Table of Contents
Taylor, N.L. 1990. The true clovers. p. 177-182. In: J. Janick and J.E.
Simon (eds.), Advances in new crops. Timber Press, Portland, OR.
The True Clovers*
Norman L. Taylor
- Table 1
- Table 2
- Table 3
- Fig. 1
The true clovers (Trifolium) include about 248 leguminous species, many
of which are important forage', throughout the world. They are located in
three main centers of diversity: Eurasia, South Africa, and the Americas. The
four most important species in the United States are red clover (Trifolium
pratense L.), white clover (T. repens L.), crimson clover (T.
incarnatum L.) and alsike clover (T. hybridum L). In addition to
their value for livestock feed, they fix elemental nitrogen, and thus are
important soil improvers, and help control soil erosion The more important
species in the U.S. were introduced with the early settlers from Europe, and
have been adapted to U.S. conditions by natural selection, breeding, and
introduction of genes for various characteristics, the most important of which
is probably disease resistance. This report summarizes the more important
activities in introduction, preservation, evaluation, and enhancement of the
clovers in the United States in the last two decades.
In a review of genetic vulnerability and germplasm resources of the clovers in
1976, Taylor et al. (1977) concluded that the trend in number and diversity of
cultivars of the cultivated species was downward, perhaps because of emphasis
on other crops and narrowing of gene bases by breeding. However, because the
cultivated clovers are mostly cross pollinated, heterozygous, and
heterogeneous, they were not considered to be pa particularly vulnerable at
that time. Germplasm resources were thought to be in a more hazardous position
because the many accessions of these species were haphazardly assembled and
were not systematically evaluated. It was, recommended that evaluation and use
of accession should be established by a committee of experts knowledgeable in
clover germplasm. It is pleasing to report that many of the recommendations of
the report have been acted upon, and that germplasm resources are now in a much
Of most importance was the establishment of a Clover and Special Purpose Legume
(CSPL) Advisory Committee to rate germplasm activities. A Trifolium
curator was appointed and the seed stocks available in the United States were
surveyed and assembled at the Curator location and at the Southern Regional
Plant Introduction Station for the annual, and at the Northeastern Plant
Introduction Station for the perennial legumes. Seeds of about 215 taxa of
clovers were assembled at Lexington, Kentucky. The committee concluded that
adequate numbers of accessions of the cultivated species were available, but
accessions of some perennial species were rare. Plant exploration was
initiated to increase the number of accessions of rare taxa- The first of these
explorations was in Greece, Italy, and Crete in 1977 (Gillett and Smith 1985).
Thirty-six species represented by 291 accessions were added to the collection
as a result of the exploration. The second exploration was in Romania in 1984
(Taylor and Rumbaugh 1986) to collect perennial species. Forty-two accessions
were collected representing 13 different species. P.I. numbers were assigned
and chromosome numbers determined (Taylor and Rumbaugh 1986). However, no new
species were added to the collection, and it was determined that Romania is not
a prime center of diversity of Trifolium. Accessions of species were
added that could contribute genes different from those previously available.
The CSPL Advisory Committee concluded that the prime center for future
collection of new perennial species is in the dinaric alps of Yugoslavia and
Albania. Because it is difficult to enter Albania, an exploration trip to
Yugoslavia was proposed for the late summer of 1988 and was completed shortly
before this conference. In addition to these formal exploration efforts, the
curator and others have collected in the United States and foreign countries.
One of the prime centers for diversity is Turkey, where clovers were collected
as a part of the Medicago exploration trip in the 1960s. Domestic
species also have been collected, particularly in the Western United States and
by informal germplasm exchange. The clovers of Ethiopia, South Africa, South
America, New Zealand and Australia also have been added to the curator and P.I.
