Mistletoe fruits with viable embryos are available from October to May. The seed can, however, only be sown to good effect during a relatively short time that is limited by a number of factors. Mistletoe seeds applied to the host in late autumn or winter are frequently eaten by tits (Grazi, 1986; Weber, 1993; Grazi and Urech, 1996). On the other hand, availability of mistletoe fruit may be greatly reduced by the beginning of the new vegetative period, as some species of birds show a preference for eating mistletoe berries in winter. Germination of the remaining seeds may also be advanced so far that the elongated hypocotyls are no longer able to connect with the host branch. The optimal time for applying mistletoe seed is from mid-March to mid-May, depending mainly on the geographical situation.
It is therefore advisable to build up a good store of mistletoe seed that may be sown later. In practice, the following method has proved effective. We harvest the fruits of healthy, robust mistletoe bushes from January onwards. The exocarp is removed, leaving the inner mesocarp around the seeds (Figure 3A) which then are placed in Petri dishes. The slimy, sticky pulp is carefully dehydrated over a period of 24-48 hours. The stabilised seeds can then be stored outside until spring. As mistletoe embryos quickly lose vitality if kept in the dark (Tubeuf, 1923), it is necessary to make sure that the seeds get enough light all the time. Care must be taken, on the other hand, to prevent the live tissues getting overheated.
Before sowing, the mistletoe seeds are covered with water so that the dried-up mesocarp swells up again and becomes slimy. Using tweezers, single or groups of seeds are transferred to the bark of host branches. They will firmly adhere within a few days because of the glue-like substances in the mesocarp. To prevent seeds from getting washed off, it is advisable to choose a time when precipitation levels are low. Seeds are applied to young branches in the host periphery. This avoids damage to the central trunk caused by the sinker of the mistletoe. In damp growth sites with dense
herbage or grass cover under the trees, the seeds should be protected from snails and slugs with light-permeable gauze strips.
Successful sowing is evident from the following:
1. Hypocotyl and holdfast stay green.
2. Host branch shows distinct swelling under the holdfast.
3. Hypocotyl comes upright.
4. Two elongated, pale green primary leaves grow from the bud on the shoot.
The last of these four steps is the most definite visible sign that the mistletoe haustorium has connected with the host's water conduit system inside the branch. The tree is mistletoe-receptive and the young mistletoe plant usually should be able to grow into a typical mistletoe bush in the years ahead.
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