A. Clonal vs. Seedling Rootstocks

Specific rootstock benefits have been taken advantage of agriculturally at both the species level and at the clonal level, depending on species and extent of horticultural domestication. Rootstock benefits at the species level include the dwarfing effect of Poncris trifoliata rootstocks on citrus, although this has been shown to be due to an asymptotic virus ( Davis & Albrigo, 1994), and the dwarfing effect of quince (Cydonia oblonga) on pear. One of the earliest reports of a specific seedling rootstock effect is found in 6th century Chinese writings of Chia Shi-yi in the book Tsee Ming Yau Su. He reported that grafting of pear onto seedlings of Pyrus betulifolia  rootstock gave better fruit than on P. phaeocarpa rootstocks. With a few crops, most notably apple, pear and grape, deliberate selection of superior rootstock genotypes maintained by clonal propagation (see Propagation of clonal rootstocks, below) has allowed for further refinement.

1. Variability associated with seedling rootstocks

Most species for which grafting is a common mode of propagation, are worked onto understocks grown from seed. A range of examples can be seen in table 1. Since outcrossing is the prominent breeding system among tree species, most of these seedling rootstocks would be highly genetically heterozygous for most characteristics. Hence, they would exhibit a great deal of phenotypic variability, i.e. lack of trueness-to-type . This would be true not only for obvious above ground characteristics like fruit size and color but also for root-system associated and other characteristics, which would influence performance of a grafted tree. Hence, for seedling rootstocks, genetic variability would exist with respect to such root system-specific characters as root architecture (depth and distribution), tolerance of edaphic environmental stresses (soil acidity, drought, salinity, etc.) and biotic stresses (root-specific diseases and pests). The genotype of a given rootstock affects not only these root-system associated characteristics, but also a number of effects by which a rootstock more directly influences the growth and performance of a scion worked onto it. These would include scion vigor (tree size), precocity, etc. In many species of trees that are commonly grafted, this range of genetic and ultimately phenotypic rootstock-associated variability of seedlings is considered acceptable horticulturally for use as rootstock material. This would include most of the seed propagated rootstocks in table 1.

2. Apomictic (clonal) seedling rootstocks

Not all clonal rootstocks require deliberate use of conventional vegetative propagation techniques (cuttage, layering, micropropagation). In the case of species that exhibit apomixis, clonal reproduction is the natural strategy by which the species reproduces. Apomixis refers to seed embryo development entirely from maternal, flower-associated tissues, without fertilization from a male (pollen) parent. Since the apomictic embryo develops entirely from maternal cells, it is genetically identical to the maternal parent, i.e. a clone of the parent tree. Apomixis is an interesting exception to genetic heterozygosity and lack of true-to-type phenotype associated with seed propagated rootstocks. Apomixis occurs in only a small minority of all plant species. In some species that exhibit apomixis, both an apomictic (clonal) embryo as well as a "normal" zygotic (sexual) embryo is produced. These species exhibit polyembryony, i.e. more than one embryo per seed. Citrus and mango are examples of apomictic, polyembryonic species. Some apple species (e.g. Malus hupahensis) produce apomictic, but monoembryonic embryos.

B. Selection and Uses of Clonal Rootstocks

In the section on Reasons for Grafting and Budding, the use of grafting to clonally propagate a selected, genetically superior individual of a species that is otherwise difficult to clonally propagate is discussed. Most deliberate clonal selection is made for one or more above ground (i.e. scion) characteristics such as flower color or fruit size, branching habit, etc. However, in a few species, including apple and grape, not only have scion varieties been selected for above ground characteristics, but also rootstock varieties have been selected for specific root system-associated characteristics such as adaptation to soil pH, abiotic (e.g. drought) and biotic stresses (diseases and insects). In addition, an important selection / breeding criterion may be the effects of rootstock on scion performance. Apple rootstocks, for example, have been selected for their ability to cause dwarfing of the scion (fruiting) variety, by inducing the above ground portion of the tree (shoot system) to go dormant sooner and hence put on less total seasonal vegetative growth.   

Although apple may not have been the first crop for which clonal rootstock selection began, no other crop has been so profoundly influenced by this practice.  This is due, at least in part, to the fact that apple is economically the second most important fruit crop in the world. It is superceded only by grapes.  Nonetheless, apple has received more attention in terms of clonal rootstock selection and breeding than grapes or any other woody crop. For this reason, apple will be given special consideration below (Section D).

C. Propagation of Clonal Rootstocks

Scions are clonally propagated by grafting, but the clonal rootstock they are grafted onto must be propagated by clonal methods other than grafting, including cuttage, layering, micropropagation, and apomictic seed. In fact, difficulty of clonal propagation of fruit tree rootstocks is one of the major limitations to rootstock selection in some cases.  According to the federally funded multi-state NC-140Fruit Tree Rootstock evaluation program,

"Knowledge of the propagation characteristics of newer rootstocks and reasons for incompatibility between cultivars and rootstocks is a continuing need. Some promising pome- and stone-fruit rootstocks presently cannot be considered for commercial use because of difficulty in rooting clonal material using existing techniques. Alternative methods of propagation only recently have begun to offer solutions to these problems. Using different propagation methods such as hardwood or softwood cuttings, which are not commonly used, may be very effective for mass production of some of the rootstocks that do not propagate well by conventional means. Expanded research with micropropagated plant material needs to be done to anticipate potential problems before widespread adoption by the fruit industry occurs. These techniques may also allow early screening of plant material for desirable characteristics such as disease and insect resistance." -- NC-140 Web site.

