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Growth of 25 accessions of Leucaena was assessed in 61 'environments' at 19 sites over a 2.5-year period to identify agronomic adaptation to environmental challenges. From analysis of variance, accessions, 'environments' and accession x environment interaction accounted for 14% 48% and 26% of the variation respectively. The large environment component of variance reflected the diversity of test environments, from non-limiting to severely limiting. Genotype x environment analysis techniques were used to identify broad and specific adaptation to environmental limitations. The FI hybrid accession L. pallida K748 x L. leucocephala K636 was comparatively very high yielding in all environments, and displayed excellent broad adaptation. Other accessions which displayed broad adaptation but at lower yield levels included L. pallida CQ3439 and L. pallida K376 x L. leucocephala K8 F5 hybrid. L. trichandra OFI 53/88 displayed specific adaptation to low temperatures in highland tropical environments. No accessions were specifically adapted to strongly acid-infertile soils or to low rainfall. Correlations between dry matter production and six environmental variables attempted to identify the major factors limiting growth of the Leucaena genus. Highly significant correlations were found for soil acidity, minimum temperature and psyllid pressure, and of these soil acidity most affected growth.

One hundred and sixteen accessions of Leucaena were assessed for resistance to the leucaena psyllid that has devastated stands of L. leucocephala pan-tropically since the mid-1980s. Damage to young leaf was assessed using a ratings scale, developed at the University of Hawaii, in replicated field trials at Brisbane, Australia and Los Banos, Philippines. The study identified a continuum of resistance among the accessions, from highly resistant to highly susceptible. Resistance levels were consistent between the two sites with few exceptions. L. collinsii subsp. collinsii, L. confertiflora, L. esculenta, L. pueblana, L. retusa, L. greggii and L. matudae were highly resistant at both sites, while L. leucocephala, L. lempirana, L. involucrata and L. multicapitula were highly susceptible at both sites. Other species were intermediate and were allocated to either resistant or susceptible categories. There was considerable variation in psyllid resistance within some species especially, L. trichandra, L. diversifolia and L. collinsii. Regression analysis was used to determine the influence of environmental variables on psyllid populations. Psyllid population outbreaks were restricted to a temperature range of 10degreesC minimum temperature and 33degreesC maximum temperature, but additional, unidentified factors influenced populations within this range. Leaf chemical characteristics were examined for their potential to confer psyllid resistance, but none were strongly correlated with accession damage ratings.

Leucaena leucocephala is an important agroforestry species pan-tropically, but relatively little is known of the forage production potential of other species in the genus. The agronomic potential of 116 accessions, represent ing the 28 species and subspecies of the Leucaena genus and several artificial hybrid accessions, was evaluated at Los Banos, Philippines and Brisbane, Australia over a 2.5–year period. Accessions were planted into replicated line plots, with 10 trees/plot spaced 50 cm apart, and with rows spaced 3 m apart. The L. pallida × L. leuco cephala KX2 F 1 hybrid accessions were highest yielding at both sites, producing dry matter (DM) yields of over 900 g/m row/month at Los Banos and approximately 320 g/m row/month at Brisbane. In the near-optimal growth conditions at Los Banos, L. leucocephala accessions were highly productive, with the best accessions producing total yields of over 500 g/m row/month. The superiority of KX2 hybrids was most pronounced at Brisbane, where high psyllid pressure during summer, and low temperatures during winter severely constrained growth of L. leu cocephala accessions. In the Brisbane environment, psyllid resistant accessions of L. pallida , L. trichandra and L. diversifolia were more productive than L. leucocephala accessions. Leucaena greggii , L. retusa , L. cuspidata , L. confertiflora , L. pulverulenta , L. pueblana and L. involucrata were of inherently low productivity in both the Brisbane and Los Banos environments. Mortality over the experimental period was very low for most species, particularly for L. leucocephala and KX2 accessions. The KX2 F1 hybrid accessions have considerable agro nomic potential as alternatives to L. leucocephala for use in tropical agroforestry.