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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


  1. INTRODUCTION
  2. THE GENUS LEUCAENA
  3. HISTORICAL USES OF LEUCAENA
    1. Fodder from Shrubby L. leucocephala, a World Vagabond
    2. Human Consumption of Seeds
    3. Green Manure—An Ancient Use of Leucaena?
  4. INTERSPECIFIC HYBRIDIZATION IN THE GENUS LEUCAENA
    1. Species Intercompatibility
    2. Research Objectives Using Interspecific Hybrids
  5. INTERSPECIFIC LEUCAENA HYBRIDS AS NEW CROPS
    1. Relationship Between Adaptability and Use
    2. Gum Production
    3. Furniture, Construction Timber and Polewood
    4. Fodder Production
    5. Shade and Support
    6. Other Uses of Species Hybrids
  6. PRODUCTION OF SEEDLESS HYBRIDS
  7. REFERENCES
  8. Table 1
  9. Table 2
  10. Fig. 1
  11. Fig. 2
  12. Fig. 3

INTRODUCTION

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 psyllid insect.

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.

THE GENUS LEUCAENA

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 phenological isolation.

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 Research Reports.

HISTORICAL USES OF LEUCAENA

Fodder from Shrubby L. leucocephala, a World Vagabond

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.

Human Consumption of Seeds

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).

Green Manure—An Ancient Use of Leucaena?

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.

INTERSPECIFIC HYBRIDIZATION IN THE GENUS LEUCAENA

Species Intercompatibility

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).

Fertility and Seediness of Interspecific Hybrids

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 1987).

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 = 108).

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%.

Research Objectives Using Interspecific Hybrids

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 and KX3).

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.

INTERSPECIFIC LEUCAENA HYBRIDS AS NEW CROPS

Relationship Between Adaptability and Use

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).

Gum Production

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 gum production

Furniture, Construction Timber and Polewood

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:
Seed parentPollen parentChromosome
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
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 eight years.

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 polewood.

Fodder Production

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.

Shade and Support

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.

Other Uses of Species Hybrids

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.

PRODUCTION OF SEEDLESS HYBRIDS

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 use.

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 commercially.

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 properties.

REFERENCES


Table 1. Currently recognized Leucaena species.

Species Abbrev. Somatic
chromosome
number
Author Date
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
zSome variability exists among chromosome counts.


Table 2. Flowering status and F1 pollen stainability of verified species hybrids.z

Pollen stainability (%) of F1
Species CO D4 D2 ES GR LA LE MA PA PU RE SA SH TR
COLL98
DIV48*9587NF15*
DIV29713NF8931NF97
ESCU9*40*
GREG
LANCNF96NF96
LEUCNF856*13*69544330*36*
MACR9898NF
PALL6958
PULVNF5*NF65NFNF
RETU64*77*14NFNF2086NF51*
SALVNF
SHANNFNFNF94NF70*NF
TRICNF88
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 staining.




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 cm.

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 by aw