The clovers of the eastern United States recently have come under considerable
scrutiny because of near extinction of several species. These include T.
stoloniferum Muhl., T. reflexum, L., T. virginicum Small ex
Small and Vail., T. carolinianum Michx., T. polymorphum Pair. ex
Lam. and Pair., and T. bejariense Moric. These species, once prevalent
in the eastern United States, are now only rarely seen. T. stoloniferum
(running buffalo clover), thought to be possibly extinct, was listed in the
Federal Register as an endangered species (Jacobs 1987), and a recovery team
has been formed. Small colonies of this clover now have been found in West
Virginia, Kentucky, and Indiana. A second stolonifereous species, as yet not
named, has been discovered in Tennessee and Virginia (Taylor 1988).
Preservation of the introduced germplasm is a continuing problem that the CSPL
advisory committee recognized early. The committee recommended that all
Trifolium seed stocks for long- and short-term storage be maintained at
below freezing temperature (-5 to -18°C), which was shown by Rincker (1974)
to be effective for T. pratense and other seeds. The curator collection
at Kentucky has been maintained at these temperatures since about 1974, and the
regional plant introduction stations have purchased equipment for
below-freezing storage that may be in place at this writing. Storage at
above-freezing temperatures has resulted in gradually declining seed viability,
necessitating periodic seed increases. About two-thirds of the species of the
genus are self-pollinated (Taylor et al. 1980) and increase of these seeds is
relatively easy because isolation is not required. Most of the self-pollinated
species are annuals, maintained at Experiment, Georgia, and periodic seed
increases have been made at that location. Recently, however, a contract was
issued to increase a number of accessions in southern California. The increase
procedure requires establishing at least 50 plants of each accession which are
mostly, if not entirely, self-pollinated and thus do not need to be isolated.
For the cross pollinated, and particularly for the perennial species, the
procedure is more difficult. The former procedure was to allow cross
pollination among accessions without isolation, but this resulted in loss of
genetic identity A more recent procedure is to conduct such increases under
cages in which bees are used for pollination. Contracts recently have been
issued to increase seeds of accessions of T. medium L. and T.
ambiguum Bieb. in northern locations where plants are established, allowed
to over-winter for vernalization, and produce seed the following year. Seed
increases are expensive but necessary to rejuvenate accessions with seeds of
low viability and to increase seed when, as is usually the case, only small
amounts of seed were originally introduced.
Prior to official evaluation efforts, examination of species of
Trifolium in the Kentucky collection revealed that annual species
generally have simple tap roots and low chromosome numbers. They are usually
self pollinating and many were introduced from a Mediterranean type climate.
Perennial species generally are taprooted, stoloniferous, or rhizomatous,
possess higher chromosome numbers than annuals, are mostly cross-pollinating
and do not have specific climate-habitat relationships. Species introduced
from Eurasia are more numerous and more diverse in base chromosome number (x =
5 to 8) than are those from other origins. Only species with diploid
chromosome numbers of 16 or higher are stoloniferous or rhizomatous.
Rhizomatous species, mostly cross-pollinated, were introduced from Eurasia,
North and South America, but not from Africa, and not often from Mediterranean
climates. Self- and cross-pollinated species occur in all regions. Different
flower colors and leafmarks are not associated with origin, climate, or other
morphological and physiological characteristics (Taylor et al. 1979).
Systematic evaluation of the many accessions of the cultivated species has
begun under contract with the Plant Introduction Service. Descriptor and
evaluation priorities have been established for T. pratense, L., T.
repens L., T. incarnatum L., T. vesiculosum Savi, T.
subterraneum L. and T. hirtum L. Evaluations that are underway
include disease and nematode resistance for T. pratense, isoflavone
content in T. subterraneum and T. hirtum, and determination of
chromosome numbers among accessions of T. medium and T. ambiguum.
It is expected that evaluation efforts will proceed as rapidly as funds are
made available, and personnel can be found with expertise to conduct the
evaluations. An unanticipated problem is that the researchers may perceive
evaluation as a service rather than a research activity.
