The European population is devided into three sub-species, that have different hosts.
- V. album L. ssp. platyspermum Kell. (ssp. album) growing on hardwood trees;
- V. album L. ssp. abietis Beck, growing on Abies sp.;
- V. album L. ssp. laxum Fick (=ssp. austriacum Wiesb. Vollmann) growing on pine trees and very rarely on spruce.
In East Asia we find V. album L. var. colaratum Ohwi (see Park, this book).
Leaf area may vary for the subspecies (Ball 1993, Singer 1958). Also the amount of embryos per berry varies. While ssp. platyspermen tends to diembryonal, the percentage of monoembryonal berries is higher in ssp. abietes and ssp. laxum. Tubeuf (1923) already pointed out that any criteria used to distinguish the subspecies can be found in the other two subspecies as well, e.g. the size of the leaves depends very much from the nutrition of the host tree and the position of the parasite within the host tree.
Nagl and Stein (1989) characterised DNA from mistletoes grown on various hosts. Small but not significant differences were found between the 2 C DNA con-tents of the three subspecies and the base composition. Significant differences were detected in the patterns of sequence organisation.
Earlier reports (Freudenberg 1968) that lignins from mistletoes grown on softwood trees (ssp. abietis and ssp. laxum) contain coniferous lignin, whereas lignin from mistletoes grown on hardwood trees (ssp. album) produce hardwood lignin could not been proven (Becker and Nimz 1974).
The pattern of flavonoid aglycones may be different for certain samples. But like morphological features there is no clear-cut distinction between the three different subspecies (Becker and Exner 1980).
The only criteria to distinguish the three subspecies by morphological criteria was described by Grazi and Urech (1981). If the ripe berries are squeezed, the seed of the ssp. abietes and ssp. laxum are easily freed from the skin and from the viscous layer. When the berries of ssp. platyspermum are squeezed, the viscous layer remains attached to the skin and the seed. The two subspecies on softwood trees can be distinguished from the appearance of their embryos. The embryo of ssp. abietes has a swollen end at the hypocotyl pole, whereas the respective embryo from ssp. laxum is cylindrical. These differences may be of advantage for the attachment of the seed to the bark of the respective host trees.
There are only very rare cases, where a natural or artificial infection was successful, but not conform to the above-mentioned host restriction. The only case in nature where a species hosted V. album ssp. laxum as well as V. album ssp. platyspermum is Genista cinerea observed by Grazi and Zemp (1985).
Some hardwood trees (e.g. beech, boxtree) are resistant to mistletoe infections; other species e.g. European elm (Ulmus campestris and U. montana) and European oak (Quercus robur and Qu. petraea) are rarely infected by mistletoe. However, if an infection is successful for one tree this tree can host several mistletoe bushes (Ramm et al., this book). A detailed list of host trees of Viscum album ssp. platyspermum is listed in Luther and Becker (1987). Hariri et al. (1991, 1992) studied different trees that were known to have different resistances to mistletoe infections. A histocytological observation demonstrated that two barriers were involved in the resistance: a mechanical one and a chemical one. The mechanical barrier concerns the thickness of the phellem (the outer part of the bark) and the location and quantity of lignified fibers. The bark of resistant trees has a thicker phellem and more superficial fibers. The chemical barrier is related to an important secretion of polyphenols, which is stimulated by the attack of the parasite. The effect of mistletoe infection on young apple trees has been studied by Preston (1977, see below).
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