Genetic Improvement of Leucaena and Acacia Koa

Sun, Weiguo
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Studies on Leucaena and Acacia koa tree improvement were undertaken in Hawaii from 1992-1996. These studies were described in two sections. The first section includes identification of high biomass yield varieties, estimation of DNA content, vegetative propagation, and forage yield management among Leucaena species and hybrids. Among interspecific hybrids of KX2 F1 (L. pallida x L. leucocephala) and 3-way crosses (A. pallida x KX3 F1 (L. diversifolia x L. leucocephala)), the best entries yielded over 16 Mg ha-1yr-1 of edible forage dry matter and over 40 Mg ha-1yr-1 for wood biomass. All these hybrids were psyllid-tolerant. Heterosis for forage yield averaged 48% (-75 to 160%) and for wood biomass averaged 85% (-99 to 223%). Among intraspecific hybrids involving six L. leucocephala accessions, the best entries were the crosses between K397, K565, and K608. They outyielded a widely planted K636 (L. leucocephala). Heterosis for biomass yield averaged 16% (-28% to 80%). A composite from the selected intraspecific hybrids was released. Nuclear DNA content varied from 1.32 to 1.74 pg/2C for diploid species and from 2.67 to 3.09 pg/2C for tetraploid species. Successful cloning method with more than 80% rooting for KIOOO and KlOOl was developed. More than 30% increase in forage yield was obtained by optimizing the harvest intervals of K636 and KX2 F1. The second section includes studies of the identification of quality seed source for reforestation and selection of superior progenies for genetic improvement among Acacia koa collections. A total of 334 koa accessions were collected from the Hawaiian Islands. These accessions were evaluated for various important traits from seed to tree growth characters. Seven field trials involving 178 accessions were established at Hamakua, Hawaii and Mauanawili, Oahu. The koa populations clearly showed great variations in these traits. These variations are essentially genetic in origin and are useful in selecting progenies for tree improvement. High quality koa seed sources from the Islands were identified. The advanced progenies based on fast growth and tree form were selected for further testing. Silviculture practice study showed that koa trees in the mixed plots grew significantly slower than trees grown alone.
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