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Dep. of Tropical Plant and Soil Science, College of Tropical Agriculture and Human Resources, Univ. of Hawaii, 3190 Maile Way, Honolulu, HI 96822. Institutional Sponsor: University of Hawaii
* Corresponding author (brewbake{at}hawaii.edu).
ABSTRACT
KX2-Hawaii (Reg. No. CV-281, PI 647963) (Leucaena hybrid) is a tropical, multipurpose, woody forage legume developed at the College of Tropical Agriculture and Human Resources (CTAHR) of the University of Hawaii. It was derived from hybrids made in 1976 between Leucaena leucocephala (Lam.) de Wit and L. pallida Britton & Rose, followed by six cycles of recurrent mass selection. Each cycle involved 2- to 4-yr growth of 
2000 plants pollarded quarterly. All trees were eliminated except self-sterile segregants with high forage-regrowth vigor and with high tolerance of the psyllid (Heteropsylla cubana Crawford). Approximately 120 remaining trees were chosen as parents for the next cycle of selection. KX2-Hawaii was released by CTAHR in 2007.
Abbreviations: CTAHR, College of Tropical Agriculture and Human Resources, University of Hawaii
KX2-Hawaii (Reg. No. CV-281, PI 647963) (Leucaena hybrid) is a tropical, multipurpose, woody, forage legume released by the College of Tropical Agriculture and Human Resources (CTAHR) of the University of Hawaii (Honolulu). It was derived from an interspecific hybrid in 1976 between Leucaena leucocephala (Lam.) de Wit and L. pallida Britton and Rose. The cultivar was released following six cycles of recurrent mass selection for self-sterility, high forage yield and resistance to the psyllid insect (Heteropsylla cubana Crawford). KX2-Hawaii is readily distinguished from the parent species by leaf and leaflet morphology, flower head color and size, and pod and seed size. It has high tolerance to the psyllid, to which its L. leucocephala parent is susceptible. It grows vigorously under cold temperatures that dwarf the L. leucocephala parent. Its forage yields greatly exceed those of the L. pallida parent and exceed L. leucocephala when under psyllid attack.
Among the 22 recognized taxa in this New World genus, Leucaena leucocephala is the only species cultivated internationally. It is the most widely used tropical woody legume, a versatile multipurpose tree valued for fodder, fuelwood, food, green manure, timber, and sustainable land management (National Academy of Sciences, 1977; Shelton and Brewbaker, 1994). Originating in Latin America, a shrubby variant of this species was introduced to the Philippines with the Spanish galleons in the 16th century and later distributed worldwide. Improved arboreal cultivars from Hawaii now dominate production. India's Bharatiya Agroindustries Foundation (BAIF) has distributed more than 25 tonnes of seeds (20 million seeds per tonne) of ‘K8’ (Brewbaker, 1975) and other improved leucaena varieties from Hawaii (Dr. N. Hegde, personal communication, 2006). The Universities of Hawaii, Queensland, and Oxford assumed leadership in germplasm collections, breeding, and distribution of improved leucaena varieties (Bray et al., 1997). Among recent releases in Australia is ‘Tarramba’, a cultivar derived from Hawaii's accession K636 and grown increasingly on the more than 150,000 ha of legume-supplemented grass pastures in Queensland (www.leucaena.net). Genetic improvements of L. leucocephala are being sought through hybridization for tolerance to psyllid insects, cold temperatures, and acid soils.
Methods
Parental Species
KX2-Hawaii was derived from a hybrid of L. leucocephala and L. pallida. Accession K8 (PI 263695) of L. leucocephala is the parent of KX2-Hawaii and was collected in 1959 by H. S. Gentry near Moyohua in Zacatecas, Mexico. It represents the arboreal subspecies glabrata (Rose) S. Zarate. A subline was released by Brewbaker (1975) as ‘Hawaiian Giant K8’ and is now cultivated worldwide for forage and wood products.
The other parent of the KX2-Hawaii population was ‘K376’ of L. pallida Britton and Rose that was collected by the author north of Oaxaca City, Mexico, in 1967. This highland Mexican species was chosen for its resistance to the psyllid insect. The psyllid was accidentally introduced into Hawaii from Florida in 1984 and continued worldwide to cause severe defoliation and yield loss in the species L. leucocephala. It is now partially controlled by predatory insects. Five of the 15 species of the genus Leucaena showed resistance to the psyllid insect in Hawaii, including Leucaena pallida and its hybrids with L. leucocephala. Leucaena pallida has the same chromosome number as L. leucocephala (2n = 104) and crosses readily with it (Brewbaker and Sorensson, 1990; Sorensson and Brewbaker, 1994). In contrast to the self-pollinated and seedy L. leucocephala, L. pallida is self-incompatible, bee-pollinated, and commonly very low in seed yield. Among CTAHR's collection of 1100 leucaena accessions, 56 were of the species L. pallida and traced to our expeditions of 1967, 1985, and 1997 in Latin America. Synonyms for L. pallida include L. dugesiana, L. oaxacana, and L. paniculata (Brewbaker, 1987), and it is believed to be an amphidiploid hybrid of L. esculenta and L. trichandra (Pan, 1985). Hughes (1997) is the taxonomic authority for this tropical genus of 22 species. All leucaena accessions and hybrids in Hawaii have been designated by the symbol K (koa haole, a local name), with species hybrids designated KX.
