African yam bean : morphology, clonal propagation and nitrogen fixation
Morphological and growth observations made on landraces of African yam bean (AYB) used in this study confirm that this species is the most morphologically variable in the genus (Potter, 1992). Morphological characters such as seed colour, stem colour, internode length, leaf size and number of leaves per plant were found to vary between landraces. Growth and development was controlled by both genotype and environment. Flowering was observed only when plants were grown at 25°C, rather than at 30°C, with a 12 h photoperiod. Tuber formation occurred only in AYBS and not in other landraces. Growth rates differed between landraces and between environments with plants grown in the soil displaying faster growth than those grown in pots. The response to the environment (pot and soil experiment) differed between landraces, i.e. AYB1 performed better than AYB2 in the pot experiment, whereas it was surpassed by AYB2 in the soil experiment. Clonal propagation protocols were developed using nodal explants/propagules to reproduce material with a high level of genetic uniformity from existing shoot meristems. Clonal propagation was investigated using macro (leafy stem cuttings) and micro (in vitro propagation from nodal stem segments) approaches. Axenic shoot cultures have been achieved from stem nodal segments sterilised with 10% "Domestos" bleach and grown in MS-based medium fortified with cytokinins. Amongst the cytokinins used, BAP (6-benzylaminopurine) was found to be more suitable than TDZ (N-phenyl-N'-1,2,3thidiazol-5-ylurea) and 2iP (6-(y, y-dimethylallylamino)purine) at both culture establishment and shoot multiplication stages, although optimisation of the protocol for shoot multiplication requires further study. There was persistent callus proliferation at both the establishment/initiation of cultures and the multiplication stage and the use of other plant growth regulators, such as GA3 (Gibberellic acid) and TIBA (2,3,5-triiodobenzoic acid), known to counter callus growth in cultures, did not give positive results. Although in vitro adventitious root formation was erratic, some shoots were able to root when exposed to auxins (IBA [indole-3-butyric acid] and NAA [αnaphthaleneacetic acid]) and were established in compost. IBA was preferable to NAA, as it induced more root formation. Overall, AYB cuttings produced adventitious roots relatively easily with or without auxins. Auxins at low concentrations induced rapid formation of roots in high numbers. Unlike in vitro rooting, adventitious rooting of cuttings was as high as 100% without any auxin treatment, suggesting a possibility of other factors involved in the rooting process in vitro. A cheap source of nitrogen for AYB is in the form of biological nitrogen fixation. AYB nodulated profusely with strains of both a slow growing Bradyrhizobium sp. and a fast growing Rhizobium sp., plants forming nitrogen fixing nodules with strains ORS302, CP279 and NGR234. Nitrogen fixed from the atmosphere accounted for 79-98% of the plant nitrogen and supported plant growth by an increase of up-to 1547% of dry matter in shoots.