed in Loranthaceae. to produce all the carbon compounds needed for its Viscaceae is regarded as the most advanced family growth. However, since mistletoes grow on tree bran- in the sandalwood order (Santalales). If the reader is ches without having direct root connections to the soil looking for more scientific information than provided they are dependent on receiving water and all inorga- in this article the following are recommended: Tu- nic nutrients from the host. Therefore the European beuf 1923, Thoday 1951, Polhill & Wiens 1998, Büs- mistletoe is called a hemi-parasite since it approxim- sing 2000, Watson 2011, Kuijt 1969, and Kuijt 2015. ately provides only about half the nutrients on its own. A B C KZ KZ M S V Fig. 4. Establishment of the holdfast of Viscum album. A, the tip of the radical expands and flattens. B, Epidermis in the contact zone differentiates to glandular cells secreting a lipid containing a adhesive compound (S). C, meristem (M) producing the intrusive organ. KZ, collapsed zones. V, host surface. 3 v ch s s m n m s v ch cu 5 μm 5 4 st v m er ls 1 μm 6 Figs. 5-7. Viscum minimum. Fig. 5 page 4. Longitu- dinal section as seen in a transmission electron mi- croscope of immature epidermal cells from a young holdfast before contact to the host is achieved. ch, chloroplast. cu, cuticle. m, mitochondrion. n, nuc- leolus in cell nucleus with strongly condensed chro- matin. s, sphaerosome (lipid body). v, vacuole. 1μm = 1/1000 mm. Fig. 6. Transmissions electron mi- 10 μm croscopy of the tip of an epidermal cell from the 7 holdfast secreting lipid containing adhesive mate- rial (ls). er, endoplasmic reticulum. m, mitochondrion. st, plastid with starch grain. v, vacuole. Fig. 7. Scanning electron microscopy of holdfast epidermal cells after contact to the host is established, but the holdfast is artificially separated from the host. A thick layer of adhesive secret (ls) with cavities possibly containing cell-wall degrading enzymes is visible. The haustorium surface, becomes expanded and flattened to form a Viscum album is attached to a host branch by means holdfast also named an adhesive disk (Fig. 4). The of a primary haustorium (Fig. 2-3) which is inter- development of the adhesive disk has been investi- preted as a modified radicle. In addition many other gated for the small South African Viscum minimum parasitic plants also have secondary haustoria. Some (Heide-Jørgensen 1989) and it is illustrated in Figs. have only secondary haustoria which develop from 5-10. When contact to the host is obtained the outer- side roots or adventitious roots. During germinati- most cell layer (the epiderm) functions as gland cells on the tip of the radicle, upon contact to the host secreting a lipidic compound. The compound glues 5 0,2 mm 8 50 μm 100 μm 50 μm 9 10 11 Figs. 8-11. Viscum minimum. Fig. 8. Scanning electron microscopy of holdfast loosened from the host to show the front of the wedge shaped intrusive organ as well as imprints of the host epidermis with stomata (arrow). The hole to the right is caused by some of the adhesive stayed attached to the host. Fig. 9. Epi-fluorescence microscopy of a young intrusive organ. Nuclei, chromosomes and the lipid-adhesive lit up and some secrete is drawn into the host. Fig. 10. Light microscopy of a young intrusive organ. Lipid-secrete is grey. Cell walls and nuclei are blue. Arrow, compressed cells. Fig. 11. Cell nuclei and tracheids of the xylem bridge lit up. 6 p vs vs h host surface v cs cs m cs c m ss ss ps 12 Fig. 12. General diagram of a Viscum haustorium. The holdfast (h) is first established, then develops the intrusive organ which after contact to the xylem of the host is called the primary sinker (ps). Then the cortical strands (cs) develop follo- wed by secondary sinkers (ss). At last the vegetative (vs) shoots appear which flower 3-4 years later. c, cambium. m, meristem (growth point). p, plu- mule. v, viscin. Fig. 13. Viscum album mother plant with a one year old vegetative shoot sprouting from a corti- 13 cal strand. the holdfast to the host (Figs. 4-7). Inside the hold- and tracheids in the wood of the host (xylem), the fast a growing point (meristem) develops. By cell di- same kind of vessels and/or tracheids differentiate visions, the meristem produces a so-called intrusive in the intrusive organ (Fig. 11). Thereby a xylem organ which penetrates the host tissue (Figs. 4 and bridge is formed between the host and the mistle- 8-10). First, the intrusive organ has to break through toe stem and water and nutrients can stream free- several cell layers in the holdfast. Its growth also cau- ly from host to parasite. Furthermore, the intrusive ses some of the holdfast cells to collapse in zones organ ramifies in the living part of the bark (cortex) (Fig. 4). The tip of the intrusive organ is wedge sha- outside the phloem. These ramifications are called ped (Fig. 8), and it penetrates the host partly by en- cortical strands (Fig. 12), and from here secondary zymatically dissolving the pectic middle lamella bet- sinkers grow into the host xylem (Fig. 12). When ween host cells and partly by compressing the host these sinkers pass the host’s growth layer (the cam- cells. The latter is possible since the hydrostatic pres- bium) they develop a meristem at the level of the sure in parasite cells always is greater than in host cambium. Thereby the sinkers are able to accomo- cells (Fig. 9-10). The parts of the parasite seen out- date the increase in thickness of the host. In Viscum side the host are called the exophyte, while the parts the xylem bridge is not accompanied by phloem. inside the host are called the endophyte. In addition to the primary shoot cortical strands When the intrusive organ (also named prima- also produce flowering shoots (Fig. 12-13). In Vi- ry sinker) reaches the water transporting vessels scum album the strands are relatively short and 7 A 14 Fig. 14. New shoots of Viscum album sprouting from cortical strands in an old apple tre (Malus do- B mesticus) at relatively short distance from the mother plant seen at right. Fig. 15A-E. Longitudinal section of apple branch and a Viscum album with 8 shoot innovations (approx. 11 year old). A. The following pictures (B-E) are pho- tographed from the opposite side and mirrored verti- cally so the left exophyte is not visible in the section series. Also note the host twig is dead distally (right) to the mistletoe. This is caused by drainage of the host for water and nutrients by the haustorium.

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