Population Development
KX2-Hawaii is an open-pollinated population representing six cycles of recurrent selection from hybrids made in 1976. For each breeding cycle, approximately 2000 trees were planted and coppiced quarterly over a period of two or more years to allow selection based on the following major criteria: (i) psyllid resistance, (ii) forage regrowth vigor, and (iii) self-sterility due to S allele–type incompatibility. Recurrent selection cycles were established at the Waimanalo Research Station of the University of Hawaii at sea level on Oahu in 1985 (F1 trees), 1987 (F2 trees, Cycle 1 of selection), and then in 1989, 1992, 1996, 2003, and 2007 (Cycle 6). Additional plantings at an elevation of 800 m grew very well (Austin et al., 1997) but failed to produce seeds. Approximately 120 trees were selected each cycle to serve as seed parents (other trees were killed), and panmixia is assumed. Seediness is an undesirable trait associated with the highly self-fertile common variety of L. leucocephala and led to our emphasis on self-sterility. Some self-fertile trees continue to segregate in KX2-Hawaii as a result of the competition interaction among S alleles characteristic of many polyploid plant species (Brewbaker, 1954). Seed harvest will continue among selected trees in Cycle 6 from orchards at CTAHR's Waimanalo Research Station on Oahu, Hawaii.
Characteristics of KX2-Hawaii
Yield and Psyllid Resistance
Interspecific hybrids of L. leucocephala and L. pallida were extremely vigorous vegetatively, with the large leaves and smaller leaflets of the L. pallida parent (Sorensson and Brewbaker, 1994; Austin et al., 1995; Brewbaker and Sun, 1999). Early cycles of selection of this hybrid were distributed as ‘KX2’ in the late 1980s to seven countries to examine forage yields and psyllid resistance. It was later distributed internationally by the Australia Council of International Agricultural Research. Its psyllid resistance was confirmed, and it outyielded L. leucocephala under psyllid infestations; for example, it yielded more than 10-fold that of L. leucocephala entries in a 9-mo trial in Queensland under psyllid attack (Shelton and Brewbaker, 1994, Fig. 2.1.3). Austin et al. (1995) reported forage dry matter yields per cutting in Hawaii on an 11-wk cycle under psyllid infestations of 6.4 t ha–1 for KX2 and 1.9 t ha–1 for L. leucocephala, with a psyllid rating of 1.6 and 6.5 for KX2 and L. leucocephala, respectively (where 1 = resistant and 9 = completely defoliated). Allometric equations that best predicted total and forage dry matter yields of KX2 annually in these trials were 5.37 x 10–5 x h2.714 and 1.66 x 10–4 x h2.365, respectively, where h = height of regrowth in centimeters. Despite its tolerance of psyllids, none of the 57 accessions of L. pallida in Hawaii achieved the forage yield of KX2 or its K8 parent in these trials. Austin et al. (1997) later reported that forage phenolic levels of KX2 were only slightly higher than in L. leucocephala and did not appear to be associated with significant loss of palatability or digestibility by cattle. However, L. pallida is much higher in tannins, and some segregation can be expected within KX2-Hawaii.
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An outstanding F1 hybrid of L. leucocephala cv. K636 and L. pallida cv. K748 averaged 2.4 t ha–1 per cutting in these trials. Animal-grazing trials of kikuyugrass (Pennisetum clandestinum Hochst. ex Chiov.) pastures supplemented with KX2-Hawaii are ongoing in this region. At an elevation of 900 m in Haiti, the dry biomass yields of a Florida selection of KX2 averaged about 3 t ha–1 yr–1 between 1997 and 2001, somewhat less than the best entries of L. leucocephala (Isaac et al., 2006). Plants were in hedgerows spaced 4 m apart and interplanted with corn (Zea mays L.), and psyllid infestation was not reported.
Botanical Characteristics
KX2-Hawaii grows to a mature height of about 13 m in 4 yr, with regrowth of 3 to 5 m per year (Fig. 1
). It flowers within 2 yr and matures seed in 90 d. Its wood is comparable to other leucaena hybrids and should be excellent fuelwood. It is harvestable as high-value hardwood (specific gravity 0.6) at 25 cm diameter (breast height) on a 10- to 12-yr cycle. It is known to survive continuous coppicing for many years and appears to be highly drought tolerant.
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Seed of KX2-Hawaii is available from Hawaii Foundation Seeds, Dep. of Tropical Plant and Soil Science, CTAHR, University of Hawaii, 3190 Maile Way, Honolulu, HI 96822.
Acknowledgments
The author acknowledges with great respect the significant contributions made toward the development of KX2-Hawaii and related leucaena hybrids by former students and colleagues Drs. M.T. Austin, R. Corrales, F.J. Pan, H.M. Shelton, X.B. Shi, C.T. Sorensson, W.G. Sun, and R.W. Wheeler. Thanks are also expressed to colleagues in Australia, India, Indonesia, Mexico, Philippines, and Thailand who assisted the Nitrogen Fixing Tree Association at the University of Hawaii to evaluate KX2-Hawaii during its development, notably to H.M. Shelton and S. Dalzell of the University of Queensland.
Footnotes
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Received for publication September 24, 2007.
References
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