Enhancement activities long underway for the major cultivated species have led
to the release of a large number of cultivars, germplasms, and gene markers
(see Taylor 1985, Clover Science and Technology for a review, of these
activities). In terms of "New Crops" to which this paper is primarily
addressed, introduction efforts have resulted in some cultivar and germplasm
releases (Table 1). Perhaps the most notable of these introductions in recent
years is T. vesiculosum (arrowleaf clover), introduced from Italy in
1963. Three different introductions varying in maturity date resulted in three
cultivars, 'Amclo', 'Meechee', and 'Yuchi' (Miller and Wells 1985). Knight and
Watson (1977) estimated the acreage of arrowleaf clover at 200,000 ha in 1980.
Rose clover (T. hirtum) is another introduction that achieved a fairly
important position in United States agriculture. According to Love (1985),
seed of the 'Wilton' cultivar developed in Australia was certified in
California in 1949. It has found a significant place in California range
sowings, but estimates of acreage are not available. Other introductions of
clover that are of some value in the United States are listed in Table 1.
Several other introduced clovers have become naturalized in the United States
and occasionally make significant contributions to pasture and hay crops.
Little or no breeding has been done with these species, which include ball
clover (T. nigrescens), hop clovers (T. campestre Shreb., T.
aureum L., and T. dubium Sibth.), lappa clover (T. lappaceum
L.), cluster clover (T. glomerulatuum L.), bigflower clover (T.
michelelianum Savi.) and rabbitfoot clover (T. arvense L.) (Knight
1985). About 10 native range species clovers of Western United States are
locally important as forage for sheep, cattle, and deer (Crampton 1985).
Much of the interest in plant introduction has stemmed from the possibility of
hybridizing perennial wild clovers with the cultivated species, and then to
develop new crops for agriculture. Barriers to interspecific hybridization are
quite difficult to overcome among species closely related to red clover (Taylor
et al. 1980, Rubtsov and Komkova 1983). Only four species have been hybridized
with red clover, and only one of these was fertile (Table 2). The first hybrid
was a mating of T. pratense (2n = 2x = 14) with T. diffusum L. (2n
= 2x = 16), an annual species. The hybrid was sterile at the diploid level but
fertile as an amphidiploid (Taylor et al. 1963). Another annual species, T.
pallidum L., (2n =2x = 16) was successfully hybridized with tetraploid
T. pratense (2n = 4x = 28) to produce a sterile sesquidiploid
(Armstrong and Cleveland 1970). In the third hybrid, embryo rescue was
necessary for the hybridization of red clover with the perennial T. sarosiense Hazsl. (2n = 6x = 48) (Phillips et al. 1982), but
unfortunately the hybrid is sterile and has remained so even when doubled with
colchicine. Tetraploid T. pratense (2n = 4 x = 28) also has been
crossed with T. medium (2n = 10x = 80) to produce a sterile hybrid
Among species closely related to red clover, T. sarosiense has been
hybridized with T. medium and T. alpestre L. has been hybridized
with T. heldreichianum (Gib. and Belli) Hausskn. and T. rubens
L., all perennial species (Quesenberry and Taylor 1976, 1977, 1978) (Table 2).
The transfer of genes from these species to red clover might be possible if the
sterility of the T. sarosiense x T. pratense hybrid could be
overcome. The perennial species possess perennial tap roots and/or rhizomes
and are resistant to several virus diseases, but the annual species seem to
possess few useful characteristics.
It is possible to construct a diagram showing natural affinities based on
successful crosses among species closely related to red clover (Fig. 1).
According to this diagram and Table 1, red clover is most closely related to
the annual species, T. diffusum, and somewhat less closely related to
T. pallidum, T. sarosiense and T. medium. Trifolium
sarosiense is closely related to T. medium and T. alpestre,
which in turn are closely related to T. rubens and T.
