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.
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.
- These specific rootstock effects,
whether they are characteristics of the root system per se, or influences
of the rootstock on the scion, 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.), and in the section on Size control, below.
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).
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.
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.
- Size control is relative,
not absolute
- The relative effects of
several commonly used apple rootstocks are shown graphically in the
associated figure,
and numerically, along with other characteristics in the Apple Rootstock Table
- It is important to note
that the actual size of any given apple tree depends not only on the
rootstock genotype but also on the genotype of the scion variety,
and also on soils and other environmental characteristics. In fact
even the relative sizes shown in the figure and table are not inviolate:
in some cases even the relative size may depend on environmental conditions.
Hence the size classes shown in the figure and table should be used
only as a guide not a blueprint.
- Mechanism of dwarfing by selected
rootstocks. Although the hormonal regulation of the dwarfing process is
not clearly resolved in the scientific literature, it is understood that
dwarfing rootstocks act by inducing the scion portion of the tree to stop
growing (enter the early stages of dormancy) earlier in the summer than
the same fruit variety on a non dwarfing rootstock. Hence, total seasonal
growth is reduced.
(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:
- Resistance to insect pests
such as wooly aphid.
- Resistance to bacterial diseases
such as fire blight caused by Erwinia amylovora.
- Resistance to fungal diseases
such as root rot (Phytophthora sp).
- Absence of burr knots (see
b.(1), below)
- Absence of suckering.
- Tolerance of adverse soil
conditions such as drought, acidity, waterlogging, etc.
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).
- This was the first of several
apple rootstock improvement programs which have continued up to the present
day. The objective was to systematically evaluate and select the best
of the many clonal rootstocks being used throughout Europe.
- In 1912, R. Wellington, director
of the East Malling Research Station in Kent, England, brought together
71 collections of Paradise apple rootstocks acquired from Britain, France,
Holland, and Germany. Wellington was succeeded by RG Hatton in 1914. Initially,
these rootstock collections were clonally multiplied to generate sufficient
numbers of each to begin orchard evaluation. The first report of
this work was published in1917, with the description and release of the
first nine "Malling" rootstocks.
- These original 9 clonal rootstocks
were introduced with a new systematic naming system, Malling I, Malling
II ... through Malling IX. For example, Malling IX was the new name given
to the original Jaune de Metz clone selected in France in 1879. Subsequently,
in 1938, the "Malling" was shortened to "EM", and the roman numerals were
changed to Arabic numerals, but more recently "EM" was changed back to
"M" for "Malling" (not to be confused with "MM" for the more recent breeding
program, "Malling Merton" that is described below).
- Size control (rootstock effect
on scion) was the principal selection criterion used in the selection
of these original East Malling rootstocks. They were categorized as very
dwarf, semidwarf, semistandard, and standard.
- In addition to size control,
other desirable characteristics were considered in the selection of these
rootstocks including precocity (earliness of bearing), prolific fruiting
(effect on fruiting of the scion variety), and ease of propagation. To
the extent that these early selections were descended from the original
Paradise apples, they were inherently more or less easy to propagate vegetatively,
mostly by layering. In fact, a tendency towards excessive spontaneous
rooting, know as burr knots, eventually came to be recognized as a problem
since it sometimes resulted in partial or complete stem girdling. This
tendency to form burr knots has been actively selected against in more
recent apple rootstock breeding programs such as the one at Geneva, NY
(described below), with the trade off being that such selections are more
difficult to clonally propagate.
Later
releases from the East Malling program
- Between 1918 and 1970, eighteen
more rootstocks were released from the EM program, bringing the total
to 27 clones. EM 26 was released in 1959 and EM 27 in 1970. Some of these
later releases were not simply selections of preexisting European rootstocks
but rather the result of deliberate breeding. Most of the breeding
work, however, was conducted in collaboration with the John Innes Horticultural
Research Station at Merton, England, which was considered a separate rootstock
improvement program known as Malling-Merton (see below).
- Widespread adoption by growers
of these EM rootstocks did not occur until the 1930s and 1040s, and have
remained an important component of modern apple production.
(2).
Malling-Merton Rootstocks (England)
- The Malling-Merton breeding
program that got underway in 1928 was motivated in part by a serious pest,
known as the wooly apple aphid (Eriosoma lanigerum), which was
threatening apple production in Australia and New Zealand.
- The East Malling research
station collaborated with the Long Ashton station at Merton, England to
cross the highly wooly-apple-aphid-susceptible East Malling rootstocks
with the wooly-aphid-resistant Northern Spy apple. The goal was to select
seedling genotypes that combined the size control and other favorable
characteristics of the East Malling series with the Northern Spy resistance
to wooly apple aphid. From a total of 3758 seedlings only 15 were eventually
selected for release in 1952. These 15 rootstocks were named MM 101 through
MM115.
(3).
East Malling Long Ashton (EMLA) Rootstocks (England)
- The EM and MM series rootstocks
had accumulated a heavy load of asymptotic viruses, which reduced tree
vigor and fruit yield.
- This program was not involved
in breeding per se, but rather in virus elimination, accomplished through
"meristem culture", a form of micropropagation (tissue culture).
- An interesting outcome was
that the rootstock clones free of specific viruses were 10 to 15% more
vigorous (less dwarfing).
(4).
Budagovsky Rootstocks (former Soviet Union)
- Extreme cold is a major limiting
factor for apple production in the region formerly known as the Soviet
Union.
- The Budagovsky series was
developed by crossing of M8 and a number of cold hardy domestic apples
from that region.
- The Budagovsky or B series
of apple rootstocks selected for cold hardiness and the full range of
size control.
(5).
Polish Rootstocks
- Cold hardiness and size control
were the objectives in breeding the Polish (P) series rootstocks
(6).
Geneva Rootstocks (Eastern United States): USDA-ARS / Cornell University apple rootstock breeding project
- Initiated in 1968 by Dr. James Cummins
at the NY Agricultural Research station at Geneva, NY.
- Original objective was to
develop rootstocks better adapted than the EM, MM and others previously
available, to the biotic stresses (soils, climate) of New York. In the
1970's emphasis shifted to include resistance to biotic stresses including
root rot (Phytophthora cactorium), fireblight (Erywinia amylovora),
particularly in the context of modern high-density orchard planting systems.
- Unique features of the program
are described by the project Web site (link above):
"All
seedlings are subjected to rigorous screening programs to eliminate
disease susceptible individuals prior to planting in the field. Rootstock
genotypes are selected on the basis of abiotic stress tolerance, disease
resistance, productivity, and precocity in orchard tests. This breeding
protocol contrasts sharply with those of other breeding programs,
which have concentrated effort on improving the stoolbed and nursery
performance of rootstock cultivars and have employed the much narrower
range of traditional rootstock genotypes as parents of breeding populations.
As a result, the USDA-ARS/Cornell University rootstock breeding program
is recognized worldwide as the most promising source of new, disease
resistant and productive rootstock genotypes for the future."
- Four rootstocks have been
commercially released so far, but availability is limited, i.e. there
is a need for rapid clonal propagation. Characteristics of these rootstocks
are listed in the Apple Rootstock Table. Many of these General
rootstocks are available from Cummins Nursery.
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.
e.
Additional information about clonal apple and other fruit tree rootstocks
and modern fruit tree production can be found at:
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