1. Cuttings

In relatively easy-to-clone species, such as grapes and some roses, rootstock cultivars are rooted from cuttings either directly in the field (rose hardwood cuttings in Kenya), where grafting will subsequently occur, or cuttings are rooted in a greenhouse, as is the case for non-hardy, florists' roses such as these grown in Colombia. Other crops, for which clonal rootstocks are sometimes propagated from cuttings, include plum, peach, and cherry.

2. Layering

For species that will not root easily from cutting, layering is often practiced, because roots are able to form before the propagule (branch, shoot, etc.) is detached (cut) from the parent plant. Apple is relatively difficult to root from cuttings, and layering is by far the most important method for clonal propagation of this crop.

3. Apomictic Seed

Many citrus species and mango varieties, used as rootstocks, are apomictic and polyembryonic, as described above. Hence clonal rootstock propagation in this case is simply a matter of seed germination.

4. Micropropagation

Micropropagation (tissue culture) for clonal propagation of some fruit tree rootstocks has been practiced for years by breeders and/or for research applications, but it has proven to be too expensive to be widely commercially viable.

D. A Detailed Look at Clonal Apple Rootstocks

1. History of Clonal Apple Rootstocks

a. Ancient history

The domestication of apple began thousands of years BC, chiefly from wild Malus pumila, indigenous to the Caucasus Mountains which lie between the Black and Caspian Seas (sometimes said to form the eastern boundary of Europe). These were carried by Caucasians south to the Eastern Mediterranean, where they were eventually used by both the Greeks and Romans, who grafted selected clones onto seedlings or clonal rootstocks from cuttings ( Westwood, 1993). The Greek philosopher Theophrastis (370 BC) wrote of a dwarf apple variety called "spring apple" (Malus pumila), brought by Alexander the Great from Asia Minor to Greece. He also wrote of the practice of grafting apples. Given the fact that these early-domesticated apples were relatively easy to root, it is possible that "Spring Apple" may have been the earliest dwarfing clonal rootstock.  Some of the early apple varieties survived the breakup of the Roman Empire, and eventually found their way all over Europe.  

b. European history

(1). Paradise apple.

Eventually apple varieties derived from the spring apple became known, at least by the 15th century in France, as the Paradise apple ( Tukey, 1964), and it is likely that it was from these that some of the early dwarfing rootstock selections were made.  Most of these variants would have been selected  by observant plantsmen, from seedlings, and easily vegetatively propagated from cuttings or layering. Numerous selections were made of the Paradise apple, and since no systematic naming conventions were in effect, a great deal of confusion arose.   It is not entirely clear when the transition was made from Paradise apples grown on their own roots to the use of Paradise as a dwarfing rootstock for other apples. The famous gardens at Versailles in France, during the reign of Louis XIV (1643-1715) included both apple and pear trees worked onto the dwarfing Paradise rootstocks.  In 1879, in France, a seedling was selected for its dwarfing effect on fruit varieties grafted onto it, and given the name Jaune de Metz (Metz's Yellow), and clonally propagated. Over a century later, this clone and many other pre-existing European genotypes served as the starting point for modern apple rootstock selection and breeding.

(2). Doucin apple

A second group of dwarf apple rootstocks that became common in Europe is the Doucin apple, first mentioned in the European literature during the early 16th century.  Doucin apple differs from Paradise in two important ways. First, although somewhat dwarf, it was significantly more vigorous than Paradise, and second, it was not so easily clonally propagated as Paradise.

2. Modern Era of Apple Rootstock Improvement

The modern era of apple production may be said to have begun in England around the turn of the century.  The impetus was a need to bring order to the confusing state of affairs with respect to the many rootstock clones descended from the original Paradise and Doucin apples. With the name Paradise, for example, being applied by nurserymen to what were obviously more than one clonal rootstock genotype, consumers simply could not know what they were getting when they purchased "Paradise". Early 20th century clonal rootstock selection and breeding is described below in the sections on the East Malling and the Malling Merton rootstock improvement programs.

a. Selection criteria for clonal apple rootstocks (see NC-140 Web site for a tabular comparison of these characteristics among the currently most common clonal apple rootstocks)

(1). Size Control

Clonal apple rootstock selection has been directed at and is perhaps best known (but not limited to) size control, i.e. the ability of a rootstock genotype to reduce vegetative growth and hence tree size of the scion (fruiting) variety. Indeed, more or less "dwarf" apple trees are a prominent feature of modern apple production. The earliest modern systematic attempt to categorize apple rootstocks with respect to their effect on the vigor and overall size of fruiting varieties grafted onto them was the East Malling program described below.