Interspecific hybridization of white clover has been more successful than with
red clover. Hybrids that have been produced are listed in Table 3. White
clover has been hybridized with the annual species T. xerocephalum
(Frenzl.) at the diploid level and with T. nigrescens (Viv.) both at
the diploid and tetraploid levels to produce partially fertile hybrids. White
clover also has been hybridized with the perennial species, T.
occidentale (Coombe) and T. uniflorum L. and with T. ambiguum
at the tetraploid level (Table 3). The most promising of these hybrids
involves T. ambiguum in which one hybrid plant is partially fertile
(Williams and Verry 1981) and F2 populations and backcrosses to white clover
have been produced. The transfer of perenniality and virus disease resistance
from T. ambiguum to T. repens has proved to be extremely
difficult. Research continues using colchicine to double the chromosome number
of the hybrid, irradiating of the F1 to aid in transfer of genes to white
clover by backcrossing, and evaluating virus resistance in backcross
In summary, although much remains to be accomplished in accessing the genetic
variation available in the clovers, research is underway, and the wide
diversity exhibited in the germplasm resources of the clovers is rapidly being
cataloged and preserved for future generations.
- Armstrong, K.C. and R.W. Cleveland. 1970. Hybrids of Trifolium pratense
x Trifolium pallidum. Crop Sci. 10:354-357.
- Baltensperger, A.A., C.E. Watson, M.A. Smith, S.D. McLean and R.E. Gaussoin.
1982. Registration of Fresa strawberry clover. Crop Sci. 22:1260.
- Brewbaker, J.L. and W.F. Keim. 1953. A fertile interspecific hybrid in
Trifolium (4n T. repens L. 4n T. nigrescens Viv.) Amer.
- Chen. C.C. and P.B. Gibson. 1970. Meiosis in two species of Trifolium
and their hybrids. Crop Sci. 10:188-189
- Crampton, B. 1985. Nature range clovers. p. 579-590. In: N.L. Taylor (ed.).
Clover science and technology., ASA Monog. 25. Madison, WI.
- Gillett, J.M. and R.R. Smith. 1985. Germplasm exploration and preservation. p.
445-456. In: N.L. Taylor (ed.) Clover science and technology., ASA Monog. 25.
- Gibson, P.B. and G. Beinhart. 1969. Hybridization of T. occidentale with
two other species of clover. J. Hered. 60:93-96.
- Gibson, P.B., C.C. Chen, J.T. Gillingham and O.W. Barnett. 1971. Interspecific
hybridization of Trifolium uniflorum L. Crop Sci. 11:895-899.
- Jacobs, J. 1987. Endangered and threatened wildlife and plants; determination
of endangered status for Trifolium stoloniferum (running buffalo
clover). Fed. Reg. 52:21478-21480.
- Kazimerski, T. and E.M. Kazimerska. 1968. Investigations of hybrids of the
genus Trifolium L. Sterile hybrid of T. repens x T.
xerocephalum Frenze. Acta Soc. Bot. Pol. 37:549-560.
- Knight, W.E. 1985. Miscellaneous Annual Clovers. p. 547-562. In: N.L. Taylor
(ed.). Clover science and technology., ASA Monog. 25. Madison, WI.
- Knight, W.E. and V.H. Watson. 1977. Update on crop plant developments: Legume
variety developments and seed needs in Southwestern United States. Proc. 23rd
Ann. ASTA Farm Seed Conf. 8 Nov., 1977. Kansas City. p. 8-26.
- Love, R.M. 1985. Rose clover. p. 535-546. In: N.L. Taylor (ed.). Clover science
and technology., ASA Monog. 25. Madison, WI.
- Merker, A. 1984. Hybrids between Trifolium medium and Trifolium
pratense. Hereditas 69:301-302.
- Miller, J.D. and H.D. Wells. 1985. Arrowleaf clover. p. 503-514. In: N.L Taylor
(ed.). Clover science and technology., ASA Monog. 25. Madison, WI.
- Peterson, M.L., J.E. Street and V.P. Osterli. 1962. Salina strawberry clover.
Calif. Agric. Exp. Stn. Leafl. 146.
- Phillips, G.C., G.B. Collins and N.L. Taylor. 1982. Interspecific hybridization
of red clover Trifolium pratense L. cultivar Kenstar with Trifolium
sarosiense using in-vitro embryo rescue. Theor. Appl. Genet. 62:17-24.
