INTRODUCTION Parasitic plants constitute about 1% of all angiosperm species and ~ 40% of them parasitize the aboveground parts of their host plants ( Norton & Carpenter 1998 ). Mistletoes are the predominant group of angiosperm shoot parasites. They are capable of attaching to and penetrating into
The xylem-tapping mistletoe, Phoradendron, forms vessel-to-vessel connections with its dicotyledonous host, Juglans. Contact vessels of the host may become embolized as evidenced by the presence of tyloses. Tyloses may also occur in parasite vessels contiguous with embolized host vessels. Differences between parasite and host in nuclear size, shape and intensity of staining make it possible to determine the origin of the tyloses present in contact vessels. Host-formed tyloses occluded not only emboli zed host vessels, but also those of the parasite with which they were continuous. Some parasite vessels may terminate in vessel members lacking perforations at the juncture with host vessels; these vessels lacked tyloses. In parasitic angiosperms in general tyloses within haustoria may be formed by either host or parasite. Several functional roles are reported for tyloses including; absorption of nutrients, defence against invasion of the parasite, and the occlusion of embolized vessels, as exemplified in the present study.
[German version] (ἰξία/ixía and ἰξός/ixós, also the name for birdlime made from mistletoe berries, and στελίς/stelís and ὑφέαρ/hyphéar in Greek dialects, Latin viscus or viscum). Of the two genuses in the family Loranthaceae, Theophrastus (H. plant. 3,7,6 and 3,16,19) knows as ixía only the true
The importance of direct tracheary element connections between mistletoes and their hosts is controversial. Direct connections have been reported for four genera within the Viscaceae. In the past such connections were considered essential for the movement of water and nutrients from host to parasite. In contrast, recent authors have suggested that the apoplastic continuum provided by the walls of contiguous host and parasite parenchyma cells is the main pathway for the transfer of water and nutrients, with direct connections playing at best a minor role in uptake. Our analysis of the parasite (Phoradendron)/host(Juglans) interface suggests otherwise. Parenchyma predominates at the interface for both host (71%) and parasite (95%). While direct tracheary element connections are less frequent than other types of connections, they occurred in all sinkers analyzed. Further, direct connections were much more abundant in host latewood (3.6%) than in earlywood (1.8%). This, and other evidence indicates that both pathways of transfer are important. We suggest that the apoplastic continuum provided by parasite parenchyma provides for selective uptake of nutrients, whereas direct connections provide for bulk flow between host and parasite.
Ashkenazi, S. 1995. Acacia trees in the Negev and Arava, Israel: a review following reported large-scale mortality. HaKeren HaKeyemet L'Israel, Jerusalem (in Hebrew, English summary).
Bowie, M., Ward, D. 2003. The effects of mistletoes on the water status and survival of
Parasitic plants are capable of causing a variety of effects to their hosts, including alterations in the process of wood formation. However, the majority of studies dealing with parasitic plant anatomy have focused on the host–parasite interface and the direct action of the haustorium, which is the organ responsible for attaching the parasite to the host. Considering this gap, we studied the anatomical and functional effects caused by a mistletoe species, Phoradendron crassifolium (Santalaceae), on the wood anatomy of the host tree Tapirira guianensis (Anacardiaceae). Both parasitized and non-parasitized branches were collected from host trees. Traditional wood anatomy procedures were employed, along with functionality experiments using the ascent of safranin solution through the xylem. Prior to the analysis, all sampled branches were divided in “upstream” and “downstream” portions, considering the direction of xylem sap flow inside the plant body. This design was chosen in order to avoid biased results derived from normal ontogeny-related wood anatomical and functional changes. Our results showed that infested wood expressed a higher density of embolized vessels, narrower vessel lumen diameter, higher vessel density, taller and wider rays, and fibers with thinner cell walls. All these responses were most conspicuous in the downstream sections of the parasitized branches. We propose that the wood anatomical and functional alterations were induced by the combination of water stress caused by water use by the parasite and consequent low turgor in differentiating cambial derivates; by unbalanced auxin/cytokinin concentrations originating at the infestation region due to phloem disruptions caused by the parasite’s penetration and action; and by higher than usual ethylene levels. Further analysis of hydraulic conductivity and hormonal changes in host branches are necessary to test this hypothesis.
Secondary xylem characteristics were compared in four species of Phoradendron Nutt. (Viscaceae) native to California. All have extremely short, thick-walled vessel elements with simple perforation plates. They also share high vessel density, radial vessel arrangement, thick-walled fibres, and multiseriate, heterocellular rays. The fibres show considerable intrusive growth. Features of the vessel elements (i.e. vessel dimensions, arrangement, type of wall sculpturing) and calcium oxalate crystals in the ray parenchyma cells are useful diagnostic traits to separate species. Grooved vessel walls are shared by the morphologically similar P. villosum and P. macrophyllum. Differences between these two species may reflect contrasting drought response strategies pursued by respective hosts. Vulnerability and mesomorphy ratios of the wood of P. californicum are higher than those of P. pauciflorum and P. macrophyllum. Phoradendron pauciflorum has the most xeromorphic wood of the four species studied.
(the Old Irish drúitheach means ‘impressive,’ ‘piercing,’ ‘influential’); (b) the other conjectural derivation, ‘oak expert,’ goes back to Pliny the Elder, who also describes the importance of mistletoe,...
All genera in the mistletoe family Viscaceae develop a secondary protective covering, the cuticular epithelium, that replaces the epidermis. The cuticular epithelium also occurs in the Eremolepidaceae and some genera within the related family Santalaceae. This secondary covering, unlike the periderm, lacks lenticels or their functional equivalent. We suggest that the cuticular epithelium provides a greater degree of control over transpirational water loss in older plant parts. The cuticular epithelium may arise in the epidermis, subepidermal layer, or in deeper tissues of the stem. Bark strands of the endophytic system, where they are in contact with either nonliving host tissues or the external environment, also form a cuticular epithelium. The epidermal feature, stomatal orientation, was also studied. All genera in the Viscaceae and Eremolepidaceae have stomata with a transverse orientation. The presence or absence of a cuticular epithelium and stomatal orientation are vegetative characters with potential taxonomic value.
[German version] The druid figure best known at present, Getafix (Fr. Panoramix; Ger. Miraculix), is distinguished by the fact that he does not divulge any of his secret teachings. And not only that - - apart from the traditional cutting of mistletoe with a golden sickle passed down to us in Pliny