|Table of Contents
Brewbaker, J.L. and C.T. Sorensson. 1990. New tree crops from interspecific
Leucaena hybrids. p. 283-289. In: J. Janick and J.E. Simon (eds.),
Advances in new crops. Timber Press, Portland, OR.
New Tree Crops from Interspecific Leucaena Hybrids
James L. Brewbaker and Charles T. Sorensson
- THE GENUS LEUCAENA
- HISTORICAL USES OF LEUCAENA
- Fodder from Shrubby L. leucocephala, a World Vagabond
- Human Consumption of Seeds
- Green ManureAn Ancient Use of Leucaena?
- INTERSPECIFIC HYBRIDIZATION IN THE GENUS LEUCAENA
- Species Intercompatibility
- Research Objectives Using Interspecific Hybrids
- INTERSPECIFIC LEUCAENA HYBRIDS AS NEW CROPS
- Relationship Between Adaptability and Use
- Gum Production
- Furniture, Construction Timber and Polewood
- Fodder Production
- Shade and Support
- Other Uses of Species Hybrids
- PRODUCTION OF SEEDLESS HYBRIDS
- Table 1
- Table 2
- Fig. 1
- Fig. 2
- Fig. 3
Leucaenas (genus Leucaena, Mimosoideae-Leguminosae) are among the most
versatile of trees. Few tropical trees have matched leucaena's ability to
provide quality fuelwood, fodder, polewood, green manure, shade, erosion
control and other useful products in the tropics and subtropics. The N-fixing
capability of these legumes is also of importance, particularly in low-input
farming systems. Currently 2-5 million hectares worldwide are estimated to be
planted to leucaenas.
Natural evolution has created impressive variability in this genus, but the
majority of leucaena production has until recently been limited to L.
leucocephala, the only species distributed pantropically. Both its shrubby
and arboreal forms are high-yielding and coppice readily. Both tend to produce
heavy seed crops, making them weedy and diverting fixed carbon from the desired
wood or fodder. Additionally, this species is often heavily damaged by a
Artificial hybridization has greatly expanded the way farmers can exploit these
multipurpose trees by creating species hybrids with unique growth habits,
ecological adaptations, wood and fodder quality, pest resistance and high
productivity. The most significant of these hybrids are those which are
seedless and have high psyllid resistance.
Leucaena Bentham has had more than 50 species ascribed to it, but most
are viewed as synonyms. Thirteen species are widely recognized (Table 1).
Taxonomic distinctions between the species are strongly supported by geographic
and ecological distributions, diverse tree and pollen morphology (e.g.,
leaflets per leaf range from 30-13,000+) as well as chromosome counts. New
taxa can be expected; the most recently validated species is L.
salvadorensis Standley, rediscovered by Hughes (1988). Natural hybrids
have been only infrequently discovered, evidently due to geographic and
Leucaena species are native to the neotropics from Texas to Peru. They
have colonized such diverse regions as Alpine, Texas, which has three months of
snow during winter, and Panamanian rainforests. They range from hot and
Mexican and Central American lowlands to the highlands (2000-3000 in
elevation). Some species grow as low (5 m), highly forked shrubs while others
grow as trees up to 20 m in height with 80 cm diameter at breast height (dbh).
All are marked by high-quality foliage attractive to herbivores and by dense
wood favored for fuel. Post-Colombian introductions of cattle and goats
decimated many natural stands in regions which were dominated by leucaenas as
evidenced by landmarks, villages, mountains and states (e.g., Oaxaca, Mexico)
which were named after leucaena.
Several seed collection trips by Brewbaker and colleagues resulted in almost a
thousand collections of leucaena species being grown in Hawaii since 1962.
Leucaena seed collection programs of Oxford Forestry Institute (OFI) in England
and the Commonwealth Scientific and Industrial Research Organization (CSIRO) in
Australia have expanded this world collection. The Nitrogen Fixing Tree
Association founded and promotes the International Leucaena Trial (ILT) program
of varietal and management trials, and publishes the annual journal Leucaena
L. leucocephala is pantropical, the result of distribution via Spanish
galleons from Mexico to Southeast Asia in the early 16th century (Brewbaker
1987). Leucaenas served as fodder and bedding for the animals which the
Spaniards shipped. Unfortunately only the shrubby strain of L.
leucocephala was involved. This "common" form seeded abundantly and
aggressively colonized much of the tropics, notably on sub-humid alkaline
soils, especially coralline islands. By the late 19th century, its value as a
shade crop for the new coffee and cacao plantations of Asia promoted further
international distribution and planting.