- Quesenberry, K.H. and N.L. Taylor. 1976. Interspecific hybridization in
Trifolium L., Sect. Trifolium Zoh. I. Diploid hybrids T.
alpestre L., T. rubens L., T. heldreichianum Hausskii., and
T. noricum Wulf. Crop Sci. 16:382-386.
- Quesenberry, K.H. and N. L. Taylor. 1977. Interspecific hybridization in
Trifolium L., Sect Trifolium Zoh. II. Fertile polyploid hybrids
between T. medium L. and T. sarosiense Hazsl. Crop Sci.
- Quesenberry, K.H. and N.L. Taylor. 1978. Interspecific hybridization in
Trifolium L. Section Trifolium Zoh. III. Partially fertile
hybrids of T. sarosiense Hazsl. x 4X T. alpestre L. Crop Sci.
- Rincker, C.M. 1974. Effect of frequent thawing on viability of red clover seed
in cold storage. Crop Sci. 14:749-750.
- Rubtsov, M.I. and T.N. Komkova. 1983. Interspecific hybridization of clover.
Skh. Biol. 2:55-58.
- Rumbaugh, M.D. 1988. Notice of release of ARS-2678 Kura Clover. USDA-ARS-Soil
Cons. Service. Utah Agr. Expt. Sta. Germplasm Release Statement.
- Taylor, N.L. 1980. Clovers. p. 261-272. In: W.R. Fehrand, H.H. Hadley (eds.).
hybridization of crop plants. American Society of Agronomy. Madison, WI.
- Taylor, N.L. 1988. The Native Clovers of Eastern United States. Proc.
Trifolium Conf.. 10:28-29.
- Taylor, N.L., P.L. Cornelius and R.E. Sigafus. 1982. Registration of Ky M-1
zigzag clover germplasm. Crop Sci. 22:1278-1279.
- Taylor, N.L., P.B. Gibson and W.E. Knight. 1977. Genetic vulnerability and
germplasm resources of the true clovers. Crop Sci. 17:632-634.
- Taylor, N.L., R.F. Quarles and M.K. Anderson. 1980. Methods of overcoming
interspecific barriers in Trifolium. Euphytica 29:441-450.
- Taylor, N.L., K.H. Quesenberry and M.K. Anderson. 1979. Genetic system
relationships in Trifolium. Econ. Bot. 33:431-441.
- Taylor, N.L. and M.D. Rumbaugh. 1986. Survey and collection of Trifolium
germplasm in Romania1984. Plant Genet. Res. Cent. Ethiopia-Int. Livest.
Cent. Africa 13:32-39.
- Taylor, N.L., W.H. Stroube, G.B. Collins and W.A. Kendall. 1963. Interspecific
hybridization of red clover (Trifolium pratense L.) Crop Sci.
- Townsend, C.E. 1971. Registration of C-1 zigzag clover germplasm. Crop Sci.
- Townsend, C.E. 1975. Registration of C-2 kura clover germplasm. Crop Sci.
- Weihing, R.M. 1962. Selecting persian clover for hard seed. Crop Sci.
- Williams, E.C. and I.M. Verry. 1981. A partially fertile hybrid between
Trifolium repens and T. ambiguum. New Zeal. J. Bot. 19:1-7.
- Yamada, T. and H. Fukaoto. 1985. Application of ovule culture to interspecific
hybridization between Trifolium repens and T. ambiguum. Proc.
Int. Grass. Congr. 15:241-243.
- Zohary, M. and D. Heller. 1984. The genus Trifolium. The Israel Academy
of Sciences and Humanities. Jerusalem, Israel.
*The investigation reported in this paper (No. 88-3-201) is in connection with
a project of the Kentucky, Agricultural Experiment Station and is published
with approval of the Director.
