Rootstocks for temperate fruit trees

Rootstocks for temperate tree fruit crops are traditionally propagated by relatively slow vegetative methods of from seed which often results in non-uniform material.
Tissue culture is a sophisticated method of vegetative propagation which has the potential for rapid multiplication to produce large populations of uniform material.
At present, few commercial micropropagation laboratories have fruit rootstock in their assortment despite the fact that micropropagation of these plants started in the mid 1970s.
Since then the tissue culture of most rootstock for apple, pear, plum, peach, apricot, etc. has been investigated and commercial micropropagation methods have been elaborated.
What is the reason for this present situation ? Is it technical or economic in nature ?
In theory, there are many pathways leading to the regeneration of plants from plant tissues placed in vitro but commercial micropropagation of rootstock is focused on the stimulation of shoot branching and, subsequently, their rooting before transferring to non-sterile conditions.
All commercial micropropagation methods for temperate zone fruits and their rootstock consist of a few, easy to distinguish stages:

• initiation of sterile cultures;
• building up the number of shoots grown in vitro by stimulation of their branching and, subsequently, dividing clumps into single shoots;
• rooting of shoots.

The details of technologies applied for particular species and cultivars can differ, but cultures of all of them start from a 0.5-1 cm long shoot tip. The best time for culture initiation is the beginning of intensive shoot growth - just after bud break at springtime.
Younger mother plants provide better growth of initial explants in vitro. Shoot tips must be disinfested before placing in sterile conditions. During the first six to 16 weeks of culture, they establish themselves in vitro, elongate and, sometimes, branch. The coefficient of this stage of micropropagation (ratio of explants obtained at the end of stage to explants placed in culture) varies from 0.1 to 0.9.
Once established in culture, shoots are forced to branch with the aim to increase the amount of sterile plant material. They are subdivided into single shoots or small clumps of shoots at four to eight week intervals and transplanted onto similar media. This procedure is repeated until there is enough plant material to cover the requirements of customers. An average coefficient of each subculture at this stage of micropropagation is three.
Because of lack of juvenility, rooting of shoots should not be, usually, attempted earlier than after fifth or sixth subculture. Shoot cuttings are rooted in vitro on sterile media. The efficiency and duration of rooting stage varies but on average 70% of shoots will root at in three to four weeks.
The acclimatisation of plants obtained in vitro to greenhouse conditions is the most critical stage of micropropagation. With proper care and protection 80-90% of plants will survive this stage and they should be ready for planting out in a nursery after two to four months of growth in multipots.
The existing micropropagation methods are efficient enough to produce about one million rootstocks in plugs from one shoot tip in two years. However, economic reality prevents the spread of large scale micropropagation of this type of crop. Rootstocks must compete with ornamental plants whose average plug price is $0.60-0.80. Because micropropagated rootstock must grow at least a year in a field before they can be grafted or budded, it is too high a price to be accepted by the average fruit tree producer.
However, micropropagated rootstock are ideal for establishing mother stocks, especially when in vitro propagation has been coupled with virus and bacteria certification programs. It has been proved that rootstock plants which originated from tissue culture are more vigorous and produce more layers and cuttings which, due to juvenility effect, root better even in the case of difficult-to-root species such as Pyrus communis.
The additional advantage of micropropagation is the possibility of rapid propagation of rootstock released from breeding programs and plants which have been imported and passed quarantine procedures.
Since the technical basis for the micropropagation of temperate zone fruit rootstocks exists, the final say concerning the application of those methods belongs to fruit tree producers. Micropropagation of rootstocks for grafting must wait for the implementation of robotics but plants of in vitro origin should be used for the establishment of mother blocks even at the present economic reality. The improved health status and cropping of stoolbeds and hedges should not only recover the initial costs but also bring additional profit for well managed nurseries.