For centuries, Mexican and Central American naives supplemented their diets
with protein-rich seeds from young pods of at least four Leucaena
species. Seeds were eaten fresh or boiled. L. esculenta, named for its
large mild-tasting seeds, is the best known species for this use, and is
commonly grown in Mexican midlands today for its seeds and as a green manure
crop. L. esculenta seeds heavily in these regions, but has not produced
seed well in Hawaii or Taiwan. Other species whose seeds are eaten include
L. leucocephala, L. macrophylla and L. pallida (Zarate 1984).
Leucaena issues contain 1-5% mimosine (Arora and Joshi 1984), a thermolabile
amino acid which is readily destroyed by heat treatment. A degradation
product, dihydroxypyridine, also has toxic properties (Brewbaker 1987b).
Leucaenas were associated with most pre-Columbian Indian civilizations located
between Honduras and Southern Mexico, which all depended on maize as their main
food staple. During fallow periods, legume trees could have restored fertility
rapidly to maize-cropped soils in areas such as the Yucatan peninsula
(Gomez-Pompa et al. 1988). The regularity of maize-based civilization collapse
(Olmec, Maya, Zapotec, Teotihuacan, Anasazi) probably related to co-evolved
maize diseases or pests (Brewbaker 1979), rather than to soil depletion.
Leucaenas are found associated with the ruins of most Mayan ceremonial centers
today. Were they recognized and used as green manure crops, as well as sources
of wood for construction and fuel? The high response of maize to applied
nitrogen, organic or inorganic, is known to all maize farmers, and would hardly
have eluded the sophisticated maize growers among Maya or Anasazi.
We have been attempting since 1981 to produce seed via hand-pollination of all
possible species hybrids among thirteen Leucaena species (182
combinations, including diploid and tetraploid L. diversifolia). One
hundred and seventy-one (94%) of these species combinations have been tested,
and 148 (87%) involved crosses of at least seven inflorescences (average 16.1
florets pollinated per inflorescence). A total of 2941 inflorescences (47,102
florets) have been hand cross-pollinated. Self-compatible species [L.
diversifolia (4x) and L. leucocephala] were hand-emasculated between
3 and 5 AM on the morning of anthesis.
The ability of species to produce interspecific hybrid seed is summarized in
Table 2. Hybrid verification was morphological and/or chromosomal (Sorensson
1987). The genus is highly intercompatible (46%), with 79 combinations
producing F1 interspecific seed, 15 producing abortive seed and 76 incompatible
combinations. Fifty-seven of the 78 combinations (74%) producing
apparently-viable seeds were grown and verified; in addition one more hybrid
was selected among open pollinated progenies: L. macrophylla x L.
diversifolia (2x). Unverified hybrid combinations either did
not germinate because of damage from seed beetles (Araecerus levipennis
Jordan) or died a few days after germination, before verification was possible.
Interspecific compatibility in the genus appears likely to increase with
further testing to nearly 70% (Sorensson 1987).
Pollen stainability of 58 species hybrids is shown in Table 2. Twenty-one
hybrids (38%) have not yet flowered, some of which were stunted. Twenty-six
hybrids (45%) produced seeds via open pollination; average seed production of
these was estimated to be half that of L. leucocephala (Sorensson
Only thirteen hybrids (22%) have flowered without setting any pods (Table 2).
Of these, six have flowered well and are represented by numerous trees to show
they are seedless (Fig. 2). The relatively low proportion of seed sterile
hybrids is probably related to the buffering effect of high chromosome numbers
in the genus. Eight of the thirteen seedless hybrids (62%) are triploids, four
are diploid (31 %) and one is an unusual tetraploid produced from 2x-4x mating,
L. retusa x L. diversifolia (F1 is 2n = 4x =
Unpublished research by Sorensson and Nagahara suggested that irregular
chromosome pairing in triploid and diploid hybrids was the basis for abnormal
size and staining of pollen whose tubes did not grow successfully in vitro.
L. retusa x L. leucocephala (80 chromosomes) produced F2 progeny
from open pollination even though its pollen did not grow successfully in
vitro. Mean pollen stainability of the 37 hybrids which have flowered was 59%,
and mean pollen stainability of seedless hybrids was 33%.
