Variation in yield and compostion of bark chemical extracts of Prunus africana (Hook f.) Kalk and its domestication in Kenya

Abstract

Due to its importance, Prunus africana has in the recent past, drawn a lot of concern. The species is an important medicinal tree as its bark produces an extract used in the treatment of benign prostatic hyperplasia (BPH) and prostate gland hypertrophy (PGH) among other uses. The dwindling of the species abundance in the wild, especially from the regions where it is commercially exploited for its bark necessitates for its domestication. The first step in tree domestication is identification of superior or desired gennplasm. In this study, the bark extracts from different forest zones (Kakamega, Timboroa, Kobujoi, Kinale and Eburu natural forests) in the country, were compared in terms of yield and chemical composition, with an aim of identifying the zones with superior gennplasm, hence recommending them as good planting material sources. Extraction was done using Debat (1974) procedure, while chemical composition was determined using Gas Liquid Chromatography (GLC) and Thin Layer Chromatography (TLC) procedures. The optimum age classes to harvest the bark for medicinal purposes in terms of the extract yield and bark thickness were also determined. A farm etlmobotanical survey was also made with the help of a questionnaire for the assessment of distribution, uses and constraints to domestication of Prunus africana in the study area. The survey results showed that, in both Kobujoi and Kakamega, farmers had a total of 78 tree species in various agroforestry systems. These mostly constitute the high value tree species. Kobujoi, however, had more tree species than Kakamega. Kobujoi also recorded more farmers who had Prunus africana on their farms than Kakamega forest reserve. Boundary planting was the most preferred pattem of planting in both areas. In Kakamega, the species occurrence near homesteads was not encouraged. More Prunus africana on farms in both Kobujoi and Kakamega areas were planted (with the main seedling source being wildings) rather than grew naturally. The multipurpose nature of the species acts as an incentive for its domestication on farms. Several constraints, however, limit its inclusion by fanners in their agroforestry systems. In Kobujoi, the species competition with the agricultural crops (53%) was the main constraint while in Kakamega, certain customs and beliefs associated with the species discouraged the farmers (39%) from cultivating it. Other obstacles include lack of seedlings, small land sizes, land/ tree tenure, slow growth of the species, drought and browsing by the grazing animals. However, despite the constraints, the majority of farmers in Kobujoi (97%) and Kakamega (56%) had plans to cultivate Prunus africana trees in future. The age of Prunus africana had an effect on bark yield. At Kakamega, a 43 year and 53 year old forest plantation had significantly higher (P<0.05) yield than a younger (17 year old) plantation. At Kobujoi, a similar trend was also observed whereby an 18 year and 20 year old plantation forest had significantly higher (P<0.05) bark extract yield than a 13 year old. Similar observations were made in relation to the species bark thickness and trees age. Kenyan populations further showed variations in terms of the yield and the chemical composition of Prunus africana's bark extract from different provenances. Timboroa provenance generally had the highest yield while Kobujoi had the lowest yield. The yield ranged from 0.73% to 1.90% extract yieldlbark powder by weight. However, the GC analysis showed that Kakamega's crude bark extract had the highest number of compounds followed by Kinale's while Kobujoi's has the least number of volatile compounds. Trees from Kinale have higher Sitosterol content than the rest of the provenances. When TLC was done, Kinale provenance showed the highest number of compounds, (11) closely followed by Kobujoi and Kakamega provenances with 10 compounds each while Timboroa and Eburu provenances had 8 compounds each