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Australian National University (1993)

Geographical variation and physiological studies in Faidherbia albida (Del.) A. Chev. syn (Acacia albida del.)

Marunda, Crispen Takuranei

Titre : Geographical variation and physiological studies in Faidherbia albida (Del.) A. Chev. syn (Acacia albida del.)

Auteur : Marunda, Crispen Takuranei

Université de soutenance : Australian National University

Grade : Master of Science (MS) 1993

Description
Faidherbia albida (DEL.) A. Chev. syn (Acacia albida DEL.) is widely distributed in Africa and parts of the Middle East. Its use in traditional agroforestry systems is well known and its potential in reforesting degraded lands is being evaluated in arid regions of many countries. Geographic pattern of variation of the species in Africa was studied to provide a basis for initiating a genetic improvement strategy. The influence of different environments on phenology and growth, and root morphology and size were studied to understand the growth patterns of the species. To test for the extent of genetic variation, twenty provenances were grown in a glasshouse and twenty-four traits assessed. There was strong evidence of genetic variation between provenances and to some extent, between families-within-provenances. Most of the traits showed a north to south trend, with south-east Africa provenances having high growth rates (in · terms of height, root collar diameter and dry weight production) whereas, north-west provenances had qualities of adaptive significance (in terms of late leaf senescence, large root to shoot and leaf to stem ratios). A few traits showed random variation. The patterns of variation were strongly related to the temperature of seed origin and the type of ecosystem in which the species grows naturally. There were strong negative relations between growth traits and those of adaptive significance. Canonical variate analysis showed that the provenances of F. albida from Africa could e grouped into three regions. The groups fall into upland ecosystem of north-west Africa, the riparian ecosystem of south-east Africa and the Sudan/Ethiopian highlands and adjacent Lodwar valley in Kenya (proposed centre of origin). Most of the traits had high heritability values which indicated that most of the variation was genetic in origin. Based on the observed variation, a breeding strategy which involves delineating provenances, collecting seed, setting up provenance trials and seed production areas was proposed. The effects of daylength and water stress on the phenology and growth habits of provenances from the three regions were examined. Long daylength and water stress caused defoliation to occur, with those from the Sudan/Ethiopian highlands defoliating the most, followed by those from the riparian ecosystem and lastly, by those from the upland ecosystem. Height and root collar diameter extension were significantly reduced in seedlings exposed to long daylength and water stress. The result showed that defoliation of F. albida trees is linked to daylength and moisture and that defoliation reduced growth rate. Provenances from the upland ecosystem had the largest root length to stem height ratio, showing that they invested more biomass in root extension than those from either the centre of origin or those from the riparian ecosystem. Upland sources also had high specific root length values which suggested high root activity. The observations showed that upland provenances were better adapted to survive in drought conditions than either those from the riparian ecosystems.

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