Recent worldwide damage to L. leucocephala by psyllids attests to the
probable narrow gene base of this species, and we treat our hybrid program
largely as a way to broaden the gene base of L. leucocephala. Because
it is tetraploid (2n = 104), as are other high-yielding species L.
diversifolia and L. pallida, our breeding has largely been at the
tetraploid level. These three tetraploid species produce excellent
single-cross hybrids in all combinations, and open-pollinated F2s from these
planted in multinational sites have generated much interest (cultivars KX1, KX2
Three-way hybrids among the tetraploid species have shown promise for
broadening the gene base from which breeders can make selections. Some of
Sorensson's three-way hybrids were found to be self-compatible (the female
parent was self-incompatible), allowing a single seed descent breeding
approach. Tetraploid three-way hybrids were also derived through unreduced
gametes between two tetraploid species and the frost-tolerant diploid species
L. retusa (Sorensson and Brewbaker 1987). The use of unreduced gametes
and of colchicine for chromosome doubling may facilitate gene transfer from
diploid species to L. leucocephala.
Many tropical environments are hostile to L. leucocephala. These
include regions above 500-1000 m elevation with mean annual temperatures below
22°C, areas where temperatures remain below freezing for more than a few
hours at a time, and sites with acid and/or high aluminum soils below pH 5.
Interspecific hybrids afford possible solutions to these problems. For
example, hybrids of L. diversifolia (4x) x L. leucocephala
averaged 4.5 m/year/height increment in a two-year period at Waimea, Hawaii
(850 m elevation, mean annual temperature 17°C). Two- and three-way hybrids
involving L. leucocephala and frost-tolerant L. retusa are being
checked for frost resistance.
Hutton (1984) tested several species and hybrids for acid soil tolerance in
Colombia and Brazil, and L. diversifolia, L. lanceolata and L.
shannoni (all 2n = 52) showed potential as parents. His current
program is largely based on hybrids derived from triploid L.
diversifolia x L. leucocephala (F1 is 2n = 3x = 78).
Legumes produce the major gums used in foods and other industries. Gum arabic
from Acacia senegal is particularly well known. L. leucocephala
has infrequently produced a translucent tan gum under stress. Gummosis was
first observed in India following attack by Fusarium semitectum, and was
later seen in Hawaii following attacks by Phytophthora drechsleri and
wood-boring beetles. Gum production was sporadic, low yielding, and was often
associated with wood dieback
Analysis of several leucaena gums has revealed that they have the closest match
to gum arabic of any gums tested from a hundred or so tropical trees. Although
toxicity and related studies are needed, leucaena gum may have potential for
use as a substitute for gum arabic (Anderson 1986).
L. leucocephala x L. esculenta hybrids segregated trees which
exuded gum copiously, and have not had wood dieback. Nine hybrid trees of this
pedigree were grown at Waimanalo, Hawaii for four years during which
approximately a third failed to produce gum, another third exuded gums
sporadically and another third exuded gums heavily (Fig. 1). These high gum
yielders exceeded the mean annual per-tree gum production (250 g/tree) of gum
arabic by Acacia senegal. Gum production appeared as balls or drippings
from mature bark, and was heaviest in the dry season. Hybrids of this type are
seedless, have good vigor and psyllid resistance, and could prove promising for
Like the arboreal forms of L. leucocephala, a number of species hybrids
appear to produce the thick straight boles required for use in furniture or
construction timber. L. pulverulenta x L. leucocephala was
preferred in Indonesia about 50 years ago for its straight boles, and fast
growth at cooler upland sites. Early studies in Hawaii showed it to be a
vigorous hybrid (Gonzalez et al. 1967), although its psyllid susceptibility now
curtails its use in lowland sites. Hybrids which have potential to form trees
with dbh of 30 cm and straight boles of 5 m include the following:
Silvicultural practice should include dense planting (e.g., 10,000 stems/ha)