(2). Other important selection criteria for clonal apple rootstocks include, but are not necessarily limited to, the elimination of some of the "problems" discussed below. Some of these include:

Many of these selection criteria are discussed in more detail in the section on Reasons for grafting and budding (Section F. Grafting to achieve independent optimization of component genotypes - Specific Rootstock / Interstock Benefits). Many of these are also the object of current, ongoing apple rootstock improvement programs discussed below.

b. Problems with clonal apple rootstocks

The problem with breeding apple trees or any crop is that seedlings selected for one desirable characteristic often have many characteristics that are not desirable. Hence it takes screening for multiple characteristics over multiple generations to obtain anything approaching the "perfect" genotype. This problem is particularly acute for trees since they typically have long generation times (years).

(1). Burr knots

Burr knots are tight clusters of preformed adventitious root primordia that occur on the stems of susceptible apple genotypes. Since they are associated with ease of clonal propagation (layering and cuttage primarily), they were actively selected for in early rootstock breeding programs.  Today, however, they are actively selected against since they are considered more of a problem than a blessing. Since normal stem / root xylem and phloem conduction is interrupted by the tightly massed cluster of adventitious root primordia that make up a burr knots, complete or partial stem girdling, with ultimately fatal consequences, may occur.

(2). Wooly apple aphid (Eriosoma lanigerum)

Wooly apple aphid is an insect pest that can cause considerable damage to apple nursery stock of susceptible genotypes.  The early East Malling rootstock selections (M9, etc., described below) were especially susceptible to wooly apple aphid, but since this pest was not a problem in Europe, at the time, wooly apple aphid susceptibility was not considered in seedling selection. However, in subsequent years, apple producers in Australia and New Zealand, where wooly apple aphid was a serious problem, reported that many of the early East Malling (EM) rootstocks were highly susceptible to the pest. Hence, resistance to wooly apple aphid became one of the primary goals of the subsequent Malling Merton (MM) breeding program that began in 1928. Since Northern Spy apple was known to be resistant to wooly apple aphid, it was used as a parent in crosses with the EM rootstocks.  Only seedlings that were wooly apple aphid resistant and that had other desirable characteristics (especially various levels of size control) were selected for and ultimately released as MM rootstocks. Wooly apple aphid resistance is still a selection criterion of modern rootstock breeding programs described below.

(3). Suckering

Suckering refers to the formation and growth of adventitious shoots from the root system of an established tree. In the case of apple varieties grafted onto clonal rootstocks, the suckers are genetically identical to the rootstock genotype, and hence genetically different from the scion variety, and not at all suited for fruit production (many produce small crabapple-like fruit). Hence, suckers must be removed to prevent them from growing to the point when they would become part of the mature fruit bearing canopy of the tree and give rise to off-type apples. Furthermore, suckers of fireblight susceptible rootstocks may become infected with this bacterial pathogen to the detriment of the entire tree. Manual removal of suckers is labor intensive and hence it is desirable to select against suckering during the process of apple rootstock seedling selection.

(4). Brittleness

Some dwarfing rootstocks like M9 tend to exhibit brittle wood due to relatively short fusiform initial cells  (cells in the vascular cambium that give rise to wood fibers) resulting in shorter, minimally interlocking wood fibers between stock and scion. Consequently, breakage may occur under a heavy crop load.

The following Web sites have additional information about common apple and other tree fruit rootstock resistance to various "problems":

c. Rootstock breeding programs around the world

(1). East Malling Rootstocks (England)

The East Malling apple rootstock program involved evaluation and selection of existing rootstocks (not breeding per se).

Later releases from the East Malling program

(2). Malling-Merton  Rootstocks (England)

(3). East Malling Long Ashton (EMLA) Rootstocks (England)

(4). Budagovsky Rootstocks (former Soviet Union)

(5). Polish Rootstocks

(6). Geneva Rootstocks (Eastern United States): USDA-ARS / Cornell University apple rootstock breeding project

G65 (M27 x 'Beauty' crabapple)

G16 (Ottowa3 x Malus floribunda)

G11 (M.26 x Robusta 5 Crabapple )

G30 (Robusta 5 x M9)

Three additional Geneva rootstocks are still under evaluation including CG.41, CG.935, and CG.210

(7). Ongoing long term evaluation in North America of clonal rootstocks from many sources - NC-140 project

Most rootstock breeding programs have been primarily concerned with selection and breeding of rootstocks for a particular region with its associated set of environmental and biological (diseases and pests) stresses and constraints. Nonetheless, many of them, particularly the EM and MM series rootstocks, are used outside the region they were developed for. In an attempt to better understand rootstock performance under a wide range of conditions and to allow an objective comparison among rootstocks from different programs under a given set of conditions, the US Department of Agriculture has funded the NC-140 Regional Hatch project. NC-140 involves evaluation of many different rootstocks at many different locations. The NC-140 Web site contains a great deal of useful information about modern research in apple rootstock improvement.  

d. Propagation of clonal apple rootstocks - this topic is reviewed in the autotutorial on Commercial Apple Production via Grafting and Budding

e. Additional information about clonal apple and other fruit tree rootstocks and modern fruit tree production can be found at:


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