Table 1. Recent introduction of species of Trifolium that have
resulted in cultivar and or germplasm releases.
zA = annual; P = perennial
|Species||Common name||Longevityz ||Origin ||USA releases ||Referencesy|
|T. vesiculosum Savi ||arrowleaf ||A ||Italy ||Amclo, Meecher Yuchi cultivars
|T. ambiguum Bieb. ||kura ||P ||USSR, Asia minor ||C-2 and ARS-2678
germplasms ||(2, 3)|
|T. medium L. ||zigzag ||P ||Europe ||C-1 and M-1 germplasms ||(4, 5)|
|T. alexandrinum L. ||berseem ||A ||Syria, Egypt ||Bigbee cultivar ||(6)|
|T. resupinatum L. ||Persian ||A ||Asia Minor ||Abon cultivar ||(7)|
|T. hirtum L. ||rose ||A ||Asia Minor ||Wilton cultivarx ||(8)|
|T. fragiferum L. ||strawberry ||P ||Europe, Asia Minor ||Salina and Fresa
cultivars ||(9, 10)|
y(1) Miller & Wells 1985; (2) Townsend 1975; (3) Rumbaugh 1988; (4)
Townsend 1971; (5) Taylor et al. 1982; (6) Knight 1985; (7) Weihing 1962; (8)
Love 1985; (9) Peterson et al. 1962; (10) Baltensperger et al. 1982.
xReleased in Australia and seed certified in California.
Table 2. Interspecific crosses among species closely related to red
|Female ||Male ||Somatic chromosome no. of hybrid ||Fertility ||Referencey|
|T. pratense ||(2n=14)||T. diffusum|| (2n=16)||15||
|T. pratense||(2n=28)||T. diffusum ||(2n=32) ||30||
|T. pratense||(2n=28)||T. pallidum||(2n=16)|| 22||
|T. sarosiense||(2n=48)||T. alpestre ||(2n=32) ||40||
|T. medium||(2n=72)||T. sarosiense ||(2n=48)z ||58-60||
|T. alpestre||(2n=16)||T. heldreichianum ||(2n=16) ||16||
|T. alpestre||(2n=16)||T. rubens ||(2n=16) ||16 ||partial||
|T. sarosiense ||(2n=48)||T. pratense ||(2n=14) ||31||
|T. medium ||(2n=80)||T. pratense ||(2n=28) ||54 ||sterile||
y(1) Taylor et al. 1963; (2) Armstrong and Cleveland 1970; (3) Quesenberry and
Taylor 1978; (4) Quesenberry and Taylor 1977; (5) Quesenberry and Taylor 1976;
(6) Phillips et al. 1982; (7) Merker 1984.
Table 3. Interspecific crosses involving Trifolium repens.
z(1) Chen and Gibson 1970; (2) Brewbaker and Keim 1953; (3) Gibson and Beinhart
1969; (4) Kazimerski and Kazinierska 1968; (5) Gibson et al. 1971; (6) Williams
and Verry 1981; (7) Yamada and Fukasoka 1985.
|Female ||Male ||Somatic chromosome no. of hybrid ||Fertility ||Referencez|
|T. repens ||(2n=32) ||T. nigrescens ||(2n=16) ||24 ||Partial
|T. repens ||(2n=32) ||T. nigrescens ||(2n=32) ||32 ||Partial||
|T. repens ||(2n=32) ||T. occidentale ||(2n=32)||32||
|T. repens ||(2n=32) ||T. xerocephalum ||(2n=16) ||24
|T. uniflorum ||(2n=32) ||T. repens ||(2n=32) ||32 ||Partial||
|T. repens ||(2n=32) ||T. ambiguum ||(2n=48) || || ||(7)|
|T. repens ||(2n=32) ||T. ambiguum ||(2n=32) ||32 ||Partial||
|T. repens ||(2n=32) ||T. uniflorum |
|32 ||Partial ||(5)|
|T. repens ||(2n=32) ||T. occidentale |
x T. uniflorum
|32 ||Partial ||(5)|
Fig. 1. Crossing affinities among species closely related to red
Last update August 26, 1997