with thinning for fuelwood at one and three years, and harvest after six to
|Seed parent||Pollen parent||Chromosome|
number of F1
|L. leucocephala|| x ||L. pallida ||104|
|L. leucocephala ||x ||L. diversifolia ||104||(Fig. 3)|
|L. pulverulenta ||x ||L. diversifolia ||80||(Fig. 2)|
|L. diversifolia (2x) ||x ||L. diversifolia (4x) ||78|
|L. diversifolia ||x ||L. leucocephala ||78|
Polewood is commonly used in the production of vine crops (black pepper,
passion fruit, pole beans) where long, straight, thin poles are preferred. The
hybrid L. diversifolia (4x) x L. pallida is psyllid resistant and
grows as a pseudo-shrub with many long straight branches. High-density
planting of the five hybrids listed above would also produce stems suitable for
Fresh herbage yields of L. leucocephala (40-80 fresh t/ha-yr) matched
or-exceeded those of other tropical legumes when moisture was not limiting
(Brewbaker 1987b). Psyllid resistance of hybrids like L. leucocephala x
L. pallida, however, exceeds that of any L. leucocephala,
permitting higher fodder yields under psyllid attack. L. leucocephala x
L. pulverulenta (Gupta et al. 1987) and L. diversifolia (4x) x
L. leucocephala (R.A. Bray personal comm.) both have outyielded L.
leucocephala cut for fodder under certain conditions.
Short heavily forked trees are preferred for herbivore browsing. Most
accessions of the tetraploid species L. pallida are low forking and they
confer this trait to some of their hybrids with L. leucocephala, some
low shrubby dwarfs also result (Sorensson 1987). Low mimosine contents
(0.5-1.0%) would enable leucaena fodder to be fed in higher quantities to
nonruminants like chickens, horses and tilapia. Examples of Leucaena
species and hybrids with low mimosine are L. diversifolia and L.
pulverulenta and their hybrids.
An important use of leucaenas is as a shade or nurse tree in plantations of
coffee, cacao, quinine or tea, or as supports for vine crops. L.
leucocephala's seediness is a major deterrent to this use, as its weediness
raises management costs. Some coffee and tea plantations in Indonesia graft
seedless leucaena clones for use as nurse crops. Our pollen analysis of two of
their clones suggests they are aneuploids derived from triploid species hybrids
of L. diversifolia (2x) x L. leucocephala or L.
pulverulenta x L. leucocephala.
Non-seedy or seedless clones are attractive options for most of leucaena's wood
uses, including fuelwood, pulpwood, roundwood, charcoal, parquet, and
craftwood. Hybrids such as L. retusa x L. esculenta make
attractive home ornamentals. Vegetatively propagated clones of
self-incompatible species which are attractive to honeybees, like L.
lanceolata and L. shannoni, should have longer flowering seasons due
to inhibition of seed set.
Commercialization of seedless leucaenas requires economic methods of production
by vegetative or seed propagules. Rooting of vegetative cuttings of leucaenas
has only been successful in temperate greenhouses. Mericlone and other tissue
culture techniques have similarly succeeded only under experimental laboratory
conditions (Brewbaker 1987b). Grafting has been used with some success in
cooler regions of Indonesia. These methods must be adapted for large-scale
A second method of producing seedless leucaenas exploits the
self-incompatibility (SI) characteristic of all diploid species and of L.
pallida. A SI species would be cloned and used as a female to hybridize
with SC species. Species would be interplanted at an appropriate ratio of seed
to pollen-parent and allowed to produce interspecific hybrid seed through
open-pollination. Promising hybrids that lend themselves to this approach
include the triploid hybrids L. diversifolia (2x) x L.
leucocephala, and L. pulverulenta x L. diversifolia (4x).
A third method to produce seedless leucaenas could involve the
self-incompatibility system. Inbreds would be produced through
"pseudo-self-fertility" treatments not yet applied to leucaena (although rare
selfs have been identified, Sorensson 1987), and S allele homozygotes
identified. When two such homozygotes are planted in isolation, all seeds
produced by both parents are of a single S allele heterozygote, e.g.,
S1S2. When these seeds are grown in isolation as a plantation, seedless
progeny will result. Gamete sterility of the triploid type is not required,
thus expanding the species and species hybrids that could be exploited
Marketing of hybrid seeds is a prerequisite for a successful leucaena seed
industry. L. leucocephala and other species have not attracted seed
industries because heavy and early seeding limits profitability. Our proposed
technologies for producing seed of seedless hybrids have the double benefit of
providing the profit incentive needed to spur a hybrid seed industry, and
providing high-yielding ecologically acceptable hybrids with unique and useful
- Anderson, D.M.W. 1986. Gum exudation by Leucaena leucocephala. Leucaena
Res. Rpt 7:108-109. (Nitrogen Fixing Tree Assoc., Waimanalo, Hawaii).
- Arora, S.K. and U.N. Joshi. 1984. Chemical composition of leucaena seeds.
Leucaena Res. Rpt. 5:16-17.
