Potential use of cassava starch and banana juice in the in vitro micropropagation of banana (Musa spp.)
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In Uganda, banana is a major staple food as well as an important cash crop in the local economy. It also serves as one of the most important food security crops of Uganda. In spite of this, banana cultivation faces constraints which include bacterial wilt disease, fungal diseases, nematode infestations, and virus infections. The above production constraints are aggravated by the lack of quality and inadequate quantities of planting material. The inadequate availability of planting materials can be minimized through production of high quality and disease-free uniform planting material using tissue culture. However, micropropagation of banana is a capital-intensive industry, and in some cases the unit cost per plant is high and unaffordable by the resource poor farmers. The high cost is largely attributed to cost of in vitro media components and specifically additives like agar and sugar. The main objective of this study was to evaluate the potential of cassava starch and banana juice as an alternative low-cost gelling agent and energy source, respectively, in the culture medium for banana in vitro micropropagation. Cassava starch was extracted from fresh cassava root tubers of AKENA and TM 2961 varieties and their physico-chemical characteristics were evaluated. As a gelling supplement in Murashige and Skoog (MS) medium, various amounts of cassava starch in combination with agar were evaluated for their ability to support in vitro growth of Kisansa, Yangambi (KM 5) and FHIA 17 banana cultivars. Banana juice was also extracted from banana cultivars Mbidde, Kayinja and KM 5 and quality analysis carried out. As an energy supplement in MS medium, various amounts of banana juice were evaluated for in vitro growth of Nakabululu, Nakinyika and Kabula banana cultivars. A combination of the most appropriate type and amount of cassava starch and banana juice were then supplemented to MS media and their ability to support in vitro growth of banana cultivars was evaluated. The response in terms of shoot height and proliferation rate was recorded for all banana cultivars under different media treatments. Analysis of variance (ANOVA) was carried out to ascertain differences in the growth response, and principle component analysis in order to determine components of banana juice that were critical in supporting growth. MS media supplemented with TM 2961 cassava starch significantly (P<0.001) influenced the number of shoots and shoot height Kisansa, KM 5 and FHIA 17 banana cultivars compared to AKENA cassava starch. The mean number of shoots and shoot height of banana cultivars ‘Kisansa’, ‘KM 5’ and ‘FHIA 17’ was highest when cultured on MS medium supplemented with combination of 50 g/l TM 2961 cassava starch and 2 g/l agar compared to other combinations in the experiment; and was not significantly different from media supplemented with 7 g/l agar. This combination reduced the cost of using agar by 54.49 %. The mean number of shoots and shoot height of banana cultivars ‘Nakabululu’, ‘Nakinyika’ and ‘Kabula’ was highest when cultured on MS medium supplemented with 50 ml/l Mbidde juice, 20, 30, 40 and 50 ml/l Kayinja juice and 40 and 50 ml/l Km 5 juice compared to other concentrations used in the study. Principle component analysis showed the contribution of various juice components to in vitro banana cultivar response. These were sucrose and fructose in the first principle component and, potassium and phosphorus in the second principle component. There were cost implications towards use of respective substitute volumes of juice as energy source. These were in terms of either cost increments (+) or reduction (-). These were +18.39 % for Mbidde juice, -51.16 %, -26.75 %, -2.33 %, +22.09 % respectively for Kayinja juice and –0.17 % and +24.78 % respectively for Km 5 juice. The mean number of shoots and shoot height of banana cultivars ‘Mpologoma’, ‘KM 5’ and ‘Nfuuka’ was highest when cultured on MS medium supplemented with a combination of the most appropriate gelling agent and 40 ml/l Km 5 juice (or 50 ml/l Kayinja juice) compared with other concentrations of banana juice. Compared with the use of 7 g/l agar and 30 g/l table sugar as media supplements, a combination of 50 g/l TM 2961 cassava starch and 2 g/l agar and either 40 ml/l Km 5 juice (or 50 ml/l Kayinja juice) reduced the cost by 53.20 % (or 52.67 %). Cassava starch with small amounts of agar improve its gelling ability and hence able to support in vitro growth. In vitro bananas responded more on cassava starch gelled media than agar gelled media. Therefore, cassava is a locally available and cheap gelling agent that can be used to substitute agar. On the other hand, in spite of various cost implications, banana juice was found to be a potential energy source for in vitro micropropagation of bananas. However, the study indicated that a combination of the most appropriate cassava starch plus agar (50 g/l TM 2961 and 2 g/l Agar) and banana juice (40 ml/l Km 5 juice or 50 ml/l Kayinja juice) could substitute agar and sugar as gelling and energy sources, respectively, as supplements in MS media for in vitro growth of banana.