Sexual and Asexual Propagation Techniques for Domestication of Allanblackia stuhlmannii (Engl.) Engl. in North-Eastern Tanzania

Abstract

Allanblackia stuhlmannii (Engl.) Engl. is a fruit-bearing tree species in the family Clusiaceae growing in eastern Arc Mountains, Tanzania. The seed contains edible, high quality oil used for production of food and cosmetics and is desired by oil-manufacturing companies. Exploitation of this species in the wild for oil production with little or no attention to its conservation and with current lack of propagation knowledge, will threaten it and its habitat. To satisfy the nut demand for industrial use, domestication which is currently constrained by propagation techniques due to poor and long germination and rooting periods, poor scion-rootstock healing, dioecism and slow growth of the species, is necessary. There is therefore need to develop sexual and asexual propagation methods that would allow the multiplication and selection of superior genotypes. The objective of the present study was to investigate the requirements for seeds germination and success rate of grafting in Allanblackia stuhlmannii under two nursery conditions, that is, highland and lowland environments. This research investigated the factors affecting seeds germination and success rate of grafting under nursery conditions in Allanblackia stuhlmannii. The first experiment evaluated methods for seed germination including: (i) sowing methods, (ii) application of germination enhancers, and (iii) physical treatments of seeds. The second experiment assessed three grafting techniques and two rootstock types under nursery conditions using scions from five mother tree sources. The fruits were collected from the ground and the seeds extracted after storing fruits for 6 weeks. For the seed germination study, a randomized complete block design with a factorial split plot arrangement with 25 seeds per treatment in each of 16 plots per block or site was adopted. The treatments were seeds sowing methods (sand in a room, sand in nursery and polybags in a room), seeds source trees (L19, L26, L28 and L62), scarification (intact or removal of seed coat) and chemical treatments (GA3, AccelegrowTM, fungicide and water). For the grafting study, five mother trees were used as scions sources (L19, L26, L28, L62 and L64), three grafting methods (top cleft, T-budding and side veneer), and two rootstocks types (rootstocks with/without leaves) were tested and monitored under nursery conditions. Two nurseries; Longuza (lowland at 180m a.s.l.) and Kwamkoro (highland at 995m a.s.l.) located at Amani Nature Reserve block in the East Usambara Mountains, Tanzania were used as study sites. Longuza nursery recorded the highest seed germination with 77.8%. The mean germination rate was 56.5% with scarified seeds recording 62.8% and un-scarified seeds at 50.2%. The type of sowing method influenced the rate and capacity of seeds germination but seeds scarification and seeds sources (mother trees) had no significant effect on the germination. There was a significant difference in germination between nurseries regardless of environmental (light or dark) conditions that seeds were sowed in. Grafted seedlings achieved survival of top cleft (43%), side veneer (14%) and T-budding (0%) methods, 8 weeks after grafting. There was no significant difference on survival of grafts between rootstocks with or without leaves. However, there were significant differences in survival of scions between grafting methods and scions source (mother trees). From the results, seeds with their coats removed tended to be higher germination by 22.5% compared to seeds with their coats was left intact especially when incubated in polythene bags. Hence, scarification is necessary for rapid and uniform seed germination. The Longuza nursery had higher germination capacity by 42.6% compared to Kwamkoro nursery, hence providing ideal conditions for seeds propagation in this species. In the grafting experiments, top-cleft tended to have a higher healing by 29% compared side-cleft, hence a superior graft method in this species. This research provides input into development of propagation protocols towards domestication, conservation and management for this species.