- Brewbaker, J.L. 1979. Diseases of maize in the wet lowland tropics and the
collapse of the classic Maya civilization. Econ. Bot. 33:101-118.
- Brewbaker, J.L. 1987a. Species in the genus Leucaena. Leucaena Res. Rpt.
- Brewbaker, J.L. 1987b. Leucaena: A multipurpose tree genus for tropical
agroforestry. p. 289-323 In: H.A. Steppler and P.K.R. Nair (eds.).
Agroforestry: A decade of development. ICRAF, Nairobi Kenya.
- Brewbaker, J.L., R.A. Wheeler, and C.T. Sorensson. 1988. Pysllid tolerant
highland leucaena yields. Leucaena Res. Rpt 8:11-13.
- Gomez-Pompa, A., J.S. Flores and D.A. Bainbridge. 1988. Yucatec Maya resource
management, Agroforestry in the dry tropics. Unpublished data, Univ.
California, Riverside, CA.
- Gonzalez, V, J.L. Brewbaker and D.E. Hamill. 1967. Leucaena cytogenetics in
relation to the breeding of low mimosine lines. Crop Sci. 7:140-143.
- Gupta, V.K., N. Kewalramani and V.S. Upadhyay. Evaluation of leucaena species
and hybrids in relation to growth and chemical composition. Leucaena Res. Rpt.
- Hughes, C.E. 1988. A new leucaena from Guatemala. Leucaena Res. Rpt
- Hutton, E.M. 1984. Breeding and selecting leucaena for acid tropical soils.
Pesquisa Agropecuaria Brasileria 19:263-274.
- Sorensson, C.T. 1987. Interspecific hybridization in Leucaena Bentham.
MS thesis, Univ of Hawaii at Manoa.
- Sorensson, C.T. and J.L. Brewbaker. 1987. Utilizing unreduced gametes for
production of novel hybrids of Leucaena species. Leucaena Res. Rpt.
- Zarate, S.P. 1984. Revision del genero Leucaena Benth. de Mexico (In
Spanish). Anales del Inst de Biologia, Univ. of Mexico. Serie Botanics. UNAM,
Table 1. Currently recognized Leucaena species.
zSome variability exists among chromosome counts.
|Species ||Abbrev. ||Somatic |
|L. collinsii ||COLL ||56z ||Britton & Rose ||1928|
|L. diversifolia ||DIV2,4 ||52, 104 ||Bentham ||1842|
|L. esculenta ||ESCU ||52 ||(Moc & Sesse) Bentham ||1875|
|L. greggii ||GREG ||56 ||S. Watson ||1888|
|L. lanceolata ||LANC ||52 ||S. Watson ||1886|
|L. leucocephala ||LEUC ||104 ||(Lam.) de Wit ||1842|
|L. macrophylla ||NMCR ||52z ||Bentham ||1844|
|L. pallida ||PALL ||104 ||Britton & Rose ||1928|
|L. pulverulenta ||PULV ||56 ||(Schlecht) Bentham ||1842|
|L. retusa ||RETU ||56 ||Bentham ||1852|
|L. salvadorensis ||SALV ||56 ||Standley ||1928|
|L. shannoni ||SHAN ||52 ||Donn. Smith ||1914|
|L. trichodes ||TRIC ||52 ||(Jacq.) Bentham ||1842|
Table 2. Flowering status and F1 pollen stainability of verified
zNF = Not flowered yet. Seedless hybrids noted with an asterisk (*). Pollen
stainability is the mean of 200+ pollen grain samples, stained in cotton
blue/lactophenol and based on grains with normal diameter and complete
| ||Pollen stainability (%) of F1|
|Species ||CO ||D4 ||D2 ||ES ||GR ||LA ||LE ||MA ||PA ||PU ||RE ||SA ||SH ||TR|
||Fig. 1. Gum (12 cm length) from mature bark of L. leucocephala
K8 x L. esculenta K138. Estimated annual production of gum from this
tree is one kilogram.
||Fig. 2. Seedy L. diversifolia K156 (left) and seedless triploid
L. pulverulenta x L. diversifolia hybrid (right, F1 is 2n
= 3x = 80). Both trees are five years old. Vertical bar = 10
Fig. 3. Bole of a seven-year old L. diversifolia K186 x L.
leucocephala K8 (F1 is 2n = 4x = 104). Bole is 28 cm
diameter at breast height.
Last update October 2, 1997